Canonicalize expressions that may be used as constants

tests/language/const_string_test.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 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'));
   Expect.isTrue(identical('abcd', "abcd"));
   Expect.isTrue(identical("ab\"cd", 'ab"cd'));
   Expect.isTrue(identical('ab\'cd', "ab'cd"));
 
   // String concatenation works even when quotes are different.
   Expect.isTrue(identical("abcd", "ab" "cd"));
   Expect.isTrue(identical("abcd", "ab" 'cd'));
   Expect.isTrue(identical("abcd", 'ab' 'cd'));
   Expect.isTrue(identical("abcd", 'ab' "cd"));
 
   // Or when there are more than 2 contatenations.
   Expect.isTrue(identical("abcd", "a" "b" "cd"));
   Expect.isTrue(identical("abcd", "a" "b" "c" "d"));
   Expect.isTrue(identical('abcd', 'a' 'b' 'c' 'd'));
   Expect.isTrue(identical("abcd", "a" "b" 'c' "d"));
   Expect.isTrue(identical("abcd", 'a' 'b' 'c' 'd'));
   Expect.isTrue(identical("abcd", 'a' "b" 'c' "d"));
 
   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"));
   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"));
   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.

 }