diff --git a/src/conf.h b/src/conf.h
index 094e57df..3e240d5b 100644
--- a/src/conf.h
+++ b/src/conf.h
@@ -863,11 +863,11 @@ int upx_test_overlap       ( const upx_bytep buf,
 
 
 #include "util/snprintf.h"   // must get included first!
+#include "util/util.h"
 #include "options.h"
 #include "except.h"
 #include "bele.h"
 #include "console/console.h"
-#include "util/util.h"
 // xspan
 #include "util/raw_bytes.h"
 #include "util/xspan.h"
diff --git a/src/help.cpp b/src/help.cpp
index 9ccb6ce4..c48c069a 100644
--- a/src/help.cpp
+++ b/src/help.cpp
@@ -109,7 +109,7 @@ struct PackerNames {
         names[names_count].sname = pb->getName();
         names_count++;
     }
-    static bool visit(PackerBase *pb, void *user) {
+    static tribool visit(PackerBase *pb, void *user) {
         PackerNames *self = (PackerNames *) user;
         self->add(pb);
         return false;
diff --git a/src/main.cpp b/src/main.cpp
index 0dd4ec58..72d6ce46 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -679,8 +679,11 @@ static int do_option(int optc, const char *arg) {
         break;
     case 632:
         opt->win32_pe.compress_resources = 1;
-        if (mfx_optarg && mfx_optarg[0])
-            getoptvar(&opt->win32_pe.compress_resources, 0, 1, arg);
+        if (mfx_optarg && mfx_optarg[0]) {
+            int value;
+            getoptvar(&value, 0, 1, arg);
+            opt->win32_pe.compress_resources = bool(value);
+        }
         // printf("compress_resources: %d\n", opt->win32_pe.compress_resources);
         break;
     case 633:
diff --git a/src/options.h b/src/options.h
index 35030722..cce2894e 100644
--- a/src/options.h
+++ b/src/options.h
@@ -163,8 +163,8 @@ struct Options final {
     struct {
         int compress_exports;
         int compress_icons;
-        int compress_resources;
-        signed char compress_rt[25]; // 25 == RT_LAST
+        TriBool<upx_int8_t> compress_resources;
+        TriBool<upx_int8_t> compress_rt[25]; // 25 == RT_LAST
         int strip_relocs;
         const char *keep_resource;
     } win32_pe;
diff --git a/src/p_com.cpp b/src/p_com.cpp
index fbb558d5..75b166fd 100644
--- a/src/p_com.cpp
+++ b/src/p_com.cpp
@@ -58,7 +58,7 @@ const int *PackCom::getFilters() const {
 //
 **************************************************************************/
 
-bool PackCom::canPack() {
+tribool PackCom::canPack() {
     byte buf[128];
 
     fi->readx(buf, sizeof(buf));
@@ -197,7 +197,7 @@ void PackCom::pack(OutputFile *fo) {
 //
 **************************************************************************/
 
-int PackCom::canUnpack() {
+tribool PackCom::canUnpack() {
     if (!readPackHeader(128)) // read "ph"
         return false;
     if (file_size_u <= ph.c_len)
diff --git a/src/p_com.h b/src/p_com.h
index b43f7f60..3f0660de 100644
--- a/src/p_com.h
+++ b/src/p_com.h
@@ -46,8 +46,8 @@ public:
     virtual void pack(OutputFile *fo) override;
     virtual void unpack(OutputFile *fo) override;
 
-    virtual bool canPack() override;
-    virtual int canUnpack() override;
+    virtual tribool canPack() override;
+    virtual tribool canUnpack() override;
 
 protected:
     virtual Linker *newLinker() const override;
diff --git a/src/p_djgpp2.cpp b/src/p_djgpp2.cpp
index a371fc48..af0d6f24 100644
--- a/src/p_djgpp2.cpp
+++ b/src/p_djgpp2.cpp
@@ -197,7 +197,7 @@ void PackDjgpp2::stripDebug() {
 //
 **************************************************************************/
 
-bool PackDjgpp2::canPack() {
+tribool PackDjgpp2::canPack() {
     if (!readFileHeader())
         return false;
     if (is_dlm(fi, coff_offset))
@@ -346,7 +346,7 @@ void PackDjgpp2::pack(OutputFile *fo) {
 //
 **************************************************************************/
 
-int PackDjgpp2::canUnpack() {
+tribool PackDjgpp2::canUnpack() {
     if (!readFileHeader())
         return false;
     if (is_dlm(fi, coff_offset))
diff --git a/src/p_djgpp2.h b/src/p_djgpp2.h
index 7744abfe..5aaf98bf 100644
--- a/src/p_djgpp2.h
+++ b/src/p_djgpp2.h
@@ -50,8 +50,8 @@ public:
     virtual void pack(OutputFile *fo) override;
     virtual void unpack(OutputFile *fo) override;
 
-    virtual bool canPack() override;
-    virtual int canUnpack() override;
+    virtual tribool canPack() override;
+    virtual tribool canUnpack() override;
 
 protected:
     void handleStub(OutputFile *fo);
diff --git a/src/p_exe.cpp b/src/p_exe.cpp
index d32a1e26..1ad2d752 100644
--- a/src/p_exe.cpp
+++ b/src/p_exe.cpp
@@ -229,7 +229,7 @@ int PackExe::readFileHeader() {
     return UPX_F_DOS_EXE;
 }
 
-bool PackExe::canPack() {
+tribool PackExe::canPack() {
     if (fn_has_ext(fi->getName(), "sys")) // dos/sys
         return false;
     if (!readFileHeader())
@@ -572,7 +572,7 @@ void PackExe::pack(OutputFile *fo) {
 //
 **************************************************************************/
 
-int PackExe::canUnpack() {
+tribool PackExe::canUnpack() {
     if (!readFileHeader())
         return false;
     const unsigned off = ih.headsize16 * 16;
diff --git a/src/p_exe.h b/src/p_exe.h
index d4f2f5ac..daee2cad 100644
--- a/src/p_exe.h
+++ b/src/p_exe.h
@@ -46,8 +46,8 @@ public:
     virtual void pack(OutputFile *fo) override;
     virtual void unpack(OutputFile *fo) override;
 
-    virtual bool canPack() override;
-    virtual int canUnpack() override;
+    virtual tribool canPack() override;
+    virtual tribool canUnpack() override;
 
     // unpacker capabilities
     virtual bool canUnpackVersion(int version) const override {
diff --git a/src/p_lx_elf.cpp b/src/p_lx_elf.cpp
index 64d379b6..f8a13cb6 100644
--- a/src/p_lx_elf.cpp
+++ b/src/p_lx_elf.cpp
@@ -2411,7 +2411,7 @@ bool PackLinuxElf32::calls_crt1(Elf32_Rel const *rel, int sz)
     return false;
 }
 
-int PackLinuxElf32::canUnpack() // bool, except -1: format known, but not packed
+tribool PackLinuxElf32::canUnpack() // bool, except -1: format known, but not packed
 {
     if (checkEhdr(&ehdri)) {
         return false;
@@ -2506,7 +2506,7 @@ PackLinuxElf32::canPackOSABI(Elf32_Ehdr const *ehdr)
     return true;  // good so far
 }
 
-bool PackLinuxElf32::canPack()
+tribool PackLinuxElf32::canPack()
 {
     union {
         unsigned char buf[MAX_ELF_HDR_32];
@@ -2589,7 +2589,7 @@ bool PackLinuxElf32::canPack()
             max_offset = UPX_MAX(max_offset, get_te32(&phdr->p_filesz) + get_te32(&phdr->p_offset));
         }
     }
-    if (canUnpack() > 0) {
+    if (canUnpack()) {
         throwAlreadyPacked();
     }
     // We want to compress position-independent executable (gcc -pie)
@@ -2952,7 +2952,7 @@ proceed: ;
     return true;
 }
 
-int PackLinuxElf64::canUnpack() // bool, except -1: format known, but not packed
+tribool PackLinuxElf64::canUnpack() // bool, except -1: format known, but not packed
 {
     if (checkEhdr(&ehdri)) {
         return false;
@@ -2966,7 +2966,7 @@ int PackLinuxElf64::canUnpack() // bool, except -1: format known, but not packed
     return false;
 }
 
-bool
+tribool
 PackLinuxElf64::canPack()
 {
     union {
@@ -3024,7 +3024,7 @@ PackLinuxElf64::canPack()
             max_offset = UPX_MAX(max_offset, get_te64(&phdr->p_filesz) + get_te64(&phdr->p_offset));
         }
     }
-    if (canUnpack() > 0) {
+    if (canUnpack()) {
         throwAlreadyPacked();
     }
     // We want to compress position-independent executable (gcc -pie)
@@ -7407,7 +7407,7 @@ PackLinuxElf32x86::~PackLinuxElf32x86()
 {
 }
 
-int PackLinuxElf32x86::canUnpack() // bool, except -1: format known, but not packed
+tribool PackLinuxElf32x86::canUnpack() // bool, except -1: format known, but not packed
 {
     if (super::canUnpack()) {
         return true;
diff --git a/src/p_lx_elf.h b/src/p_lx_elf.h
index 77e85744..d59e401f 100644
--- a/src/p_lx_elf.h
+++ b/src/p_lx_elf.h
@@ -46,7 +46,7 @@ public:
     /*virtual void buildLoader(const Filter *);*/
     virtual int getVersion() const override { return 14; } // upx-3.96 cannot upack, for instance
     virtual bool canUnpackVersion(int version) const override { return (version >= 11); }
-    virtual int  canUnpack() override { return super::canUnpack(); } // bool, except -1: format known, but not packed
+    virtual tribool canUnpack() override { return super::canUnpack(); } // bool, except -1: format known, but not packed
 
 protected:
     virtual const int *getCompressionMethods(int method, int level) const override;
@@ -140,8 +140,8 @@ protected:
     virtual void PackLinuxElf32help1(InputFile *f);
     virtual int checkEhdr(Elf32_Ehdr const *ehdr) const;
     virtual bool canPackOSABI(Elf32_Ehdr const *);
-    virtual bool canPack() override;
-    virtual int  canUnpack() override; // bool, except -1: format known, but not packed
+    virtual tribool canPack() override;
+    virtual tribool canUnpack() override; // bool, except -1: format known, but not packed
 
     // These ARM routines are essentially common to big/little endian,
     // but the class hierarchy splits after this class.
@@ -311,8 +311,8 @@ public:
 protected:
     virtual void PackLinuxElf64help1(InputFile *f);
     virtual int checkEhdr(Elf64_Ehdr const *ehdr) const;
-    virtual bool canPack() override;
-    virtual int  canUnpack() override; // bool, except -1: format known, but not packed
+    virtual tribool canPack() override;
+    virtual tribool canUnpack() override; // bool, except -1: format known, but not packed
 
     virtual void pack1(OutputFile *, Filter &) override;  // generate executable header
     virtual void asl_pack2_Shdrs(OutputFile *, unsigned pre_xct_top);  // AndroidSharedLibrary processes Shdrs
@@ -606,7 +606,7 @@ public:
     virtual const char *getName() const override { return "linux/i386"; }
     virtual const char *getFullName(const options_t *) const override { return "i386-linux.elf"; }
     virtual const int *getFilters() const override;
-    virtual int  canUnpack() override; // bool, except -1: format known, but not packed
+    virtual tribool canUnpack() override; // bool, except -1: format known, but not packed
 
 protected:
     virtual void pack1(OutputFile *, Filter &) override;  // generate executable header
diff --git a/src/p_lx_exc.cpp b/src/p_lx_exc.cpp
index 9aea093e..24f83c19 100644
--- a/src/p_lx_exc.cpp
+++ b/src/p_lx_exc.cpp
@@ -495,7 +495,7 @@ int PackLinuxI386::checkEhdr(const Elf_LE32_Ehdr *ehdr) const
 //
 **************************************************************************/
 
-bool PackLinuxI386::canPack()
+tribool PackLinuxI386::canPack()
 {
     if (exetype != 0)
         return super::canPack();
diff --git a/src/p_lx_exc.h b/src/p_lx_exc.h
index 6730baf5..484801c8 100644
--- a/src/p_lx_exc.h
+++ b/src/p_lx_exc.h
@@ -54,7 +54,7 @@ public:
     virtual const int *getFilters() const override;
     virtual void buildLoader(const Filter *) override;
 
-    virtual bool canPack() override;
+    virtual tribool canPack() override;
 
 protected:
     virtual void pack1(OutputFile *, Filter &) override;  // generate executable header
diff --git a/src/p_lx_interp.cpp b/src/p_lx_interp.cpp
index a354e9a1..bd03d262 100644
--- a/src/p_lx_interp.cpp
+++ b/src/p_lx_interp.cpp
@@ -64,7 +64,7 @@ PackLinuxElf32x86interp::~PackLinuxElf32x86interp()
 {
 }
 
-bool PackLinuxElf32x86interp::canPack()
+tribool PackLinuxElf32x86interp::canPack()
 {
     if (opt->o_unix.make_ptinterp) {
         return true;
diff --git a/src/p_lx_interp.h b/src/p_lx_interp.h
index 2a7e4f91..4c464fee 100644
--- a/src/p_lx_interp.h
+++ b/src/p_lx_interp.h
@@ -49,7 +49,7 @@ public:
     virtual const char *getName() const override { return "linux/elfi386"; }
     virtual const char *getFullName(const options_t *) const override { return "i386-linux.elf.interp"; }
 
-    virtual bool canPack() override;
+    virtual tribool canPack() override;
     virtual void unpack(OutputFile *fo) override;
 
 protected:
diff --git a/src/p_lx_sh.cpp b/src/p_lx_sh.cpp
index 3780db4e..82a1850d 100644
--- a/src/p_lx_sh.cpp
+++ b/src/p_lx_sh.cpp
@@ -122,7 +122,7 @@ bool PackLinuxI386sh::getShellName(char *buf)
         return false;
     for (int j = 0; nullptr != shname[j]; ++j) {
         if (0 == strcmp(shname[j], bname + 1)) {
-            bool const s = super::canPack();
+            bool const s = bool(super::canPack());
             if (s) {
                 opt->o_unix.blocksize = blocksize = file_size;
             }
@@ -138,7 +138,7 @@ bool PackLinuxI386sh::getShellName(char *buf)
 }
 
 
-bool PackLinuxI386sh::canPack()
+tribool PackLinuxI386sh::canPack()
 {
 #if defined(__linux__)  //{
     // only compress i386sh scripts when running under Linux
diff --git a/src/p_lx_sh.h b/src/p_lx_sh.h
index 23b30629..541ae8e5 100644
--- a/src/p_lx_sh.h
+++ b/src/p_lx_sh.h
@@ -54,7 +54,7 @@ public:
     virtual void pack1(OutputFile *fo, Filter &ft) override;
     virtual off_t pack3(OutputFile *fo, Filter &ft) override;
 
-    virtual bool canPack() override;
+    virtual tribool canPack() override;
     // virtual void unpack(OutputFile *fo) { super::unpack(fo); }
     virtual bool canUnpackVersion(int version) const override
         { return (version >= 11); }
diff --git a/src/p_mach.cpp b/src/p_mach.cpp
index 205efb96..76f3947f 100644
--- a/src/p_mach.cpp
+++ b/src/p_mach.cpp
@@ -1615,7 +1615,7 @@ void PackMachBase<T>::unpack(OutputFile *fo)
 
 // The prize is the value of overlay_offset: the offset of compressed data
 template <class T>
-int PackMachBase<T>::canUnpack()
+tribool PackMachBase<T>::canUnpack()
 {
     unsigned const lc_seg = lc_seg_info[sizeof(Addr)>>3].segment_cmd;
     fi->seek(0, SEEK_SET);
@@ -1924,7 +1924,7 @@ upx_uint64_t PackMachBase<T>::get_mod_init_func(Mach_segment_command const *segp
 }
 
 template <class T>
-bool PackMachBase<T>::canPack()
+tribool PackMachBase<T>::canPack()
 {
     unsigned const lc_seg = lc_seg_info[sizeof(Addr)>>3].segment_cmd;
     fi->seek(0, SEEK_SET);
@@ -2484,7 +2484,7 @@ void PackMachFat::unpack(OutputFile *fo)
     }
 }
 
-bool PackMachFat::canPack()
+tribool PackMachFat::canPack()
 {
     struct Mach_fat_arch const *const arch = &fat_head.arch[0];
 
@@ -2552,7 +2552,7 @@ bool PackMachFat::canPack()
     return true;
 }
 
-int PackMachFat::canUnpack()
+tribool PackMachFat::canUnpack()
 {
     struct Mach_fat_arch const *const arch = &fat_head.arch[0];
 
diff --git a/src/p_mach.h b/src/p_mach.h
index 165c5017..899d517f 100644
--- a/src/p_mach.h
+++ b/src/p_mach.h
@@ -756,8 +756,8 @@ public:
     virtual void pack1_setup_threado(OutputFile *const fo) = 0;
     virtual void unpack(OutputFile *fo) override;
 
-    virtual bool canPack() override;
-    virtual int canUnpack() override;
+    virtual tribool canPack() override;
+    virtual tribool canUnpack() override;
     virtual upx_uint64_t get_mod_init_func(Mach_segment_command const *segptr);
     virtual unsigned find_SEGMENT_gap(unsigned const k, unsigned pos_eof);
 
@@ -1257,8 +1257,8 @@ protected:
     virtual void list() override;
 
 public:
-    virtual bool canPack() override;
-    virtual int canUnpack() override;
+    virtual tribool canPack() override;
+    virtual tribool canUnpack() override;
 
 protected:
     // loader core
diff --git a/src/p_ps1.cpp b/src/p_ps1.cpp
index cc4ce216..a269e58d 100644
--- a/src/p_ps1.cpp
+++ b/src/p_ps1.cpp
@@ -220,7 +220,7 @@ bool PackPs1::checkFileHeader() {
 //
 **************************************************************************/
 
-bool PackPs1::canPack() {
+tribool PackPs1::canPack() {
     byte buf[PS_HDR_SIZE - sizeof(ps1_exe_t)];
 
     if (!readFileHeader())
@@ -614,7 +614,7 @@ void PackPs1::pack(OutputFile *fo) {
 //
 **************************************************************************/
 
-int PackPs1::canUnpack() {
+tribool PackPs1::canUnpack() {
     if (!readFileHeader())
         return false;
     if (!readPackHeader(CD_SEC))
diff --git a/src/p_ps1.h b/src/p_ps1.h
index 0724ac07..416364a3 100644
--- a/src/p_ps1.h
+++ b/src/p_ps1.h
@@ -50,8 +50,8 @@ public:
     virtual void pack(OutputFile *fo) override;
     virtual void unpack(OutputFile *fo) override;
 
-    virtual bool canPack() override;
-    virtual int canUnpack() override;
+    virtual tribool canPack() override;
+    virtual tribool canUnpack() override;
 
 protected:
     void putBkupHeader(const byte *src, byte *dst, unsigned *len);
diff --git a/src/p_sys.cpp b/src/p_sys.cpp
index 46246cb7..4913d0a3 100644
--- a/src/p_sys.cpp
+++ b/src/p_sys.cpp
@@ -40,7 +40,7 @@ static const CLANG_FORMAT_DUMMY_STATEMENT
 //
 **************************************************************************/
 
-bool PackSys::canPack() {
+tribool PackSys::canPack() {
     byte buf[128];
 
     fi->readx(buf, sizeof(buf));
diff --git a/src/p_sys.h b/src/p_sys.h
index da34a9c9..cd255ead 100644
--- a/src/p_sys.h
+++ b/src/p_sys.h
@@ -41,7 +41,7 @@ public:
     virtual const char *getName() const override { return "dos/sys"; }
     virtual const char *getFullName(const Options *) const override { return "i086-dos16.sys"; }
 
-    virtual bool canPack() override;
+    virtual tribool canPack() override;
 
 protected: // dos/com overrides
     virtual unsigned getCallTrickOffset() const override { return 0; }
diff --git a/src/p_tmt.cpp b/src/p_tmt.cpp
index 051b3c36..a4b3b3db 100644
--- a/src/p_tmt.cpp
+++ b/src/p_tmt.cpp
@@ -150,7 +150,7 @@ int PackTmt::readFileHeader() {
 #undef H
 }
 
-bool PackTmt::canPack() {
+tribool PackTmt::canPack() {
     if (!readFileHeader())
         return false;
     return true;
@@ -265,7 +265,7 @@ void PackTmt::pack(OutputFile *fo) {
         throwNotCompressible();
 }
 
-int PackTmt::canUnpack() {
+tribool PackTmt::canUnpack() {
     if (!readFileHeader())
         return false;
     fi->seek(adam_offset, SEEK_SET);
diff --git a/src/p_tmt.h b/src/p_tmt.h
index 97fd9a6c..b9277920 100644
--- a/src/p_tmt.h
+++ b/src/p_tmt.h
@@ -48,8 +48,8 @@ public:
     virtual void pack(OutputFile *fo) override;
     virtual void unpack(OutputFile *fo) override;
 
-    virtual bool canPack() override;
-    virtual int canUnpack() override;
+    virtual tribool canPack() override;
+    virtual tribool canUnpack() override;
 
 protected:
     int readFileHeader();
diff --git a/src/p_tos.cpp b/src/p_tos.cpp
index eaeb30e7..204e0373 100644
--- a/src/p_tos.cpp
+++ b/src/p_tos.cpp
@@ -342,7 +342,7 @@ static bool check_relocs(const byte *relocs, unsigned rsize, unsigned image_size
 //
 **************************************************************************/
 
-bool PackTos::canPack() {
+tribool PackTos::canPack() {
     if (!readFileHeader())
         return false;
 
@@ -673,7 +673,7 @@ void PackTos::pack(OutputFile *fo) {
 //
 **************************************************************************/
 
-int PackTos::canUnpack() {
+tribool PackTos::canUnpack() {
     if (!readFileHeader())
         return false;
     if (!readPackHeader(768))
diff --git a/src/p_tos.h b/src/p_tos.h
index d752d518..11d91c24 100644
--- a/src/p_tos.h
+++ b/src/p_tos.h
@@ -46,8 +46,8 @@ public:
     virtual void pack(OutputFile *fo) override;
     virtual void unpack(OutputFile *fo) override;
 
-    virtual bool canPack() override;
-    virtual int canUnpack() override;
+    virtual tribool canPack() override;
+    virtual tribool canUnpack() override;
 
     virtual void fileInfo() override;
 
diff --git a/src/p_unix.cpp b/src/p_unix.cpp
index 0ad711dd..a9dfbe2d 100644
--- a/src/p_unix.cpp
+++ b/src/p_unix.cpp
@@ -67,7 +67,7 @@ PackUnix::~PackUnix()
 }
 
 // common part of canPack(), enhanced by subclasses
-bool PackUnix::canPack()
+tribool PackUnix::canPack()
 {
     if (exetype == 0)
         return false;
@@ -555,7 +555,7 @@ unsigned PackUnix::unpackExtent(unsigned wanted, OutputFile *fo,
 **************************************************************************/
 
 // The prize is the value of overlay_offset: the offset of compressed data
-int PackUnix::canUnpack()
+tribool PackUnix::canUnpack()
 {
     int const small = 32 + sizeof(overlay_offset);
     // Allow zero-filled last page, for Mac OS X code signing.
diff --git a/src/p_unix.h b/src/p_unix.h
index 0c705249..a492afc8 100644
--- a/src/p_unix.h
+++ b/src/p_unix.h
@@ -55,8 +55,8 @@ public:
     virtual void pack(OutputFile *fo) override;
     virtual void unpack(OutputFile *fo) override;
 
-    virtual bool canPack() override;
-    virtual int  canUnpack() override; // bool, except -1: format known, but not packed
+    virtual tribool canPack() override;
+    virtual tribool canUnpack() override; // bool, except -1: format known, but not packed
     int find_overlay_offset(MemBuffer const &buf);
 
 protected:
diff --git a/src/p_vmlinx.cpp b/src/p_vmlinx.cpp
index 7751c7d0..106c82dc 100644
--- a/src/p_vmlinx.cpp
+++ b/src/p_vmlinx.cpp
@@ -143,7 +143,7 @@ typename T::Shdr const *PackVmlinuxBase<T>::getElfSections()
 }
 
 template <class T>
-bool PackVmlinuxBase<T>::canPack()
+tribool PackVmlinuxBase<T>::canPack()
 {
     fi->seek(0, SEEK_SET);
     fi->readx(&ehdri, sizeof(ehdri));
@@ -553,7 +553,7 @@ void PackVmlinuxBase<T>::pack(OutputFile *fo)
 }
 
 template <class T>
-int PackVmlinuxBase<T>::canUnpack()
+tribool PackVmlinuxBase<T>::canUnpack()
 {
     fi->seek(0, SEEK_SET);
     fi->readx(&ehdri, sizeof(ehdri));
diff --git a/src/p_vmlinx.h b/src/p_vmlinx.h
index b3cc90b0..3471f082 100644
--- a/src/p_vmlinx.h
+++ b/src/p_vmlinx.h
@@ -83,9 +83,9 @@ protected:
     virtual int getStrategy(Filter &/*ft*/);
     virtual bool is_valid_e_entry(Addr) = 0;
     virtual bool has_valid_vmlinux_head() = 0;
-    virtual bool canPack() override;
+    virtual tribool canPack() override;
     virtual void pack(OutputFile *fo) override;
-    virtual int canUnpack() override;  // bool, except -1: format known, but not packed
+    virtual tribool canUnpack() override;  // bool, except -1: format known, but not packed
     virtual void unpack(OutputFile *fo) override;
     virtual unsigned write_vmlinux_head(
         OutputFile *fo,
diff --git a/src/p_vmlinz.cpp b/src/p_vmlinz.cpp
index 8025a0c5..8e0cbb40 100644
--- a/src/p_vmlinz.cpp
+++ b/src/p_vmlinz.cpp
@@ -93,7 +93,7 @@ int PackVmlinuzI386::getStrategy(Filter &/*ft*/)
 }
 
 
-bool PackVmlinuzI386::canPack()
+tribool PackVmlinuzI386::canPack()
 {
     return readFileHeader() == getFormat();
 }
@@ -671,7 +671,7 @@ void PackBvmlinuzI386::pack(OutputFile *fo)
 // unpack
 **************************************************************************/
 
-int PackVmlinuzI386::canUnpack()
+tribool PackVmlinuzI386::canUnpack()
 {
     if (readFileHeader() != getFormat())
         return false;
@@ -735,7 +735,7 @@ int PackVmlinuzARMEL::getStrategy(Filter &/*ft*/)
     return (opt->no_filter ? -3 : ((opt->filter > 0) ? -2 : 2));
 }
 
-bool PackVmlinuzARMEL::canPack()
+tribool PackVmlinuzARMEL::canPack()
 {
     return readFileHeader() == getFormat();
 }
@@ -1029,7 +1029,7 @@ void PackVmlinuzARMEL::pack(OutputFile *fo)
         throwNotCompressible();
 }
 
-int PackVmlinuzARMEL::canUnpack()
+tribool PackVmlinuzARMEL::canUnpack()
 {
     if (readFileHeader() != getFormat())
         return false;
diff --git a/src/p_vmlinz.h b/src/p_vmlinz.h
index e658d8a5..a19b8819 100644
--- a/src/p_vmlinz.h
+++ b/src/p_vmlinz.h
@@ -51,8 +51,8 @@ public:
     virtual void pack(OutputFile *fo) override;
     virtual void unpack(OutputFile *fo) override;
 
-    virtual bool canPack() override;
-    virtual int canUnpack() override;
+    virtual tribool canPack() override;
+    virtual tribool canUnpack() override;
 
 protected:
     virtual int readFileHeader();
@@ -142,8 +142,8 @@ public:
     virtual void pack(OutputFile *fo) override;
     virtual void unpack(OutputFile *fo) override;
 
-    virtual bool canPack() override;
-    virtual int canUnpack() override;
+    virtual tribool canPack() override;
+    virtual tribool canUnpack() override;
 
 protected:
     virtual int readFileHeader();
diff --git a/src/p_w32pe_i386.cpp b/src/p_w32pe_i386.cpp
index 689fbc89..532301a9 100644
--- a/src/p_w32pe_i386.cpp
+++ b/src/p_w32pe_i386.cpp
@@ -75,7 +75,7 @@ int PackW32PeI386::readFileHeader() {
 // pack
 **************************************************************************/
 
-bool PackW32PeI386::canPack() {
+tribool PackW32PeI386::canPack() {
     if (!readFileHeader())
         return false;
     checkMachine(ih.cpu);
diff --git a/src/p_w32pe_i386.h b/src/p_w32pe_i386.h
index 20923691..1bd094eb 100644
--- a/src/p_w32pe_i386.h
+++ b/src/p_w32pe_i386.h
@@ -51,7 +51,7 @@ public:
     virtual void setOhHeaderSize(const pe_section_t *osection) override;
     virtual void pack(OutputFile *fo) override;
 
-    virtual bool canPack() override;
+    virtual tribool canPack() override;
 
 protected:
     virtual int readFileHeader() override;
diff --git a/src/p_w64pe_amd64.cpp b/src/p_w64pe_amd64.cpp
index 6ee9f4a9..a9f5f178 100644
--- a/src/p_w64pe_amd64.cpp
+++ b/src/p_w64pe_amd64.cpp
@@ -65,7 +65,7 @@ Linker *PackW64PeAmd64::newLinker() const { return new ElfLinkerAMD64; }
 // pack
 **************************************************************************/
 
-bool PackW64PeAmd64::canPack() {
+tribool PackW64PeAmd64::canPack() {
     if (!readFileHeader())
         return false;
     checkMachine(ih.cpu);
diff --git a/src/p_w64pe_amd64.h b/src/p_w64pe_amd64.h
index ac1e9296..a0a18fff 100644
--- a/src/p_w64pe_amd64.h
+++ b/src/p_w64pe_amd64.h
@@ -50,7 +50,7 @@ public:
     virtual void setOhHeaderSize(const pe_section_t *osection) override;
     virtual void pack(OutputFile *fo) override;
 
-    virtual bool canPack() override;
+    virtual tribool canPack() override;
 
 protected:
     virtual void buildLoader(const Filter *ft) override;
diff --git a/src/p_w64pe_arm64.cpp b/src/p_w64pe_arm64.cpp
index fa70c2a7..9451bb59 100644
--- a/src/p_w64pe_arm64.cpp
+++ b/src/p_w64pe_arm64.cpp
@@ -59,7 +59,7 @@ const int *PackW64PeArm64::getFilters() const { return nullptr; }
 // pack
 **************************************************************************/
 
-bool PackW64PeArm64::canPack() {
+tribool PackW64PeArm64::canPack() {
     if (!readFileHeader())
         return false;
     checkMachine(ih.cpu);
@@ -106,7 +106,7 @@ void PackW64PeArm64::pack(OutputFile *fo) {
 // pack
 **************************************************************************/
 
-bool PackW64PeArm64EC::canPack() {
+tribool PackW64PeArm64EC::canPack() {
     if (!readFileHeader())
         return false;
     checkMachine(ih.cpu);
diff --git a/src/p_w64pe_arm64.h b/src/p_w64pe_arm64.h
index d0c37d18..c8cf4d7d 100644
--- a/src/p_w64pe_arm64.h
+++ b/src/p_w64pe_arm64.h
@@ -50,7 +50,7 @@ public:
     virtual void setOhHeaderSize(const pe_section_t *osection) override;
     virtual void pack(OutputFile *fo) override;
 
-    virtual bool canPack() override;
+    virtual tribool canPack() override;
 
 protected:
     virtual void buildLoader(const Filter *ft) override;
@@ -74,7 +74,7 @@ public:
     virtual const char *getName() const override { return "win64/arm64ec"; }
     virtual const char *getFullName(const Options *) const override { return "arm64ec-win64.pe"; }
 
-    virtual bool canPack() override;
+    virtual tribool canPack() override;
 };
 
 /* vim:set ts=4 sw=4 et: */
diff --git a/src/p_wcle.cpp b/src/p_wcle.cpp
index 912c738e..ec8e03ac 100644
--- a/src/p_wcle.cpp
+++ b/src/p_wcle.cpp
@@ -112,7 +112,7 @@ void PackWcle::handleStub(OutputFile *fo) {
         Packer::handleStub(fi, fo, le_offset);
 }
 
-bool PackWcle::canPack() {
+tribool PackWcle::canPack() {
     if (!LeFile::readFileHeader())
         return false;
     return true;
@@ -731,7 +731,7 @@ void PackWcle::decodeEntryTable() {
     ientries = nullptr;
 }
 
-int PackWcle::canUnpack() {
+tribool PackWcle::canUnpack() {
     if (!LeFile::readFileHeader())
         return false;
     fi->seek(exe_offset + ih.data_pages_offset, SEEK_SET);
diff --git a/src/p_wcle.h b/src/p_wcle.h
index f7cb4689..a525efc1 100644
--- a/src/p_wcle.h
+++ b/src/p_wcle.h
@@ -48,8 +48,8 @@ public:
     virtual void pack(OutputFile *fo) override;
     virtual void unpack(OutputFile *fo) override;
 
-    virtual bool canPack() override;
-    virtual int canUnpack() override;
+    virtual tribool canPack() override;
+    virtual tribool canUnpack() override;
 
 protected:
     virtual void handleStub(OutputFile *fo);
diff --git a/src/p_wince_arm.cpp b/src/p_wince_arm.cpp
index 47d93f25..0cbb360e 100644
--- a/src/p_wince_arm.cpp
+++ b/src/p_wince_arm.cpp
@@ -111,7 +111,7 @@ void PackWinCeArm::processTls(Interval *) // pass 1
 // pack
 **************************************************************************/
 
-bool PackWinCeArm::canPack() {
+tribool PackWinCeArm::canPack() {
     if (!readFileHeader())
         return false;
     checkMachine(ih.cpu);
diff --git a/src/p_wince_arm.h b/src/p_wince_arm.h
index 39939cd4..9078efd7 100644
--- a/src/p_wince_arm.h
+++ b/src/p_wince_arm.h
@@ -55,7 +55,7 @@ public:
     virtual void setOhHeaderSize(const pe_section_t *osection) override;
     virtual void pack(OutputFile *fo) override;
 
-    virtual bool canPack() override;
+    virtual tribool canPack() override;
 
 protected:
     virtual void buildLoader(const Filter *ft) override;
diff --git a/src/packer.h b/src/packer.h
index 3b4461e3..3f9f3de9 100644
--- a/src/packer.h
+++ b/src/packer.h
@@ -57,12 +57,14 @@ public:
     virtual const int *getCompressionMethods(int method, int level) const = 0;
     virtual const int *getFilters() const = 0;
 
-    // canPack() should throw a cantPackException eplaining why it
-    // cannot pack a recognized format.
-    virtual bool canPack() = 0;
-    // canUnpack() can return -1 meaning "format recognized, but file
-    // is definitely not packed". See packmast.cpp try_unpack().
-    virtual int canUnpack() = 0;
+    // canPack() should throw a cantPackException eplaining why it cannot pack
+    // a recognized format.
+    // canPack() can return -1 to stop early; see class PackMaster
+    virtual tribool canPack() = 0;
+    // canUnpack() should throw a cantUnpackException eplaining why it cannot pack
+    // a recognized format.
+    // canUnpack() can return -1 to stop early; see class PackMaster
+    virtual tribool canUnpack() = 0;
 
     // PackMaster entries
     virtual void assertPacker() const = 0;
@@ -164,10 +166,10 @@ protected:
     void verifyOverlappingDecompression(byte *o_ptr, unsigned o_size, Filter *ft = nullptr);
 
     // packheader handling
-    virtual int patchPackHeader(void *b, int blen);
-    virtual bool getPackHeader(const void *b, int blen, bool allow_incompressible = false);
-    virtual bool readPackHeader(int len, bool allow_incompressible = false);
-    virtual void checkAlreadyPacked(const void *b, int blen);
+    virtual int patchPackHeader(void *b, int blen) final;
+    virtual bool getPackHeader(const void *b, int blen, bool allow_incompressible = false) final;
+    virtual bool readPackHeader(int len, bool allow_incompressible = false) final;
+    virtual void checkAlreadyPacked(const void *b, int blen) final;
 
     // loader core
     virtual void buildLoader(const Filter *ft) = 0;
diff --git a/src/packmast.cpp b/src/packmast.cpp
index 3ea59a55..c303614b 100644
--- a/src/packmast.cpp
+++ b/src/packmast.cpp
@@ -89,37 +89,38 @@ PackMaster::~PackMaster() noexcept {
 //
 **************************************************************************/
 
-static bool try_can_pack(PackerBase *pb, void *user) may_throw {
+static tribool try_can_pack(PackerBase *pb, void *user) may_throw {
     InputFile *f = (InputFile *) user;
     try {
         pb->initPackHeader();
         f->seek(0, SEEK_SET);
-        if (pb->canPack()) {
+        tribool r = pb->canPack();
+        if (r) {
             if (opt->cmd == CMD_COMPRESS)
                 pb->updatePackHeader();
             f->seek(0, SEEK_SET);
-            return true;
+            return true; // success
         }
+        if (r.isOther()) // aka "-1"
+            return r;    // canPack() says the format is recognized and we should stop early
     } catch (const IOException &) {
         // ignored
     }
     return false;
 }
 
-static bool try_can_unpack(PackerBase *pb, void *user) may_throw {
+static tribool try_can_unpack(PackerBase *pb, void *user) may_throw {
     InputFile *f = (InputFile *) user;
     try {
         pb->initPackHeader();
         f->seek(0, SEEK_SET);
-        int r = pb->canUnpack();
-        if (r > 0) {
+        tribool r = pb->canUnpack();
+        if (r) {
             f->seek(0, SEEK_SET);
-            return true;
-        }
-        if (r < 0) {
-            // FIXME - could stop testing all other unpackers at this time
-            // see canUnpack() in packer.h
+            return true; // success
         }
+        if (r.isOther()) // aka "-1"
+            return r;    // canUnpack() says the format is recognized and we should stop early
     } catch (const IOException &) {
         // ignored
     }
@@ -141,8 +142,11 @@ PackerBase *PackMaster::visitAllPackers(visit_func_t func, InputFile *f, const O
     if (o->debug.debug_level)                                                                      \
         fprintf(stderr, "visitAllPackers: (ver=%d, fmt=%3d) %s\n", pb->getVersion(),               \
                 pb->getFormat(), #Klass);                                                          \
-    if (func(pb.get(), user))                                                                      \
-        return pb.release();                                                                       \
+    tribool r = func(pb.get(), user);                                                              \
+    if (r)                                                                                         \
+        return pb.release(); /* success */                                                         \
+    if (r.isOther())                                                                               \
+        return nullptr; /* stop early */                                                           \
     ACC_BLOCK_END
 
     // NOTE: order of tries is important !!!
diff --git a/src/packmast.h b/src/packmast.h
index ab7709cf..16a61c25 100644
--- a/src/packmast.h
+++ b/src/packmast.h
@@ -46,7 +46,7 @@ public:
     void list();
     void fileInfo();
 
-    typedef bool (*visit_func_t)(PackerBase *pb, void *user);
+    typedef tribool (*visit_func_t)(PackerBase *pb, void *user);
     static PackerBase *visitAllPackers(visit_func_t, InputFile *f, const Options *, void *user)
         may_throw;
 
diff --git a/src/pefile.cpp b/src/pefile.cpp
index 7f5ee917..46cfa8a2 100644
--- a/src/pefile.cpp
+++ b/src/pefile.cpp
@@ -1795,14 +1795,14 @@ void PeFile::processResources(Resource *res) {
         return;
 
     // setup default options for resource compression
-    if (opt->win32_pe.compress_resources < 0)
+    if (opt->win32_pe.compress_resources.isOther())
         opt->win32_pe.compress_resources = !isefi;
     if (!opt->win32_pe.compress_resources) {
         opt->win32_pe.compress_icons = false;
         for (int i = 0; i < RT_LAST; i++)
             opt->win32_pe.compress_rt[i] = false;
     }
-    if (opt->win32_pe.compress_rt[RT_STRING] < 0) {
+    if (opt->win32_pe.compress_rt[RT_STRING].isOther()) {
         // by default, don't compress RT_STRINGs of screensavers (".scr")
         opt->win32_pe.compress_rt[RT_STRING] = true;
         if (fn_has_ext(fi->getName(), "scr"))
@@ -3017,7 +3017,7 @@ void PeFile32::unpack(OutputFile *fo) {
     unpack0<pe_header_t, LE32>(fo, ih, oh, 1U << 31, set_oft);
 }
 
-int PeFile32::canUnpack() {
+tribool PeFile32::canUnpack() {
     if (!canPack()) // this calls readFileHeader() and readPeHeader()
         return false;
     return canUnpack0(getFormat() == UPX_F_WINCE_ARM ? 4 : 3, ih.objects, ih.entry, sizeof(ih));
@@ -3065,7 +3065,7 @@ void PeFile64::pack0(OutputFile *fo, unsigned subsystem_mask, upx_uint64_t defau
 
 void PeFile64::unpack(OutputFile *fo) { unpack0<pe_header_t, LE64>(fo, ih, oh, 1ULL << 63, false); }
 
-int PeFile64::canUnpack() {
+tribool PeFile64::canUnpack() {
     if (!canPack()) // this calls readFileHeader() and readPeHeader()
         return false;
     return canUnpack0(3, ih.objects, ih.entry, sizeof(ih));
diff --git a/src/pefile.h b/src/pefile.h
index 5af8efb5..377d0ab4 100644
--- a/src/pefile.h
+++ b/src/pefile.h
@@ -503,7 +503,7 @@ protected:
     void pack0(OutputFile *fo, unsigned subsystem_mask, upx_uint64_t default_imagebase,
                bool last_section_rsrc_only);
     virtual void unpack(OutputFile *fo) override;
-    virtual int canUnpack() override;
+    virtual tribool canUnpack() override;
 
     virtual void readPeHeader() override;
 
@@ -564,7 +564,7 @@ protected:
     void pack0(OutputFile *fo, unsigned subsystem_mask, upx_uint64_t default_imagebase);
 
     virtual void unpack(OutputFile *fo) override;
-    virtual int canUnpack() override;
+    virtual tribool canUnpack() override;
 
     virtual void readPeHeader() override;
 
diff --git a/src/util/util.cpp b/src/util/util.cpp
index bfa6879e..2843123f 100644
--- a/src/util/util.cpp
+++ b/src/util/util.cpp
@@ -787,4 +787,60 @@ TEST_CASE("get_ratio") {
     CHECK(get_ratio(2 * UPX_RSIZE_MAX, 1024ull * UPX_RSIZE_MAX) == 9999999);
 }
 
+template <class T>
+struct TestTriBool {
+    static void test(bool expect_true, int x) noexcept {
+        CHECK(T(false) == T::False);
+        CHECK(T(true) == T::True);
+        CHECK(T(T::Other) == T::Other);
+        T a;
+        CHECK(!a);
+        CHECK(a.isStrictFalse());
+        CHECK(!a.isStrictTrue());
+        CHECK(a.isStrictBool());
+        CHECK(!a.isOther());
+        a = false;
+        CHECK(!a);
+        CHECK(a.isStrictFalse());
+        CHECK(!a.isStrictTrue());
+        CHECK(a.isStrictBool());
+        CHECK(!a.isOther());
+        a = true;
+        CHECK(a);
+        CHECK(!a.isStrictFalse());
+        CHECK(a.isStrictTrue());
+        CHECK(a.isStrictBool());
+        CHECK(!a.isOther());
+        a = T::Other;
+        if (expect_true)
+            CHECK(a);
+        else
+            CHECK(!a);
+        CHECK(!a.isStrictFalse());
+        CHECK(!a.isStrictTrue());
+        CHECK(!a.isStrictBool());
+        CHECK(a.isOther());
+        a = x;
+        if (expect_true)
+            CHECK(a);
+        else
+            CHECK(!a);
+        CHECK(!a.isStrictFalse());
+        CHECK(!a.isStrictTrue());
+        CHECK(!a.isStrictBool());
+        CHECK(a.isOther());
+    }
+};
+
+TEST_CASE("TriBool") {
+    TestTriBool<tribool>::test(false, -1);
+    TestTriBool<TriBool<upx_int8_t> >::test(false, -1);
+    TestTriBool<TriBool<upx_int64_t> >::test(false, -1);
+    //
+    TestTriBool<TriBool<upx_int8_t, 2> >::test(true, 2);
+    TestTriBool<TriBool<upx_uint8_t, 2> >::test(true, 2);
+    TestTriBool<TriBool<upx_int64_t, 2> >::test(true, 2);
+    TestTriBool<TriBool<upx_uint64_t, 2> >::test(true, 2);
+}
+
 /* vim:set ts=4 sw=4 et: */
diff --git a/src/util/util.h b/src/util/util.h
index 47539100..ab96c04f 100644
--- a/src/util/util.h
+++ b/src/util/util.h
@@ -194,6 +194,49 @@ inline void owner_delete(T (&array)[]) noexcept DELETED_FUNCTION;
 template <class T, size_t N>
 inline void owner_delete(T (&array)[N]) noexcept DELETED_FUNCTION;
 
+/*************************************************************************
+// TriBool - tri-state bool
+**************************************************************************/
+
+template <class T = int, T TOther = -1> // an enum with an underlying type and 3 values
+class TriBool {
+public:
+    // types
+    typedef T underlying_type;
+    static_assert(std::is_integral_v<underlying_type>);
+    typedef decltype(T(0) + T(0)) promoted_type;
+    static_assert(std::is_integral_v<promoted_type>);
+    static_assert(TOther != 0 && TOther != 1);
+    enum value_type : underlying_type { False = 0, True = 1, Other = TOther };
+    // constructors
+    forceinline TriBool() noexcept = default;
+    forceinline ~TriBool() noexcept = default;
+    constexpr TriBool(value_type x) noexcept : value(x) {}
+    constexpr TriBool(promoted_type x) noexcept : value(x == 0 ? False : (x == 1 ? True : Other)) {}
+    // access
+    value_type getValue() const noexcept { return value; }
+    // checks for > 0, so TOther determines if Other is false (the default) or true
+    explicit operator bool() const noexcept { return value > False; }
+    // query; this is NOT the same as operator bool()
+    bool isStrictFalse() const noexcept { return value == False; }
+    bool isStrictTrue() const noexcept { return value == True; }
+    bool isStrictBool() const noexcept { return value == False || value == True; }
+    bool isOther() const noexcept { return value != False && value != True; }
+    // "other" can mean many things, depending on usage context, so provide some alternative names:
+    // forceinline bool isDefault() const noexcept { return isOther(); } // might be misleading
+    forceinline bool isIndeterminate() const noexcept { return isOther(); }
+    forceinline bool isUndecided() const noexcept { return isOther(); }
+    // forceinline bool isUnset() const noexcept { return isOther(); } // might be misleading
+    constexpr bool operator==(TriBool other) const noexcept { return value == other.value; }
+    constexpr bool operator==(value_type other) const noexcept { return value == other; }
+    constexpr bool operator==(promoted_type other) const noexcept { return value == other; }
+protected:
+    // value
+    value_type value = False;
+};
+
+typedef TriBool<> tribool;
+
 /*************************************************************************
 // misc. support functions
 **************************************************************************/