JavaScript: a for-in variable initializer is a SyntaxError in strict mode code

Current test262 (ECMA-262 2017 Annex B.3.6) accepts `for (var x = 1 in o)` only outside strict
mode code. The parser now rejects the initializer in strict code, with the error at `in`.

crates/js/src/parser.rs

@@ -500,6 +500,11 @@ impl<'s> Parser<'s> {
             let mut decls = self.parse_var_decls(true)?;
             if decls.len() == 1 && self.at_keyword(Keyword::In) {
                 let decl = decls.pop().ok_or_else(|| self.unexpected())?;
+                // The variable of a for-in loop takes an initializer only outside strict mode
+                // code (ECMA-262 2017 Annex B.3.6).
+                if self.ctx.strict && decl.init.is_some() {
+                    return Err(self.error_at_token(&self.tok, "a for-in variable cannot have an initializer in strict mode code"));
+                }
                 return self.parse_for_in(ForInTarget::Var(decl), start);
             }
             Some(ForInit::Var(decls))

crates/js/src/tests.rs

@@ -466,6 +466,10 @@ fn strict_mode_early_errors() {
     }
     assert!(strict_err("function f(a, a) {}").message.contains("duplicate parameter"));
     ok("function f(a, a) {}");
+    let e = strict_err("for (var x = 1 in o) {}");
+    assert!(e.message.contains("initializer"), "{}", e.message);
+    assert_eq!((e.line, e.column), (2, 16));
+    ok("'use strict'; for (var x in o) {}");
 }
 
 #[test]