fix corner cases in optional_chains (#5110)

lib/ast.js

@@ -1289,13 +1289,14 @@ function must_be_expressions(node, prop, allow_spread, allow_hole) {
     });
 }
 
-var AST_Call = DEFNODE("Call", "args expression optional pure", {
+var AST_Call = DEFNODE("Call", "args expression optional pure terminal", {
     $documentation: "A function call expression",
     $propdoc: {
         args: "[AST_Node*] array of arguments",
         expression: "[AST_Node] expression to invoke as function",
         optional: "[boolean] whether the expression is optional chaining",
         pure: "[string/S] marker for side-effect-free call expression",
+        terminal: "[boolean] whether the chain has ended",
     },
     walk: function(visitor) {
         var node = this;
@@ -1316,6 +1317,7 @@ var AST_New = DEFNODE("New", null, {
     $documentation: "An object instantiation.  Derives from a function call since it has exactly the same properties",
     _validate: function() {
         if (this.optional) throw new Error("optional must be false");
+        if (this.terminal) throw new Error("terminal must be false");
     },
 }, AST_Call);
 
@@ -1338,12 +1340,18 @@ var AST_Sequence = DEFNODE("Sequence", "expressions", {
     },
 });
 
-var AST_PropAccess = DEFNODE("PropAccess", "expression optional property", {
+function root_expr(prop) {
+    while (prop instanceof AST_PropAccess) prop = prop.expression;
+    return prop;
+}
+
+var AST_PropAccess = DEFNODE("PropAccess", "expression optional property terminal", {
     $documentation: "Base class for property access expressions, i.e. `a.foo` or `a[\"foo\"]`",
     $propdoc: {
         expression: "[AST_Node] the “container” expression",
         optional: "[boolean] whether the expression is optional chaining",
         property: "[AST_Node|string] the property to access.  For AST_Dot this is always a plain string, while for AST_Sub it's an arbitrary AST_Node",
+        terminal: "[boolean] whether the chain has ended",
     },
     get_property: function() {
         var p = this.property;