[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, 8 insertions, 0 deletions

diff --git a/clang/test/SemaObjCXX/literals.mm b/clang/test/SemaObjCXX/literals.mm
index 6cdd207d57d..b62ed05f159 100644
--- a/clang/test/SemaObjCXX/literals.mm
+++ b/clang/test/SemaObjCXX/literals.mm
@@ -50,6 +50,9 @@ void x() {
 + (id)dictionaryWithObjects:(const id [])objects forKeys:(const id<NSCopying> [])keys count:(unsigned long)cnt;
 @end
 
+@interface NSString
+@end
+
 template<typename T>
 struct ConvertibleTo {
   operator T();
@@ -185,3 +188,8 @@ id value;
 void test_dictionary_colon() {
   id dict = @{ key : value };
 }
+
+void testConstExpr() {
+  constexpr NSString *t0 = @"abc";
+  constexpr NSString *t1 = @("abc");
+}