LibJS: Check AssignmentExpression LHS in parser

There are many cases which shouldn't even parse, like

null = ...
true = ...
false = ...
123 = ...
"foo" = ...

However this *is* valid syntax:

foo() = ...

So we still have to keep the current code doing a runtime check if the
LHS value is a resolvable reference. I believe this was declared valid
syntax to *in theory* allow functions returning references - though in
practice that isn't a thing.

Fixes #2204.

Libraries/LibJS/AST.h

@@ -60,6 +60,7 @@ public:
     virtual bool is_scope_node() const { return false; }
     virtual bool is_program() const { return false; }
     virtual bool is_variable_declaration() const { return false; }
+    virtual bool is_call_expression() const { return false; }
     virtual bool is_new_expression() const { return false; }
 
 protected:
@@ -583,6 +584,7 @@ public:
 
 private:
     virtual const char* class_name() const override { return "CallExpression"; }
+    virtual bool is_call_expression() const override { return true; }
 
     struct ThisAndCallee {
         Value this_value;
@@ -603,6 +605,7 @@ public:
 
 private:
     virtual const char* class_name() const override { return "NewExpression"; }
+    virtual bool is_call_expression() const override { return false; }
     virtual bool is_new_expression() const override { return true; }
 };
 

Libraries/LibJS/Parser.cpp

@@ -755,6 +755,10 @@ NonnullRefPtr<Expression> Parser::parse_secondary_expression(NonnullRefPtr<Expre
         return parse_call_expression(move(lhs));
     case TokenType::Equals:
         consume();
+        if (!lhs->is_identifier() && !lhs->is_member_expression() && !lhs->is_call_expression()) {
+            syntax_error("Invalid left-hand side in assignment");
+            return create_ast_node<ErrorExpression>();
+        }
         return create_ast_node<AssignmentExpression>(AssignmentOp::Assignment, move(lhs), parse_expression(min_precedence, associativity));
     case TokenType::Period:
         consume();

Libraries/LibJS/Tests/invalid-lhs-in-assignment.js

@@ -1,22 +1,17 @@
 load("test-common.js");
 
 try {
+    function foo() { }
+
     assertThrowsError(() => {
-        512 = 256;
+        foo() = "foo";
     }, {
         error: ReferenceError,
         message: "Invalid left-hand side in assignment"
     });
 
     assertThrowsError(() => {
-        512 = 256;
+        (function () { })() = "foo";
-    }, {
-        error: ReferenceError,
-        message: "Invalid left-hand side in assignment"
-    });
-
-    assertThrowsError(() => {
-        "hello world" = "another thing?";
     }, {
         error: ReferenceError,
         message: "Invalid left-hand side in assignment"