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

Fix __has_unique_object_representations for several cases:

- A base class with no direct nonstatic data members that inherits
  some (and is thus not empty) was treated as an empty base. Its size
  was counted as tail padding, making the trait incorrectly FALSE for
  derived classes. Use is_empty_class_type instead of checking for an
  empty field list.

- Treat unnamed bit-fields of nonzero width as padding in Clang 17+
  and g++ 13.1+ (also g++ 10.5, 11.4, 12.3 backports).

- Restrict treating volatile-qualified non-class types as lacking
  unique object representations to g++ modes before version 10, and
  no longer apply it in Clang modes.

Add regression test has-unique-object-rep-inherited-base and update
expectations for imported Clang type-traits and g++
has-unique-obj-representations3 tests.

Author: jakub-labenski

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