[Sema] Don't accept e.g. "(int *)(long)&x" as a constant expression if x is in address space != 0.

[mjacob]

Summary

The constant expression evaluator for pointers returns a base and an
offset if successful. In this case, CodeGen generates a constant pointer cast,
casting the base (with offset applied if necessary) to the destination type.
However, that is different from casting the base to an integer and then to the
destination type if the destination points to another address space as the
base. Supporting this would require a lot of work because it can't be
expressed as base + offset. Changing the whole constant expression evaluator
is not worthline for supporting this special case.

[mjacob]

ping

lib/AST/ExprConstant.cpp

if (Value.isInt()) {
       uint64_t N = Value.getInt().extOrTrunc(Size).getZExtValue();
       Result.Base = (Expr*)nullptr;
       Result.InvalidBase = false;
       Result.Offset = CharUnits::fromQuantity(N);
       Result.CallIndex = 0;
       Result.Designator.setInvalid();
       return true;
     } else {
-      // Cast is of an lvalue, no need to change value.
+      // Cast is of an lvalue, no need to change value, unless they have
+      // different address spaces.
+      if (Info.Ctx.getTargetAddressSpace(getType(Value.getLValueBase())) !=
+          Info.Ctx.getTargetAddressSpace(E->getType()->getPointeeType()))
+        return false;
       Result.setFrom(Info.Ctx, Value);
       return true;
     }
   }
   case CK_ArrayToPointerDecay:
     if (SubExpr->isGLValue()) {
       if (!EvaluateLValue(SubExpr, Result, Info))
         return false;

test/Sema/const-eval.c

 EVAL_EXPR(51, 0 != (float)1e38)
 
 // PR21945
 void PR21945() { int i = (({}), 0l); }
 
 void PR24622();
 struct PR24622 {} pr24622;
 EVAL_EXPR(52, &pr24622 == (void *)&PR24622); // expected-error {{must have a constant size}}
+
+int __attribute__((address_space(1))) addrspace1;
+int *addrspace0_ptr = (int *)(long)&addrspace1; // expected-error {{not a compile-time constant}}