Canonicalize expressions that may be used as constants

tests/language/const_string_test.dart

Index: tests/language/const_string_test.dart
diff --git a/tests/language/const_string_test.dart b/tests/language/const_string_test.dart
index 4736ca49f12501a07e6f9b1335154e43b4053c16..13f1a9cb73a76632650dc38819d36b48a28d51ac 100644
--- a/tests/language/const_string_test.dart
+++ b/tests/language/const_string_test.dart
@@ -6,6 +6,8 @@ import "package:expect/expect.dart";
 
 // Exercises compile-time string constants
 
+const yz = "y" + "z";
+
 main() {
   // Constant comparisons are independent of the quotes used.
   Expect.isTrue(identical("abcd", 'abcd'));
@@ -34,4 +36,16 @@ main() {
   Expect.isTrue(identical('a\'b\'cd', "a" "'b'" 'c' "d"));
   Expect.isTrue(identical('a"b"cd', 'a' '"b"' 'c' "d"));
   Expect.isTrue(identical("a\"b\"cd", 'a' '"b"' 'c' "d"));
+
+  const a = identical("ab", "a" + "b");
+  Expect.isTrue(a);
+
+  const b = identical("xyz", "x" + yz);
+  Expect.isTrue(b);
+
+  const c = identical("12", "1" "2");
+  Expect.isTrue(c);
+
+  const d = identical("zyz", "z$yz");
+  Expect.isTrue(d);

[Lasse Reichstein Nielsen, 2016/01/05 18:55:13]

Maybe also test without the const declarations.
An expression on the form `identical("ab", "a" + "b")` is a compile-time
constant expression in itself, and should be be the compile-time constant true.
The specification doesn't distinguish between whether it's used as a constant,
so `Expect.isTrue(identical("ab", "a" + "b"));` should also succeed.

[Lasse Reichstein Nielsen, 2016/01/05 18:56:15]

(I think it will succeed after this patch, I just want the test to cover it :)

[hausner, 2016/01/05 19:05:34]

I've talked to Gilad about this. He said that the Spec is not mandating that
"a"+"b" be treated as a constant when not used in a context that requiring a
constant. Consider:

var a = "a"+"b";
...
identical("ab", a);

The language should (and does) not require that the value in a is canonicalized,
even though it is a compile-time constant. Thus the identical() call may return
false.

 }