[clang][Sema] Don't issue -Wcast-function-type-mismatch for enums with a matching underlying type (#87793)

Enums are passed as their underlying integral type so they're ABI compatible if the size matches.
Useful with C APIs that pass user-controlled values to callbacks that can be made type safe by using enumerations (e.g. GStreamer).

Discovered internally in some code after 999d4f8.

Author: tambry

clang/lib/Sema/SemaCast.cpp

@@ -1093,9 +1093,10 @@ static bool argTypeIsABIEquivalent(QualType SrcType, QualType DestType,
     return true;
 
   // Allow integral type mismatch if their size are equal.
-  if (SrcType->isIntegralType(Context) && DestType->isIntegralType(Context))
-    if (Context.getTypeInfoInChars(SrcType).Width ==
-        Context.getTypeInfoInChars(DestType).Width)
+  if ((SrcType->isIntegralType(Context) || SrcType->isEnumeralType()) &&
+      (DestType->isIntegralType(Context) || DestType->isEnumeralType()))
+    if (Context.getTypeSizeInChars(SrcType) ==
+        Context.getTypeSizeInChars(DestType))
       return true;
 
   return Context.hasSameUnqualifiedType(SrcType, DestType);

clang/test/Sema/warn-cast-function-type-strict.c

@@ -29,8 +29,17 @@ f8 *h;
 f9 *i;
 f10 *j;
 
+enum E : long;
+int efunc(enum E);
+
+// Produce the underlying `long` type implicitly.
+enum E2 { big = __LONG_MAX__ };
+int e2func(enum E2);
+
 void foo(void) {
   a = (f1 *)x;
+  a = (f1 *)efunc; // strict-warning {{cast from 'int (*)(enum E)' to 'f1 *' (aka 'int (*)(long)') converts to incompatible function type}}
+  a = (f1 *)e2func; // strict-warning {{cast from 'int (*)(enum E2)' to 'f1 *' (aka 'int (*)(long)') converts to incompatible function type}}
   b = (f2 *)x; /* expected-warning {{cast from 'int (*)(long)' to 'f2 *' (aka 'int (*)(void *)') converts to incompatible function type}} */
   c = (f3 *)x; /* strict-warning {{cast from 'int (*)(long)' to 'f3 *' (aka 'int (*)()') converts to incompatible function type}} */
   d = (f4 *)x; /* expected-warning {{cast from 'int (*)(long)' to 'f4 *' (aka 'void (*)()') converts to incompatible function type}} */

clang/test/Sema/warn-cast-function-type.c

@@ -19,8 +19,17 @@ f5 *e;
 f6 *f;
 f7 *g;
 
+enum E : long;
+int efunc(enum E);
+
+// Produce the underlying `long` type implicitly.
+enum E2 { big = __LONG_MAX__ };
+int e2func(enum E2);
+
 void foo(void) {
   a = (f1 *)x;
+  a = (f1 *)efunc; // enum is just type system sugar, still passed as a long.
+  a = (f1 *)e2func; // enum is just type system sugar, still passed as a long.
   b = (f2 *)x; /* expected-warning {{cast from 'int (*)(long)' to 'f2 *' (aka 'int (*)(void *)') converts to incompatible function type}} */
   c = (f3 *)x;
   d = (f4 *)x; /* expected-warning {{cast from 'int (*)(long)' to 'f4 *' (aka 'void (*)()') converts to incompatible function type}} */

clang/test/SemaCXX/warn-cast-function-type-strict.cpp

@@ -29,8 +29,17 @@ struct S
 
 typedef void (S::*mf)(int);
 
+enum E : long;
+int efunc(E);
+
+// Produce the underlying `long` type implicitly.
+enum E2 { big = __LONG_MAX__ };
+int e2func(E2);
+
 void foo() {
   a = (f1 *)x;
+  a = (f1 *)efunc; // strict-warning {{cast from 'int (*)(E)' to 'f1 *' (aka 'int (*)(long)') converts to incompatible function type}}
+  a = (f1 *)e2func; // strict-warning {{cast from 'int (*)(E2)' to 'f1 *' (aka 'int (*)(long)') converts to incompatible function type}}
   b = (f2 *)x; // expected-warning {{cast from 'int (*)(long)' to 'f2 *' (aka 'int (*)(void *)') converts to incompatible function type}}
   b = reinterpret_cast<f2 *>(x); // expected-warning {{cast from 'int (*)(long)' to 'f2 *' (aka 'int (*)(void *)') converts to incompatible function type}}
   c = (f3 *)x; // strict-warning {{cast from 'int (*)(long)' to 'f3 *' (aka 'int (*)(...)') converts to incompatible function type}}

clang/test/SemaCXX/warn-cast-function-type.cpp

@@ -28,8 +28,17 @@ struct S
 
 typedef void (S::*mf)(int);
 
+enum E : long;
+int efunc(E);
+
+// Produce the underlying `long` type implicitly.
+enum E2 { big = __LONG_MAX__ };
+int e2func(E2);
+
 void foo() {
   a = (f1 *)x;
+  a = (f1 *)efunc; // enum is just type system sugar, still passed as a long.
+  a = (f1 *)e2func; // enum is just type system sugar, still passed as a long.
   b = (f2 *)x; // expected-warning {{cast from 'int (*)(long)' to 'f2 *' (aka 'int (*)(void *)') converts to incompatible function type}}
   b = reinterpret_cast<f2 *>(x); // expected-warning {{cast from 'int (*)(long)' to 'f2 *' (aka 'int (*)(void *)') converts to incompatible function type}}
   c = (f3 *)x;