Canonicalize expressions that may be used as constants

Evaluate and canonicalize the parameters to the built-in function
identical(a,b) if they are strings. This guarantees that
identical(“ab”, “a”+”b”) evaluates to true.

Note: when the parser parses “a”+”b”, it doesn’t know whether the
expression will be used in a context that may require a
constant value. The canonicalization is thus deferred until it is
known that it is required.

BUG=25024
[email protected]

Review URL: https://codereview.chromium.org/1558033002 .

Author: mhausner

runtime/vm/parser.cc

@@ -11202,6 +11202,26 @@ AstNode* Parser::ParseStaticCall(const Class& cls,
     // source.
     if (!FLAG_warn_on_javascript_compatibility || is_patch_source()) {
       ASSERT(num_arguments == 2);
+
+      // If both arguments are constant expressions of type string,
+      // evaluate and canonicalize them.
+      // This guarantees that identical("ab", "a"+"b") is true.
+      // An alternative way to guarantee this would be to introduce
+      // an AST node that canonicalizes a value.
+      AstNode* arg0 = arguments->NodeAt(0);
+      const Instance* val0 = arg0->EvalConstExpr();
+      if ((val0 != NULL) && (val0->IsString())) {
+        AstNode* arg1 = arguments->NodeAt(1);
+        const Instance* val1 = arg1->EvalConstExpr();
+        if ((val1 != NULL) && (val1->IsString())) {
+          arguments->SetNodeAt(0,
+              new(Z) LiteralNode(arg0->token_pos(),
+                                 EvaluateConstExpr(arg0->token_pos(), arg0)));
+          arguments->SetNodeAt(1,
+              new(Z) LiteralNode(arg1->token_pos(),
+                                 EvaluateConstExpr(arg1->token_pos(), arg1)));
+        }
+      }
       return new(Z) ComparisonNode(ident_pos,
                                    Token::kEQ_STRICT,
                                    arguments->NodeAt(0),

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);
 }