[parser] error on for (async of [])

Summary: Fixes #8651

Reviewed By: SamChou19815

Differential Revision: D42187354

fbshipit-source-id: 042aa651ce1f05aaac35d83730f76b55556ee323

src/parser/statement_parser.ml

@@ -368,6 +368,11 @@ module Statement
         let leading = leading @ Peek.comments env in
         Expect.token env T_LPAREN;
         let comments = Flow_ast_utils.mk_comments_opt ~leading () in
+        let init_starts_with_async =
+          match Peek.token env with
+          | T_ASYNC -> true
+          | _ -> false
+        in
         let (init, errs) =
           let env = env |> with_no_in true in
           match Peek.token env with
@@ -429,6 +434,25 @@ module Statement
             | Some (For_expression expr) ->
               (* #sec-for-in-and-for-of-statements-static-semantics-early-errors *)
               let patt = Pattern_cover.as_pattern ~err:Parse_error.InvalidLHSInForOf env expr in
+              (match (init_starts_with_async, patt) with
+              | ( true,
+                  ( _,
+                    Pattern.Identifier
+                      {
+                        Pattern.Identifier.name =
+                          (id_loc, { Identifier.name = "async"; comments = _ });
+                        annot = _;
+                        optional = _;
+                      }
+                  )
+                ) ->
+                (* #prod-nLtPS4oB - `for (async of ...)` is forbidden because it is
+                   ambiguous whether it's a for-of with an `async` identifier, or a
+                   regular for loop with an async arrow function with a param named
+                   `of`. We can backtrack, so we know it's a for-of, but the spec
+                   still disallows it. *)
+                error_at env (id_loc, Parse_error.InvalidLHSInForOf)
+              | _ -> ());
               Statement.ForOf.LeftPattern patt
             | None -> assert false
           in

src/parser/test/flow/for_of_loops_invalid/for_async_of.js

@@ -0,0 +1 @@
+for (async of []);

src/parser/test/flow/for_of_loops_invalid/for_async_of.tree.json

@@ -0,0 +1,39 @@
+{
+  "errors":[
+    {
+      "loc":{"source":null,"start":{"line":1,"column":5},"end":{"line":1,"column":10}},
+      "message":"Invalid left-hand side in for-of"
+    }
+  ],
+  "type":"Program",
+  "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":18}},
+  "range":[0,18],
+  "body":[
+    {
+      "type":"ForOfStatement",
+      "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":18}},
+      "range":[0,18],
+      "left":{
+        "type":"Identifier",
+        "loc":{"source":null,"start":{"line":1,"column":5},"end":{"line":1,"column":10}},
+        "range":[5,10],
+        "name":"async",
+        "typeAnnotation":null,
+        "optional":false
+      },
+      "right":{
+        "type":"ArrayExpression",
+        "loc":{"source":null,"start":{"line":1,"column":14},"end":{"line":1,"column":16}},
+        "range":[14,16],
+        "elements":[]
+      },
+      "body":{
+        "type":"EmptyStatement",
+        "loc":{"source":null,"start":{"line":1,"column":17},"end":{"line":1,"column":18}},
+        "range":[17,18]
+      },
+      "await":false
+    }
+  ],
+  "comments":[]
+}

src/parser/test/flow/statement/for/for_async_of_arrow.js

@@ -0,0 +1 @@
+for (async of => {};;);

src/parser/test/flow/statement/for/for_async_of_arrow.tree.json

@@ -0,0 +1,48 @@
+{
+  "type":"Program",
+  "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":23}},
+  "range":[0,23],
+  "body":[
+    {
+      "type":"ForStatement",
+      "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":23}},
+      "range":[0,23],
+      "init":{
+        "type":"ArrowFunctionExpression",
+        "loc":{"source":null,"start":{"line":1,"column":5},"end":{"line":1,"column":19}},
+        "range":[5,19],
+        "id":null,
+        "params":[
+          {
+            "type":"Identifier",
+            "loc":{"source":null,"start":{"line":1,"column":11},"end":{"line":1,"column":13}},
+            "range":[11,13],
+            "name":"of",
+            "typeAnnotation":null,
+            "optional":false
+          }
+        ],
+        "body":{
+          "type":"BlockStatement",
+          "loc":{"source":null,"start":{"line":1,"column":17},"end":{"line":1,"column":19}},
+          "range":[17,19],
+          "body":[]
+        },
+        "async":true,
+        "generator":false,
+        "predicate":null,
+        "expression":false,
+        "returnType":null,
+        "typeParameters":null
+      },
+      "test":null,
+      "update":null,
+      "body":{
+        "type":"EmptyStatement",
+        "loc":{"source":null,"start":{"line":1,"column":22},"end":{"line":1,"column":23}},
+        "range":[22,23]
+      }
+    }
+  ],
+  "comments":[]
+}

src/parser/test/flow/statement/for/for_async_of_parens.js

@@ -0,0 +1 @@
+for ((async) of y);

src/parser/test/flow/statement/for/for_async_of_parens.tree.json

@@ -0,0 +1,35 @@
+{
+  "type":"Program",
+  "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":19}},
+  "range":[0,19],
+  "body":[
+    {
+      "type":"ForOfStatement",
+      "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":19}},
+      "range":[0,19],
+      "left":{
+        "type":"Identifier",
+        "loc":{"source":null,"start":{"line":1,"column":6},"end":{"line":1,"column":11}},
+        "range":[6,11],
+        "name":"async",
+        "typeAnnotation":null,
+        "optional":false
+      },
+      "right":{
+        "type":"Identifier",
+        "loc":{"source":null,"start":{"line":1,"column":16},"end":{"line":1,"column":17}},
+        "range":[16,17],
+        "name":"y",
+        "typeAnnotation":null,
+        "optional":false
+      },
+      "body":{
+        "type":"EmptyStatement",
+        "loc":{"source":null,"start":{"line":1,"column":18},"end":{"line":1,"column":19}},
+        "range":[18,19]
+      },
+      "await":false
+    }
+  ],
+  "comments":[]
+}

src/parser_utils/output/__tests__/js_layout_generator_test.ml

@@ -1798,6 +1798,12 @@ let tests =
            assert_statement_string ~ctxt "for(let{x,y}of z);";
            assert_statement_string ~ctxt ~pretty:true "for (let {x, y} of z);"
          );
+         ( "forof_async" >:: fun ctxt ->
+           (* the parens are required *)
+           assert_statement_string ~ctxt "for((async)of y);";
+           assert_statement_string ~ctxt ~pretty:true "for ((async) of y);";
+           assert_statement_string ~ctxt ~pretty:true "for ((/* needs parens */ async) of y);"
+         );
          ( "yield_expressions" >:: fun ctxt ->
            assert_expression_string ~ctxt "function* f(){yield}";
            assert_expression_string ~ctxt "function* f(){yield a}";

src/parser_utils/output/js_layout_generator.ml

@@ -872,6 +872,18 @@ and statement ?(pretty_semicolon = false) ~opts (root_stmt : (Loc.t, Loc.t) Ast.
                       begin
                         match left with
                         | S.ForOf.LeftDeclaration decl -> variable_declaration ~opts decl
+                        | S.ForOf.LeftPattern
+                            ( ( _,
+                                Ast.Pattern.Identifier
+                                  {
+                                    Ast.Pattern.Identifier.name =
+                                      (_, { Ast.Identifier.name = "async"; _ });
+                                    annot = Ast.Type.Missing _;
+                                    _;
+                                  }
+                              ) as patt
+                            ) ->
+                          wrap_in_parens (pattern ~opts patt)
                         | S.ForOf.LeftPattern patt -> pattern ~opts patt
                       end;
                       Atom "of";