[Sema] Make C++ functional-style cast warn about dropped qualifiers (-Wcast-qual)

Functional-style cast (i.e. a simple-type-specifier or typename-specifier
followed by a parenthesize single expression [expr.type.conv]) is equivalent
to the C-style cast, so that makes sense they have identical behavior
including warnings.

This also matches GCC https://godbolt.org/z/b8Ma9Thjb.

Reviewed By: rnk, aaron.ballman

Differential Revision: https://reviews.llvm.org/D159133

clang/docs/ReleaseNotes.rst

@@ -165,6 +165,7 @@ Improvements to Clang's diagnostics
   (`#64871: <https://github.com/llvm/llvm-project/issues/64871>`_).
   Also clang no longer emits false positive warnings about the output length of
   ``%g`` format specifier.
+- Clang now emits ``-Wcast-qual`` for functional-style cast expressions.
 
 Bug Fixes in This Version
 -------------------------

clang/lib/Sema/SemaCast.cpp

@@ -3374,6 +3374,9 @@ ExprResult Sema::BuildCXXFunctionalCastExpr(TypeSourceInfo *CastTypeInfo,
   if (auto *ConstructExpr = dyn_cast<CXXConstructExpr>(SubExpr))
     ConstructExpr->setParenOrBraceRange(SourceRange(LPLoc, RPLoc));
 
+  // -Wcast-qual
+  DiagnoseCastQual(Op.Self, Op.SrcExpr, Op.DestType);
+
   return Op.complete(CXXFunctionalCastExpr::Create(
       Context, Op.ResultType, Op.ValueKind, CastTypeInfo, Op.Kind,
       Op.SrcExpr.get(), &Op.BasePath, CurFPFeatureOverrides(), LPLoc, RPLoc));

clang/test/Sema/warn-cast-qual.c

@@ -36,6 +36,39 @@ void foo(void) {
   char *charptr = (char *)constcharptr; // expected-warning {{cast from 'const char *' to 'char *' drops const qualifier}}
   const char *constcharptr2 = (char *)constcharptr; // expected-warning {{cast from 'const char *' to 'char *' drops const qualifier}}
   const char *charptr2 = (char *)charptr; // no warning
+
+#ifdef __cplusplus
+  using CharPtr = char *;
+  using CharPtrPtr = char **;
+  using ConstCharPtrPtr = const char **;
+  using CharPtrConstPtr = char *const *;
+
+  char *fy = CharPtr(ptr);	// expected-warning {{cast from 'const char *' to 'char *' drops const qualifier}}
+  char **fy1 = CharPtrPtr(ptrptr);	// expected-warning {{cast from 'const char *const *' to 'char **' drops const qualifier}}
+  const char **fy2 = ConstCharPtrPtr(ptrptr);	// expected-warning {{cast from 'const char *const *' to 'const char **' drops const qualifier}}
+  char *const *fy3 = CharPtrConstPtr(ptrptr);	// expected-warning {{cast from 'const char *const' to 'char *const' drops const qualifier}}
+  const char **fy4 = ConstCharPtrPtr(ptrcptr);	// expected-warning {{cast from 'char *const *' to 'const char **' drops const qualifier}}
+
+  using ConstVoidPtr = const void *;
+  char *fz = CharPtr(uintptr_t(ConstVoidPtr(ptr)));	// no warning
+  char *fz1 = CharPtr(ConstVoidPtr(ptr));	// expected-warning {{cast from 'const void *' to 'char *' drops const qualifier}}
+
+  char *fvol2 = CharPtr(vol); // expected-warning {{cast from 'volatile char *' to 'char *' drops volatile qualifier}}
+  char *fvolc2 = CharPtr(volc); // expected-warning {{cast from 'const volatile char *' to 'char *' drops const and volatile qualifiers}}
+
+  using ConstIntPtrPtr = const int **;
+  using VolitileIntPtrPtr = volatile int **;
+  const int **fintptrptrc = ConstIntPtrPtr(intptrptr); // expected-warning {{cast from 'int **' to 'ConstIntPtrPtr' (aka 'const int **') must have all intermediate pointers const qualified}}
+  volatile int **fintptrptrv = VolitileIntPtrPtr(intptrptr); // expected-warning {{cast from 'int **' to 'VolitileIntPtrPtr' (aka 'volatile int **') must have all intermediate pointers const qualified}}
+
+  using ConstIntPtr = const int *;
+  const int *fintptrc = ConstIntPtr(intptr);    // no warning
+
+  char **fcharptrptr = CharPtrPtr(charptrptrc); // expected-warning {{cast from 'const char *' to 'char *' drops const qualifier}}
+
+  char *fcharptr = CharPtr(constcharptr); // expected-warning {{cast from 'const char *' to 'char *' drops const qualifier}}
+  const char *fcharptr2 = CharPtr(charptr); // no warning
+#endif
 }
 
 void bar_0(void) {
@@ -48,6 +81,12 @@ void bar_0(void) {
 
   *(int *)(&S.a) = 0; // expected-warning {{cast from 'const int *' to 'int *' drops const qualifier}}
   *(int *)(&S.b) = 0; // expected-warning {{cast from 'const int *' to 'int *' drops const qualifier}}
+
+#ifdef __cplusplus
+  using IntPtr = int *;
+  *(IntPtr(&S.a)) = 0; // expected-warning {{cast from 'const int *' to 'int *' drops const qualifier}}
+  *(IntPtr(&S.b)) = 0; // expected-warning {{cast from 'const int *' to 'int *' drops const qualifier}}
+#endif
 }
 
 void bar_1(void) {
@@ -61,4 +100,10 @@ void bar_1(void) {
 
   *(int *)(&S.a) = 0; // expected-warning {{cast from 'const int *' to 'int *' drops const qualifier}}
   *(int *)(&S.b) = 0; // no warning
+
+#ifdef __cplusplus
+  using IntPtr = int *;
+  *(IntPtr(&S.a)) = 0; // expected-warning {{cast from 'const int *' to 'int *' drops const qualifier}}
+  *(IntPtr(&S.b)) = 0; // no warning
+#endif
 }