[parser] Always return a valid var from DeclareVariableName

That way we can continue running in failure mode.

Bug: chromium:933214
Change-Id: I975901a72f615e2b7ed9955b75ce86bbcad0bbbb
Reviewed-on: https://chromium-review.googlesource.com/c/1481219Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59813}

src/parsing/expression-scope.h

@@ -56,7 +56,8 @@ class ExpressionScope {
         // with or catch scope. In those cases the proxy isn't guaranteed to
         // refer to the declared variable, so consider it unresolved.
         parser()->scope()->AddUnresolved(result);
-      } else if (var) {
+      } else {
+        DCHECK_NOT_NULL(var);
         result->BindTo(var);
       }
     }
@@ -327,9 +328,8 @@ class VariableDeclarationParsingScope : public ExpressionScope<Types> {
         //
         // This also handles marking of loop variables in for-in and for-of
         // loops, as determined by loop-nesting-depth.
-        if (V8_LIKELY(var)) {
-          var->set_maybe_assigned();
-        }
+        DCHECK_NOT_NULL(var);
+        var->set_maybe_assigned();
       }
     }
     return var;

src/parsing/preparser.h

@@ -1110,6 +1110,8 @@ class PreParser : public ParserBase<PreParser> {
     Variable* var = scope->DeclareVariableName(name, mode, was_added, kind);
     if (var == nullptr) {
       ReportUnidentifiableError();
+      if (!IsLexicalVariableMode(mode)) scope = scope->GetDeclarationScope();
+      var = scope->LookupLocal(name);
     } else if (var->scope() != scope) {
       DCHECK_NE(kNoSourcePosition, position);
       DCHECK_EQ(VariableMode::kVar, mode);

test/mjsunit/regress/regress-crbug-933214.js

+// Copyright 2019 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+assertThrows(`
+    function __v_0() {
+        function __v_2() {
+            try {
+                function* __v_0() {}
+                function __v_0() {}
+            }
+        }
+    }`, SyntaxError);