[C23] More improved type compatibility for enumerations

clang/include/clang/Basic/DiagnosticASTKinds.td

@@ -511,6 +511,14 @@ def note_odr_number_of_bases : Note<
   "class has %0 base %plural{1:class|:classes}0">;
 def note_odr_enumerator : Note<"enumerator %0 with value %1 here">;
 def note_odr_missing_enumerator : Note<"no corresponding enumerator here">;
+def note_odr_incompatible_fixed_underlying_type : Note<
+  "enumeration %0 declared with incompatible fixed underlying types (%1 vs. "
+  "%2)">;
+def note_odr_fixed_underlying_type : Note<
+  "enumeration %0 has fixed underlying type here">;
+def note_odr_missing_fixed_underlying_type : Note<
+  "enumeration %0 missing fixed underlying type here">;
+
 def err_odr_field_type_inconsistent : Error<
   "field %0 declared with incompatible types in different "
   "translation units (%1 vs. %2)">;

clang/lib/AST/ASTStructuralEquivalence.cpp

@@ -2093,6 +2093,48 @@ static bool IsStructurallyEquivalent(StructuralEquivalenceContext &Context,
       !CheckStructurallyEquivalentAttributes(Context, D1, D2))
     return false;
 
+  // In C23, if one enumeration has a fixed underlying type, the other shall
+  // have a compatible fixed underlying type (6.2.7).
+  if (Context.LangOpts.C23) {
+    if (D1->isFixed() != D2->isFixed()) {
+      if (Context.Complain) {
+        Context.Diag2(D2->getLocation(),
+                      Context.getApplicableDiagnostic(
+                          diag::err_odr_tag_type_inconsistent))
+            << Context.ToCtx.getTypeDeclType(D2)
+            << (&Context.FromCtx != &Context.ToCtx);
+        Context.Diag1(D1->getLocation(),
+                      D1->isFixed()
+                          ? diag::note_odr_fixed_underlying_type
+                          : diag::note_odr_missing_fixed_underlying_type)
+            << D1;
+        Context.Diag2(D2->getLocation(),
+                      D2->isFixed()
+                          ? diag::note_odr_fixed_underlying_type
+                          : diag::note_odr_missing_fixed_underlying_type)
+            << D2;
+      }
+      return false;
+    }
+    if (D1->isFixed()) {
+      assert(D2->isFixed() && "enums expected to have fixed underlying types");
+      if (!IsStructurallyEquivalent(Context, D1->getIntegerType(),
+                                    D2->getIntegerType())) {
+        if (Context.Complain) {
+          Context.Diag2(D2->getLocation(),
+                        Context.getApplicableDiagnostic(
+                            diag::err_odr_tag_type_inconsistent))
+              << Context.ToCtx.getTypeDeclType(D2)
+              << (&Context.FromCtx != &Context.ToCtx);
+          Context.Diag2(D2->getLocation(),
+                        diag::note_odr_incompatible_fixed_underlying_type)
+              << D2 << D2->getIntegerType() << D1->getIntegerType();
+        }
+        return false;
+      }
+    }
+  }
+
   llvm::SmallVector<const EnumConstantDecl *, 8> D1Enums, D2Enums;
   auto CopyEnumerators =
       [](auto &&Range, llvm::SmallVectorImpl<const EnumConstantDecl *> &Cont) {

clang/test/ASTMerge/enum/Inputs/enum3.c

@@ -0,0 +1,14 @@
+// [C23] missing underlying types
+enum E1 : int {
+  E1Enumerator1
+};
+
+enum E2 {
+  E2Enumerator1
+};
+
+// [C23] Incompatible underlying types
+enum E3 : long {
+  E3Enumerator1
+};
+

clang/test/ASTMerge/enum/Inputs/enum4.c

@@ -0,0 +1,14 @@
+// [C23] missing underlying types
+enum E1 {
+  E1Enumerator1
+};
+
+enum E2 : int {
+  E2Enumerator1
+};
+
+// [C23] Incompatible underlying types
+enum E3 : short {
+  E3Enumerator1
+};
+

clang/test/ASTMerge/enum/test2.c

@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -std=c23 -emit-pch -o %t.1.ast %S/Inputs/enum3.c
+// RUN: %clang_cc1 -std=c23 -emit-pch -o %t.2.ast %S/Inputs/enum4.c
+// RUN: not %clang_cc1 -std=c23 -ast-merge %t.1.ast -ast-merge %t.2.ast -fsyntax-only %s 2>&1 | FileCheck %s
+
+// FIXME: this error should not happen!
+// CHECK: error: type 'struct __NSConstantString_tag' has an attribute which currently causes the types to be treated as though they are incompatible
+// CHECK: enum3.c:2:6: warning: type 'enum E1' has incompatible definitions in different translation units
+// CHECK: enum4.c:2:6: note: enumeration 'E1' missing fixed underlying type here
+// CHECK: enum3.c:2:6: note: enumeration 'E1' has fixed underlying type here
+// CHECK: enum3.c:6:6: warning: type 'enum E2' has incompatible definitions in different translation units
+// CHECK: enum4.c:6:6: note: enumeration 'E2' has fixed underlying type here
+// CHECK: enum3.c:6:6: note: enumeration 'E2' missing fixed underlying type here
+// CHECK: enum3.c:11:6: warning: type 'enum E3' has incompatible definitions in different translation units
+// CHECK: enum3.c:11:6: note: enumeration 'E3' declared with incompatible fixed underlying types ('long' vs. 'short')
+// CHECK: 3 warnings and 1 error generated
+

clang/test/C/C23/n3037.c

@@ -402,6 +402,44 @@ _Static_assert(0 == _Generic(inner_anon_tagged.untagged, struct { int i; } : 1,
 enum { E_Untagged1 } nontag_enum; // both-note {{previous definition is here}}
 _Static_assert(0 == _Generic(nontag_enum, enum { E_Untagged1 } : 1, default : 0)); // both-error {{redefinition of enumerator 'E_Untagged1'}}
 
+// Test that enumerations with mixed underlying types are properly handled.
+enum GH150594_E1 : int { GH150594_Val1 };
+enum GH150594_E2 : int { GH150594_Val2 };
+enum GH150594_E3 { GH150594_Val3 };
+enum GH150594_E4 : int { GH150594_Val4 };
+void GH150594(void) {
+  extern enum GH150594_E1 Fn1(void); // both-note {{previous declaration is here}}
+  extern enum GH150594_E2 Fn2(void); // c17-note {{previous declaration is here}}
+  extern enum GH150594_E3 Fn3(void); // both-note {{previous declaration is here}}
+  extern enum GH150594_E4 Fn4(void); // both-note {{previous declaration is here}}
+  enum GH150594_E1 { GH150594_Val1 };
+  enum GH150594_E2 : int { GH150594_Val2 };
+  enum GH150594_E3 : int { GH150594_Val3 };
+  enum GH150594_E4 : short { GH150594_Val4 };
+  extern enum GH150594_E1 Fn1(void); // both-error {{conflicting types for 'Fn1'}}
+  extern enum GH150594_E2 Fn2(void); // c17-error {{conflicting types for 'Fn2'}}
+  extern enum GH150594_E3 Fn3(void); // both-error {{conflicting types for 'Fn3'}}
+  extern enum GH150594_E4 Fn4(void); // both-error {{conflicting types for 'Fn4'}}
+
+  // Show that two declarations in the same scope give expected diagnostics.
+  enum E1 { e1 };       // both-note {{previous declaration is here}}
+  enum E1 : int { e1 }; // both-error {{enumeration previously declared with nonfixed underlying type}}
+
+  enum E2 : int { e2 }; // both-note {{previous declaration is here}}
+  enum E2 { e2 };       // both-error {{enumeration previously declared with fixed underlying type}}
+
+  enum E3 : int { e3 };   // both-note {{previous declaration is here}}
+  enum E3 : short { e3 }; // both-error {{enumeration redeclared with different underlying type 'short' (was 'int')}}
+
+  typedef short foo;
+  enum E4 : foo { e4 };   // c17-note 2 {{previous definition is here}}
+  enum E4 : short { e4 }; // c17-error {{redefinition of 'E4'}} \
+                             c17-error {{redefinition of enumerator 'e4'}}
+
+  enum E5 : foo { e5 }; // both-note {{previous declaration is here}}
+  enum E5 : int { e5 }; // both-error {{enumeration redeclared with different underlying type 'int' (was 'foo' (aka 'short'))}}
+}
+
 // Test that enumerations are compatible with their underlying type, but still
 // diagnose when "same type" is required rather than merely "compatible type".
 enum E1 : int { e1 }; // Fixed underlying type