[ObjC] Emit a boxed expression as a compile-time constant if the

expression inside the parentheses is a valid UTF-8 string literal.

Previously clang emitted an expression like @("abc") as a message send
to stringWithUTF8String. This commit makes clang emit the boxed
expression as a compile-time constant instead.

This commit also has the effect of silencing the nullable-to-nonnull
conversion warning clang started emitting after r317727, which
originally motivated this commit (see https://oleb.net/2018/@keypath).

rdar://problem/42684601

Differential Revision: https://reviews.llvm.org/D58729

llvm-svn: 355662

1 files changed, 4 insertions, 0 deletions

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 111cffe373e..77443b67de4 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -1743,6 +1743,8 @@ static bool IsGlobalLValue(APValue::LValueBase B) {
   case Expr::CXXTypeidExprClass:
   case Expr::CXXUuidofExprClass:
     return true;
+  case Expr::ObjCBoxedExprClass:
+    return cast<ObjCBoxedExpr>(E)->isExpressibleAsConstantInitializer();
   case Expr::CallExprClass:
     return IsStringLiteralCall(cast<CallExpr>(E));
   // For GCC compatibility, &&label has static storage duration.
@@ -5794,6 +5796,8 @@ public:
   bool VisitObjCStringLiteral(const ObjCStringLiteral *E)
       { return Success(E); }
   bool VisitObjCBoxedExpr(const ObjCBoxedExpr *E) {
+    if (E->isExpressibleAsConstantInitializer())
+      return Success(E);
     if (Info.noteFailure())
       EvaluateIgnoredValue(Info, E->getSubExpr());
     return Error(E);