[Clang] treat fixed-underlying enum constants as the enumerated type in C23 to follow the spec

clang/docs/ReleaseNotes.rst

@@ -537,6 +537,7 @@ Bug Fixes in This Version
 - Fixed false-positive shadow diagnostics for lambdas in explicit object member functions. (#GH163731)
 - Fix an assertion failure when a ``target_clones`` attribute is only on the
   forward declaration of a multiversioned function. (#GH165517) (#GH129483)
+- Clang now treats enumeration constants of fixed-underlying enums as the enumerated type. (#GH172118)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaDecl.cpp

@@ -20813,10 +20813,12 @@ void Sema::ActOnEnumBody(SourceLocation EnumLoc, SourceRange BraceRange,
       NewSign = true;
     } else if (ECD->getType() == BestType) {
       // Already the right type!
-      if (getLangOpts().CPlusPlus)
+      if (getLangOpts().CPlusPlus || (getLangOpts().C23 && Enum->isFixed()))
         // C++ [dcl.enum]p4: Following the closing brace of an
         // enum-specifier, each enumerator has the type of its
         // enumeration.
+        // C23 6.7.2.2p15: For an enumerated type with fixed underlying type,
+        // the enumeration member type is the enumerated type.
         ECD->setType(EnumType);
       continue;
     } else {
@@ -20836,10 +20838,13 @@ void Sema::ActOnEnumBody(SourceLocation EnumLoc, SourceRange BraceRange,
       ECD->setInitExpr(ImplicitCastExpr::Create(
           Context, NewTy, CK_IntegralCast, ECD->getInitExpr(),
           /*base paths*/ nullptr, VK_PRValue, FPOptionsOverride()));
-    if (getLangOpts().CPlusPlus)
+    if (getLangOpts().CPlusPlus ||
+        (getLangOpts().C23 && (Enum->isFixed() || !MembersRepresentableByInt)))
       // C++ [dcl.enum]p4: Following the closing brace of an
       // enum-specifier, each enumerator has the type of its
       // enumeration.
+      // C23 6.7.2.2p15: For an enumerated type with fixed underlying type, the
+      // enumeration member type is the enumerated type.
       ECD->setType(EnumType);
     else
       ECD->setType(NewTy);

clang/test/C/C23/n3029.c

@@ -42,7 +42,8 @@ _Bool e (void) {
 }
 
 int f (void) {
-  return a0; // all-warning {{implicit conversion from 'unsigned long' to 'int' changes value from 18446744073709551615 to -1}}
+  return a0; // expected-warning {{implicit conversion from 'enum a' to 'int' changes value from 18446744073709551615 to -1}}
+             // pedantic-warning@-1 {{implicit conversion from 'unsigned long' to 'int' changes value from 18446744073709551615 to -1}}
 }
 
 unsigned long g (void) {

clang/test/Sema/c23-fixed-underlying-enum.c

@@ -0,0 +1,59 @@
+// RUN: %clang_cc1 -triple x86_64-linux -std=c23 -fsyntax-only -verify %s
+// expected-no-diagnostics
+
+#define INT64_MIN (-9223372036854775807LL - 1)
+
+void t1() {
+  {
+    enum { A };
+    enum { B };
+
+    _Static_assert(_Generic(A, typeof(B): 1, default: 0) == 1, "");
+    _Static_assert(_Generic(typeof(A), typeof(B): 1, default: 0) == 1, "");
+  }
+
+  {
+    _Static_assert(
+      _Generic(typeof(enum {A}), typeof(enum {B}): 1, default: 0) == 0, "");
+  }
+}
+
+void t2() {
+  {
+    enum : int { A };
+    enum : int { B };
+
+    _Static_assert(_Generic(A, typeof(B): 1, default: 0) == 0, "");
+    _Static_assert(_Generic(typeof(A), typeof(B): 1, default: 0) == 0, "");
+  }
+
+  {
+    _Static_assert(
+      _Generic(typeof(enum : int{A}), typeof(enum : int{B}): 1, default: 0) == 0, "");
+  }
+}
+
+void t3() {
+  {
+    enum { A = INT64_MIN };
+    enum { B = INT64_MIN };
+
+    _Static_assert(_Generic(A, __typeof__(B): 1, default: 0) == 0, "");
+    _Static_assert(_Generic(__typeof__(A), __typeof__(B): 1, default: 0) == 0, "");
+  }
+
+  {
+    enum : long long { A = INT64_MIN };
+    enum : long long { B = INT64_MIN };
+
+    _Static_assert(_Generic(A, __typeof__(B): 1, default: 0) == 0, "");
+    _Static_assert(_Generic(__typeof__(A), __typeof__(B): 1, default: 0) == 0, "");
+  }
+}
+
+void t4() {
+  enum : int { A };
+  enum : int { B };
+
+  _Static_assert(_Generic(A, typeof(B): 1, typeof(A): 2, default: 0) == 2, "");
+}

clang/test/Sema/c23-switch.c

@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -std=c23 -fsyntax-only -verify -Wswitch %s
+
+typedef enum : long { E0 } E;
+void test1(E e) {
+  auto v = E0;
+  switch (v) { } // expected-warning {{enumeration value 'E0' not handled in switch}}
+}
+
+void test2(E e) {
+  __auto_type v = E0;
+  switch (v) { } // expected-warning {{enumeration value 'E0' not handled in switch}}
+}
+
+void test3(_Bool b, E e) {
+  __auto_type v = E0;
+  if (b) {
+    v = e;
+  }
+  switch (v) { } // expected-warning {{enumeration value 'E0' not handled in switch}}
+}

clang/test/SemaCXX/bitfield-preferred-type-sizing.cpp

@@ -377,9 +377,9 @@ void write_high_constantA(S_A *s) {
 void write_high_constantB(S_B *s) {
   s->field1 = ENUM_CLASS_REF(B, B_d);
   // cpp-warning@-1 {{implicit truncation from 'B' to bit-field changes value from 3 to 1}}
-  // c-warning@-2 {{implicit truncation from 'int' to bit-field changes value from 3 to -1}}
+  // c-warning@-2 {{implicit truncation from 'enum B' to bit-field changes value from 3 to -1}}
   s->field2 = ENUM_CLASS_REF(B, B_d);
-  // c-warning@-1 {{implicit truncation from 'int' to bit-field changes value from 3 to -1}}
+  // c-warning@-1 {{implicit truncation from 'enum B' to bit-field changes value from 3 to -1}}
   s->field3 = ENUM_CLASS_REF(B, B_d);
   s->field4 = (unsigned)ENUM_CLASS_REF(B, B_d);
   // expected-warning@-1 {{implicit truncation from 'unsigned int' to bit-field changes value from 3 to 1}}
@@ -396,7 +396,7 @@ void write_high_constantB(S_B *s) {
 void write_high_constantC(S_C *s) {
   s->field1 = ENUM_CLASS_REF(C, C_d);
   // cpp-warning@-1 {{implicit truncation from 'C' to bit-field changes value from 3 to 1}}
-  // c-warning@-2 {{implicit truncation from 'unsigned int' to bit-field changes value from 3 to 1}}
+  // c-warning@-2 {{implicit truncation from 'enum C' to bit-field changes value from 3 to 1}}
   s->field2 = ENUM_CLASS_REF(C, C_d);
   s->field3 = ENUM_CLASS_REF(C, C_d);
   s->field4 = (unsigned)ENUM_CLASS_REF(C, C_d);