[C23] Fix treating unnamed records nested in different types as compatible.

clang/lib/AST/ASTContext.cpp

@@ -11581,6 +11581,12 @@ QualType ASTContext::mergeTagDefinitions(QualType LHS, QualType RHS) {
   if (LangOpts.CPlusPlus || !LangOpts.C23)
     return {};
 
+  // Nameless tags are comparable only within outer definitions. At the top
+  // level they are not comparable.
+  const TagDecl *LTagD = LHS->castAsTagDecl(), *RTagD = RHS->castAsTagDecl();
+  if (!LTagD->getIdentifier() || !RTagD->getIdentifier())
+    return {};
+
   // C23, on the other hand, requires the members to be "the same enough", so
   // we use a structural equivalence check.
   StructuralEquivalenceContext::NonEquivalentDeclSet NonEquivalentDecls;

clang/lib/AST/ASTStructuralEquivalence.cpp

@@ -1763,19 +1763,6 @@ static bool IsStructurallyEquivalent(StructuralEquivalenceContext &Context,
   // another anonymous structure or union, respectively, if their members
   // fulfill the preceding requirements. ... Otherwise, the structure, union,
   // or enumerated types are incompatible.
-
-  // Note: "the same tag" refers to the identifier for the structure; two
-  // structures without names are not compatible within a TU. In C23, if either
-  // declaration has no name, they're not equivalent. However, the paragraph
-  // after the bulleted list goes on to talk about compatibility of anonymous
-  // structure and union members, so this prohibition only applies to top-level
-  // declarations; if either declaration is not a member, they cannot be
-  // compatible.
-  if (Context.LangOpts.C23 && (!D1->getIdentifier() || !D2->getIdentifier()) &&
-      (!D1->getDeclContext()->isRecord() || !D2->getDeclContext()->isRecord()))
-    return false;
-
-  // Otherwise, check the names for equivalence.
   if (!NameIsStructurallyEquivalent(*D1, *D2))
     return false;
 

clang/test/C/C23/n3037.c

@@ -30,11 +30,24 @@ void func2(PRODUCT(int, SUM(float, double)) y) { // c17-warning {{declaration of
 
 struct foop { struct { int x; }; }; // c17-note {{previous definition is here}}
 struct foop { struct { int x; }; }; // c17-error {{redefinition of 'foop'}}
+// Test the field lookup compatibility isn't sufficient, the structure of types should be compatible.
+struct AnonymousStructNotMatchingFields { // c17-note {{previous definition is here}}
+  struct { // c23-note {{field has name '' here}}
+    int x;
+  };
+};
+struct AnonymousStructNotMatchingFields { // c23-error {{type 'struct AnonymousStructNotMatchingFields' has incompatible definitions}} \
+                                             c17-error {{redefinition of 'AnonymousStructNotMatchingFields'}}
+  int x; // c23-note {{field has name 'x' here}}
+};
+
 union barp { int x; float y; };     // c17-note {{previous definition is here}}
 union barp { int x; float y; };     // c17-error {{redefinition of 'barp'}}
 typedef struct q { int x; } q_t;    // c17-note 2 {{previous definition is here}}
 typedef struct q { int x; } q_t;    // c17-error {{redefinition of 'q'}} \
                                        c17-error-re {{typedef redefinition with different types ('struct (unnamed struct at {{.*}})' vs 'struct q')}}
+typedef struct { int x; } untagged_q_t; // both-note {{previous definition is here}}
+typedef struct { int x; } untagged_q_t; // both-error {{typedef redefinition with different types}}
 void func3(void) {
   struct S { int x; };       // c17-note {{previous definition is here}}
   struct T { struct S s; };  // c17-note {{previous definition is here}}
@@ -389,13 +402,40 @@ void nontag_both_in_params(struct { int i; } Arg1, struct { int i; } Arg2) {
   _Static_assert(0 == _Generic(__typeof__(Arg1), __typeof__(Arg2) : 1, default : 0)); // both-warning {{passing a type argument as the first operand to '_Generic' is a C2y extension}}
 }
 
-struct InnerAnonStruct {
+struct InnerUnnamedStruct {
   struct {
     int i;
   } untagged;
-} inner_anon_tagged;
+} inner_unnamed_tagged;
+_Static_assert(0 == _Generic(inner_unnamed_tagged.untagged, struct { int i; } : 1, default : 0));
 
-_Static_assert(0 == _Generic(inner_anon_tagged.untagged, struct { int i; } : 1, default : 0));
+struct InnerUnnamedStruct_same {
+  struct {
+    int i;
+  } untagged;
+};
+struct InnerUnnamedStruct_differentNaming {
+  struct {
+    int i;
+  } untaggedDifferent;
+};
+struct InnerUnnamedStruct_differentShape {
+  float x;
+  struct {
+    int i;
+  } untagged;
+  int y;
+};
+void compare_unnamed_struct_from_different_outer_type(
+    struct InnerUnnamedStruct sameOuterType,
+    struct InnerUnnamedStruct_same matchingType,
+    struct InnerUnnamedStruct_differentNaming differentFieldName,
+    struct InnerUnnamedStruct_differentShape differentType) {
+  inner_unnamed_tagged.untagged = sameOuterType.untagged;
+  inner_unnamed_tagged.untagged = matchingType.untagged; // both-error-re {{assigning to 'struct (unnamed struct at {{.*}})' from incompatible type 'struct (unnamed struct at {{.*}})'}}
+  inner_unnamed_tagged.untagged = differentFieldName.untaggedDifferent; // both-error-re {{assigning to 'struct (unnamed struct at {{.*}})' from incompatible type 'struct (unnamed struct at {{.*}})'}}
+  inner_unnamed_tagged.untagged = differentType.untagged; // both-error-re {{assigning to 'struct (unnamed struct at {{.*}})' from incompatible type 'struct (unnamed struct at {{.*}})'}}
+}
 
 // Test the same thing with enumerations (test for unions is omitted because
 // unions and structures are both RecordDecl objects, whereas EnumDecl is not).