fix(folding): correct unique object rep checks []

src/Changes

@@ -1,5 +1,35 @@
 Version 7.1
 
+10/16/26
+__has_unique_object_representations fixes
+
+A base class that declares no nonstatic data members of its own but inherits
+some (and is therefore not empty) was treated as an empty base class.  Its
+size was then counted as tail padding, so the trait was incorrectly FALSE for
+classes derived from it.  For example, with --g++:
+
+  struct A { char i; };
+  struct B : A {};
+  struct C : B {};
+  static_assert(__has_unique_object_representations(C));  // Previously
+                                                           // failed.
+
+This is now fixed.  In addition, the following version-dependent behaviors
+of g++ and Clang are now emulated:
+
+  - An unnamed bit-field of nonzero width is treated as padding (making the
+    result FALSE) in g++ 10.5, 11.4, 12.3, 13.1 and later, and in Clang 17
+    and later.  For example:
+
+      struct S { char a; char : 8; char b; };
+      // __has_unique_object_representations(S) is now FALSE in those modes.
+
+  - Volatile-qualified non-class types (e.g., "volatile int") are treated as
+    not having unique object representations only in g++ modes before
+    version 10 (previously this was done for all g++ versions, including
+    Clang modes).
+
+
 10/16/26 [GH #229]
 Improper handling of "if constexpr" substatements
 

src/folding.c

@@ -8939,6 +8939,26 @@ with cssp are trivial.
 }  /* all_copy_constructors_trivial */
 
 
+static a_boolean unnamed_bit_field_is_padding(void)
+/*
+Return TRUE if an unnamed bit-field of non-zero width is considered padding
+by type_has_unique_object_representations.  That is the case for Clang 17
+and later and for g++ 13.1 and later (the change was also backported to
+g++ 10.5, 11.4, and 12.3).
+*/
+{
+  a_boolean result = FALSE;
+
+  if (clang_mode) {
+    result = clang_version >= 170000;
+  } else if (gpp_mode) {
+    result = gnu_version >= 120300 ||
+             (gnu_version >= 110400 && gnu_version < 120000) ||
+             (gnu_version >= 100500 && gnu_version < 110000);
+  }  /* if */
+  return result;
+}  /* unnamed_bit_field_is_padding */
+
 static a_boolean type_has_unique_object_representations(
                                    a_type_ptr     type,
                                    a_targ_size_t  *after_base_members,
@@ -8999,8 +9019,7 @@ architectures for which these assumptions are not valid.
           if (bcp->offset != end_of_last_subobject) {
             /* There's padding between base class subobjects. */
             result = FALSE;
-          } else if (bcp->type->variant.class_struct_union.field_list ==
-                                                                        NULL) {
+          } else if (is_empty_class_type(bcp->type)) {
             /* An empty base class: treat it as having size 0 and do not
                check type_has_unique_object_representation, since an empty
                class will consist of nothing but padding.  If it was not
@@ -9035,6 +9054,12 @@ architectures for which these assumptions are not valid.
              same size or (for g++, not clang) if a bit-field is shorter
              than the size of the union. */
           result = FALSE;
+        } else if (field->is_bit_field && field->bit_size != 0 &&
+                   unnamed_bit_field_is_padding() &&
+                   field->source_corresp.assoc_info ==
+                                     (char *)get_unnamed_field_symbol()) {
+          /* An unnamed bit-field of non-zero width is padding. */
+          result = FALSE;
         } else if (field->offset != end_of_last_subobject ||
             field->offset_bit_remainder != bit_offset_of_end) {
           /* There's padding between nonstatic data members. */
@@ -9078,9 +9103,10 @@ architectures for which these assumptions are not valid.
         }  /* if */
       }  /* if */
     }  /* if */
-  } else if (gpp_mode && is_volatile_qualified_type(orig_type)) {
-    /* g++ treats volatile-qualified types as not having unique object
-       representations. */
+  } else if (gpp_mode && !clang_mode && gnu_version < 100000 &&
+             is_volatile_qualified_type(orig_type)) {
+    /* g++ before version 10 treats volatile-qualified non-class types as
+       not having unique object representations. */
     result = FALSE;
   } else {
     /* A non-class type. */

tests/expectations/edg_x86_64/imported.log

@@ -6872,7 +6872,6 @@ gnu/cpp/cpp1z/fold1/test.mft.cpp:fn:-DTEST_NUMBER=1 --c++17:FAIL:
 gnu/cpp/cpp1z/fold5.sft.cpp:fn:-DTEST_NUMBER=1 --c++11:FAIL:
 gnu/cpp/cpp1z/fold6.sft.cpp:fn:-DTEST_NUMBER=1 --c++17:FAIL:
 gnu/cpp/cpp1z/has-unique-obj-representations1.sft.cpp:fp:-DTEST_NUMBER=1 --c++11:FAIL:
-gnu/cpp/cpp1z/has-unique-obj-representations3.sft.cpp:fp:-DTEST_NUMBER=1 --c++11:FAIL:
 gnu/cpp/cpp1z/inh-ctor1.sft.cpp:fn:-DTEST_NUMBER=1 --c++11:FAIL:
 gnu/cpp/cpp1z/inh-ctor33.sft.cpp:lp:-DTEST_NUMBER=1 --c++11:FAIL:
 gnu/cpp/cpp1z/inh-ctor38.sft.cpp:rp:-DTEST_NUMBER=1 --c++11:FAIL:

tests/tests/imported/clang/cpp/SemaCXX/.type-traits.rto/default.1.1.txt

@@ -375,38 +375,6 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
     static_assert(!__is_trivially_destructible(AnIncompleteType[1])); // expected-error {{incomplete type}}
                                                ^
 
-"Test_name.c", line 3482: error: static assertion failed with "integrals are"
-  static_assert(__has_unique_object_representations(volatile int), "integrals are");
-  ^
-
-"Test_name.c", line 3483: error: static assertion failed with "integrals are"
-  static_assert(__has_unique_object_representations(const volatile int), "integrals are");
-  ^
-
-"Test_name.c", line 3502: error: static assertion failed with "even function pointers"
-  static_assert(__has_unique_object_representations(volatile FP), "even function pointers");
-  ^
-
-"Test_name.c", line 3503: error: static assertion failed with "even function pointers"
-  static_assert(__has_unique_object_representations(const volatile FP), "even function pointers");
-  ^
-
-"Test_name.c", line 3507: error: static assertion failed with "and pointer to members"
-  static_assert(__has_unique_object_representations(volatile PMF), "and pointer to members");
-  ^
-
-"Test_name.c", line 3508: error: static assertion failed with "and pointer to members"
-  static_assert(__has_unique_object_representations(const volatile PMF), "and pointer to members");
-  ^
-
-"Test_name.c", line 3512: error: static assertion failed with "and pointer to members"
-  static_assert(__has_unique_object_representations(volatile PMD), "and pointer to members");
-  ^
-
-"Test_name.c", line 3513: error: static assertion failed with "and pointer to members"
-  static_assert(__has_unique_object_representations(const volatile PMD), "and pointer to members");
-  ^
-
 "Test_name.c", line 3544: error: an incomplete class type is not allowed
   static_assert(!__has_unique_object_representations(AnIncompleteType));
                  ^
@@ -431,14 +399,6 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
     char d : 1;
          ^
 
-"Test_name.c", line 3777: error: static assertion failed with "Bitfield padding"
-  static_assert(!__has_unique_object_representations(UnnamedBitfield), "Bitfield padding");
-  ^
-
-"Test_name.c", line 3778: error: static assertion failed with "Bitfield padding"
-  static_assert(!__has_unique_object_representations(UnnamedBitfieldPacked), "Bitfield padding");
-  ^
-
 "Test_name.c", line 3790: warning: declared size for bit field is larger than the size of the bit field type; truncated to 8 bits
     bool b : 16;
              ^
@@ -695,5 +655,5 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
   #error Programming model currently unsupported; please add a new entry.
    ^
 
-124 errors and 1 catastrophic error detected in the compilation of "Test_name.c".
+114 errors and 1 catastrophic error detected in the compilation of "Test_name.c".
 Compilation terminated.

tests/tests/imported/clang/cpp/SemaCXX/.type-traits.rto/edg_x86_32.1.1.txt

@@ -379,38 +379,6 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
     static_assert(!__is_trivially_destructible(AnIncompleteType[1])); // expected-error {{incomplete type}}
                                                ^
 
-"Test_name.c", line 3482: error: static assertion failed with "integrals are"
-  static_assert(__has_unique_object_representations(volatile int), "integrals are");
-  ^
-
-"Test_name.c", line 3483: error: static assertion failed with "integrals are"
-  static_assert(__has_unique_object_representations(const volatile int), "integrals are");
-  ^
-
-"Test_name.c", line 3502: error: static assertion failed with "even function pointers"
-  static_assert(__has_unique_object_representations(volatile FP), "even function pointers");
-  ^
-
-"Test_name.c", line 3503: error: static assertion failed with "even function pointers"
-  static_assert(__has_unique_object_representations(const volatile FP), "even function pointers");
-  ^
-
-"Test_name.c", line 3507: error: static assertion failed with "and pointer to members"
-  static_assert(__has_unique_object_representations(volatile PMF), "and pointer to members");
-  ^
-
-"Test_name.c", line 3508: error: static assertion failed with "and pointer to members"
-  static_assert(__has_unique_object_representations(const volatile PMF), "and pointer to members");
-  ^
-
-"Test_name.c", line 3512: error: static assertion failed with "and pointer to members"
-  static_assert(__has_unique_object_representations(volatile PMD), "and pointer to members");
-  ^
-
-"Test_name.c", line 3513: error: static assertion failed with "and pointer to members"
-  static_assert(__has_unique_object_representations(const volatile PMD), "and pointer to members");
-  ^
-
 "Test_name.c", line 3544: error: an incomplete class type is not allowed
   static_assert(!__has_unique_object_representations(AnIncompleteType));
                  ^
@@ -435,14 +403,6 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
     char d : 1;
          ^
 
-"Test_name.c", line 3777: error: static assertion failed with "Bitfield padding"
-  static_assert(!__has_unique_object_representations(UnnamedBitfield), "Bitfield padding");
-  ^
-
-"Test_name.c", line 3778: error: static assertion failed with "Bitfield padding"
-  static_assert(!__has_unique_object_representations(UnnamedBitfieldPacked), "Bitfield padding");
-  ^
-
 "Test_name.c", line 3790: warning: declared size for bit field is larger than the size of the bit field type; truncated to 8 bits
     bool b : 16;
              ^
@@ -695,5 +655,5 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
   #error Programming model currently unsupported; please add a new entry.
    ^
 
-124 errors and 1 catastrophic error detected in the compilation of "Test_name.c".
+114 errors and 1 catastrophic error detected in the compilation of "Test_name.c".
 Compilation terminated.

tests/tests/imported/clang/cpp/SemaCXX/.type-traits.rto/edg_x86_32_cp.1.1.txt

@@ -363,38 +363,6 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
     static_assert(!__is_trivially_destructible(AnIncompleteType[1])); // expected-error {{incomplete type}}
                                                ^
 
-"Test_name.c", line 3482: error: static assertion failed with "integrals are"
-  static_assert(__has_unique_object_representations(volatile int), "integrals are");
-  ^
-
-"Test_name.c", line 3483: error: static assertion failed with "integrals are"
-  static_assert(__has_unique_object_representations(const volatile int), "integrals are");
-  ^
-
-"Test_name.c", line 3502: error: static assertion failed with "even function pointers"
-  static_assert(__has_unique_object_representations(volatile FP), "even function pointers");
-  ^
-
-"Test_name.c", line 3503: error: static assertion failed with "even function pointers"
-  static_assert(__has_unique_object_representations(const volatile FP), "even function pointers");
-  ^
-
-"Test_name.c", line 3507: error: static assertion failed with "and pointer to members"
-  static_assert(__has_unique_object_representations(volatile PMF), "and pointer to members");
-  ^
-
-"Test_name.c", line 3508: error: static assertion failed with "and pointer to members"
-  static_assert(__has_unique_object_representations(const volatile PMF), "and pointer to members");
-  ^
-
-"Test_name.c", line 3512: error: static assertion failed with "and pointer to members"
-  static_assert(__has_unique_object_representations(volatile PMD), "and pointer to members");
-  ^
-
-"Test_name.c", line 3513: error: static assertion failed with "and pointer to members"
-  static_assert(__has_unique_object_representations(const volatile PMD), "and pointer to members");
-  ^
-
 "Test_name.c", line 3544: error: an incomplete class type is not allowed
   static_assert(!__has_unique_object_representations(AnIncompleteType));
                  ^
@@ -419,14 +387,6 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
     char d : 1;
          ^
 
-"Test_name.c", line 3777: error: static assertion failed with "Bitfield padding"
-  static_assert(!__has_unique_object_representations(UnnamedBitfield), "Bitfield padding");
-  ^
-
-"Test_name.c", line 3778: error: static assertion failed with "Bitfield padding"
-  static_assert(!__has_unique_object_representations(UnnamedBitfieldPacked), "Bitfield padding");
-  ^
-
 "Test_name.c", line 3790: warning: declared size for bit field is larger than the size of the bit field type; truncated to 8 bits
     bool b : 16;
              ^
@@ -635,5 +595,5 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
   #error Programming model currently unsupported; please add a new entry.
    ^
 
-124 errors and 1 catastrophic error detected in the compilation of "Test_name.c".
+114 errors and 1 catastrophic error detected in the compilation of "Test_name.c".
 Compilation terminated.

tests/tests/imported/clang/cpp/SemaCXX/.type-traits.rto/edg_x86_64_cp.1.1.txt

@@ -359,38 +359,6 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
     static_assert(!__is_trivially_destructible(AnIncompleteType[1])); // expected-error {{incomplete type}}
                                                ^
 
-"Test_name.c", line 3482: error: static assertion failed with "integrals are"
-  static_assert(__has_unique_object_representations(volatile int), "integrals are");
-  ^
-
-"Test_name.c", line 3483: error: static assertion failed with "integrals are"
-  static_assert(__has_unique_object_representations(const volatile int), "integrals are");
-  ^
-
-"Test_name.c", line 3502: error: static assertion failed with "even function pointers"
-  static_assert(__has_unique_object_representations(volatile FP), "even function pointers");
-  ^
-
-"Test_name.c", line 3503: error: static assertion failed with "even function pointers"
-  static_assert(__has_unique_object_representations(const volatile FP), "even function pointers");
-  ^
-
-"Test_name.c", line 3507: error: static assertion failed with "and pointer to members"
-  static_assert(__has_unique_object_representations(volatile PMF), "and pointer to members");
-  ^
-
-"Test_name.c", line 3508: error: static assertion failed with "and pointer to members"
-  static_assert(__has_unique_object_representations(const volatile PMF), "and pointer to members");
-  ^
-
-"Test_name.c", line 3512: error: static assertion failed with "and pointer to members"
-  static_assert(__has_unique_object_representations(volatile PMD), "and pointer to members");
-  ^
-
-"Test_name.c", line 3513: error: static assertion failed with "and pointer to members"
-  static_assert(__has_unique_object_representations(const volatile PMD), "and pointer to members");
-  ^
-
 "Test_name.c", line 3544: error: an incomplete class type is not allowed
   static_assert(!__has_unique_object_representations(AnIncompleteType));
                  ^
@@ -415,14 +383,6 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
     char d : 1;
          ^
 
-"Test_name.c", line 3777: error: static assertion failed with "Bitfield padding"
-  static_assert(!__has_unique_object_representations(UnnamedBitfield), "Bitfield padding");
-  ^
-
-"Test_name.c", line 3778: error: static assertion failed with "Bitfield padding"
-  static_assert(!__has_unique_object_representations(UnnamedBitfieldPacked), "Bitfield padding");
-  ^
-
 "Test_name.c", line 3790: warning: declared size for bit field is larger than the size of the bit field type; truncated to 8 bits
     bool b : 16;
              ^
@@ -635,5 +595,5 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
   #error Programming model currently unsupported; please add a new entry.
    ^
 
-124 errors and 1 catastrophic error detected in the compilation of "Test_name.c".
+114 errors and 1 catastrophic error detected in the compilation of "Test_name.c".
 Compilation terminated.

tests/tests/imported/gnu/cpp/cpp1z/.has-unique-obj-representations3.rto/default.1.1.txt

@@ -1,16 +1 @@
 fe_only -DTEST_NUMBER=1 --gnu_version=999999 -tused -e 200 --no_wrap --no_wrap_diagnostics --c++11 Test_name.c
-"Test_name.c", line 10: error: static assertion failed with ""
-  static_assert (__has_unique_object_representations (U) == false, "");
-  ^
-"Test_name.c", line 10: note: the final comparison was 1 == 0
-  static_assert (__has_unique_object_representations (U) == false, "");
-                                                         ^
-
-"Test_name.c", line 11: error: static assertion failed with ""
-  static_assert (__has_unique_object_representations (V) == false, "");
-  ^
-"Test_name.c", line 11: note: the final comparison was 1 == 0
-  static_assert (__has_unique_object_representations (V) == false, "");
-                                                         ^
-
-2 errors detected in the compilation of "Test_name.c".

tests/tests/regressions/.has-unique-object-rep-inherited-base.rto/default.1.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=1 --c++17 --g++ --gnu_version=150100 Test_name.c

tests/tests/regressions/.has-unique-object-rep-inherited-base.rto/default.2.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=2 --c++17 --clang --clang_version=190000 Test_name.c

tests/tests/regressions/has-unique-object-rep-inherited-base.sft.cpp

@@ -0,0 +1,24 @@
+//remark:has_unique_object_representations with a base that has no direct fields
+//type:fp
+//options_all:--c++17
+//options:--g++ --gnu_version=150100:--clang --clang_version=190000
+
+// B declares no fields but is not empty (it inherits A::i), so it must not
+// be skipped as an empty base when checking C.
+struct A { char i; };
+struct B : A {};
+struct C : B {};
+struct D : C {};
+struct E {};
+struct F : E, B {};          // Real empty base is still skipped.
+struct G : B { char j; };
+struct P { int i; char c; }; // Tail padding.
+struct Q : P {};
+
+static_assert(__has_unique_object_representations(B), "B");
+static_assert(__has_unique_object_representations(C), "C");
+static_assert(__has_unique_object_representations(D), "D");
+static_assert(__has_unique_object_representations(F), "F");
+static_assert(__has_unique_object_representations(G), "G");
+static_assert(!__has_unique_object_representations(P), "P");
+static_assert(!__has_unique_object_representations(Q), "Q");