Emit CK_NoOp casts in C mode, not just C++.

[jyknight]

Summary

Previously, it had been using CK_BitCast even for casts that only
change const/restrict/volatile. Now it will use CK_Noop where
appropriate.

This is an alternate solution to r336746.

[aaron.ballman]

Patch is missing tests -- perhaps you could dump the AST and check the casting notation from the output?

[jyknight]

In D52918#1256420, @aaron.ballman wrote:

Patch is missing tests -- perhaps you could dump the AST and check the casting notation from the output?

It would appear that which casts get emitted in C mode is almost completely untested. I added tests just for this change in a new file, and left a TODO to fill it out for the remainder of those functions.

[jyknight]

Added test.

[aaron.ballman]

LGTM, thank you for adding the test and the TODO!

lib/AST/ExprConstant.cpp

bool PointerExprEvaluator::VisitCastExpr(const CastExpr *E) {
   case CK_AnyPointerToBlockPointerCast:
   case CK_AddressSpaceConversion:
     if (!Visit(SubExpr))
       return false;
     // Bitcasts to cv void* are static_casts, not reinterpret_casts, so are
     // permitted in constant expressions in C++11. Bitcasts from cv void* are
     // also static_casts, but we disallow them as a resolution to DR1312.
     if (!E->getType()->isVoidPointerType()) {
-      // If we changed anything other than cvr-qualifiers, we can't use this
-      // value for constant folding. FIXME: Qualification conversions should
-      // always be CK_NoOp, but we get this wrong in C.
-      if (!Info.Ctx.hasCvrSimilarType(E->getType(), E->getSubExpr()->getType()))
-        Result.Designator.setInvalid();
       if (SubExpr->getType()->isVoidPointerType())
         CCEDiag(E, diag::note_constexpr_invalid_cast)
           << 3 << SubExpr->getType();
       else
         CCEDiag(E, diag::note_constexpr_invalid_cast) << 2;
     }
     if (E->getCastKind() == CK_AddressSpaceConversion && Result.IsNullPtr)
       ZeroInitialization(E);

lib/Sema/SemaExpr.cpp

CastKind Sema::PrepareScalarCast(ExprResult &Src, QualType DestTy) {
   case Type::STK_BlockPointer:
   case Type::STK_ObjCObjectPointer:
     switch (DestTy->getScalarTypeKind()) {
     case Type::STK_CPointer: {
       LangAS SrcAS = SrcTy->getPointeeType().getAddressSpace();
       LangAS DestAS = DestTy->getPointeeType().getAddressSpace();
       if (SrcAS != DestAS)
         return CK_AddressSpaceConversion;
+      if (Context.hasCvrSimilarType(SrcTy, DestTy))
+        return CK_NoOp;
       return CK_BitCast;
     }
     case Type::STK_BlockPointer:
       return (SrcKind == Type::STK_BlockPointer
                 ? CK_BitCast : CK_AnyPointerToBlockPointerCast);
     case Type::STK_ObjCObjectPointer:
       if (SrcKind == Type::STK_ObjCObjectPointer)
         return CK_BitCast;
Sema::CheckAssignmentConstraints(QualType LHSType, ExprResult &RHS,
   }
 
   // Conversions to normal pointers.
   if (const PointerType *LHSPointer = dyn_cast<PointerType>(LHSType)) {
     // U* -> T*
     if (isa<PointerType>(RHSType)) {
       LangAS AddrSpaceL = LHSPointer->getPointeeType().getAddressSpace();
       LangAS AddrSpaceR = RHSType->getPointeeType().getAddressSpace();
-      Kind = AddrSpaceL != AddrSpaceR ? CK_AddressSpaceConversion : CK_BitCast;
+      if (AddrSpaceL != AddrSpaceR)
+        Kind = CK_AddressSpaceConversion;
+      else if (Context.hasCvrSimilarType(RHSType, LHSType))
+        Kind = CK_NoOp;
+      else
+        Kind = CK_BitCast;
       return checkPointerTypesForAssignment(*this, LHSType, RHSType);
     }
 
     // int -> T*
     if (RHSType->isIntegerType()) {
       Kind = CK_IntegralToPointer; // FIXME: null?
       return IntToPointer;
     }

test/Sema/c-casts.c

+// RUN: %clang_cc1 -w -ast-dump %s | FileCheck %s
+
+// The cast construction code both for implicit and c-style casts is very
+// different in C vs C++. This file is intended to test the C behavior.
+
+// TODO: add tests covering the rest of the code in
+// Sema::CheckAssignmentConstraints and Sema::PrepareScalarCast
+
+// CHECK-LABEL: FunctionDecl {{.*}} cast_cvr_pointer
+void cast_cvr_pointer(char volatile * __restrict * const * p) {
+  char*** x;
+  // CHECK: ImplicitCastExpr {{.*}} 'char ***' <NoOp>
+  x = p;
+  // CHECK: CStyleCastExpr {{.*}} 'char ***' <NoOp>
+  x = (char***)p;
+}
+
+// CHECK-LABEL: FunctionDecl {{.*}} cast_pointer_type
+void cast_pointer_type(char *p) {
+  void *x;
+  // CHECK: ImplicitCastExpr {{.*}} 'void *' <BitCast>
+  x = p;
+  // CHECK: CStyleCastExpr {{.*}} 'void *' <BitCast>
+  x = (void*)p;
+}