[Sema] Add -Wpointer-to-enum-cast and -Wvoid-pointer-to-enum-cast

GCC does not warn on casts from pointers to enumerators, while clang
currently does: https://godbolt.org/z/3DFDVG

This causes a bunch of extra warnings in the Linux kernel, where
certain structs contain a void pointer to avoid using a gigantic
union for all of the various types of driver data, such as
versions.

Add a diagnostic that allows certain projects like the kernel to
disable the warning just for enums, which allows those projects to
keep full compatibility with GCC but keeps the intention of treating
casts to integers and enumerators the same by default so that other
projects have the opportunity to catch issues not noticed before (or
follow suite and disable the warning).

Link: ClangBuiltLinux/linux#887

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D75758

Author: nathanchance

clang/include/clang/Basic/DiagnosticGroups.td

@@ -838,9 +838,13 @@ def IncompatibleExceptionSpec : DiagGroup<"incompatible-exception-spec">;
 def IntToVoidPointerCast : DiagGroup<"int-to-void-pointer-cast">;
 def IntToPointerCast : DiagGroup<"int-to-pointer-cast",
                                  [IntToVoidPointerCast]>;
-def VoidPointerToIntCast : DiagGroup<"void-pointer-to-int-cast">;
+def VoidPointerToEnumCast : DiagGroup<"void-pointer-to-enum-cast">;
+def VoidPointerToIntCast : DiagGroup<"void-pointer-to-int-cast",
+                                     [VoidPointerToEnumCast]>;
+def PointerToEnumCast : DiagGroup<"pointer-to-enum-cast",
+                                  [VoidPointerToEnumCast]>;
 def PointerToIntCast : DiagGroup<"pointer-to-int-cast",
-                                 [VoidPointerToIntCast]>;
+                                 [PointerToEnumCast, VoidPointerToIntCast]>;
 
 def Move : DiagGroup<"move", [
     PessimizingMove,

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3668,9 +3668,15 @@ def warn_int_to_void_pointer_cast : Warning<
 def warn_pointer_to_int_cast : Warning<
   "cast to smaller integer type %1 from %0">,
   InGroup<PointerToIntCast>;
+def warn_pointer_to_enum_cast : Warning<
+  warn_pointer_to_int_cast.Text>,
+  InGroup<PointerToEnumCast>;
 def warn_void_pointer_to_int_cast : Warning<
   "cast to smaller integer type %1 from %0">,
   InGroup<VoidPointerToIntCast>;
+def warn_void_pointer_to_enum_cast : Warning<
+  warn_void_pointer_to_int_cast.Text>,
+  InGroup<VoidPointerToEnumCast>;
 def ext_ms_pointer_to_int_cast : ExtWarn<
   "cast to smaller integer type %1 from %0 is a Microsoft extension">,
 InGroup<MicrosoftCast>;

clang/lib/Sema/SemaCast.cpp

@@ -2777,11 +2777,16 @@ void CastOperation::CheckCStyleCast() {
       // If the result cannot be represented in the integer type, the behavior
       // is undefined. The result need not be in the range of values of any
       // integer type.
-      unsigned Diag = Self.getLangOpts().MicrosoftExt
-                          ? diag::ext_ms_pointer_to_int_cast
-                          : SrcType->isVoidPointerType()
-                                ? diag::warn_void_pointer_to_int_cast
-                                : diag::warn_pointer_to_int_cast;
+      unsigned Diag;
+      if (Self.getLangOpts().MicrosoftExt)
+        Diag = diag::ext_ms_pointer_to_int_cast;
+      else if (SrcType->isVoidPointerType())
+        Diag = DestType->isEnumeralType() ? diag::warn_void_pointer_to_enum_cast
+                                          : diag::warn_void_pointer_to_int_cast;
+      else if (DestType->isEnumeralType())
+        Diag = diag::warn_pointer_to_enum_cast;
+      else
+        Diag = diag::warn_pointer_to_int_cast;
       Self.Diag(OpRange.getBegin(), Diag) << SrcType << DestType << OpRange;
     }
   }

clang/test/Sema/cast.c

@@ -186,3 +186,23 @@ void *intToPointerCast2(X x) {
 void *intToPointerCast3() {
   return (void*)(1 + 3);
 }
+
+void voidPointerToEnumCast(VoidPtr v) {
+  (void)(X) v; // expected-warning{{cast to smaller integer type 'X' from 'VoidPtr' (aka 'void *')}}
+  // Test that casts to void* can be controlled separately
+  // from other -Wpointer-to-enum-cast warnings.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wvoid-pointer-to-enum-cast"
+  (void)(X) v; // no-warning
+#pragma clang diagnostic pop
+}
+
+void pointerToEnumCast(CharPtr v) {
+  (void)(X) v; // expected-warning{{cast to smaller integer type 'X' from 'CharPtr' (aka 'char *')}}
+  // Test that casts to void* can be controlled separately
+  // from other -Wpointer-to-enum-cast warnings.
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wvoid-pointer-to-enum-cast"
+  (void)(X) v; // expected-warning{{cast to smaller integer type 'X' from 'CharPtr' (aka 'char *')}}
+#pragma clang diagnostic pop
+}