[flow][match] Add better parse error for multiple match args

Summary:
We don't allow multple match args (parsing as AssignmentExpression rather than Expression), to reserve the future possibilty for allowing multiple args (where they would mean the same as `match ([a, b]) {}`.

This is currently a parse error, but working on the Hermes parser version, I realized we can have a better parse error here. Rather than attempting in parse as a `match` call and then failing on the `{`, we can have a better parse error when a single argument is not supplied.

Changelog: [internal]

Reviewed By: SamChou19815

Differential Revision: D65851645

fbshipit-source-id: 5025e0db5c14cd9b46b7c7fc91e5a46b1c911a74

src/parser/expression_parser.ml

@@ -1338,15 +1338,23 @@ module Expression
       let start_loc = Peek.loc env in
       (* Consume `match` as an identifier, in case it's a call expression. *)
       let id = Parse.identifier env in
-      (* Allows trailing comma - ban? *)
+      (* Allows trailing comma. *)
       let (args_loc, args) = arguments env in
-      (match args with
       (* `match (<expr>) {` *)
-      | { Expression.ArgList.arguments = [Expression.Expression arg]; _ }
-        when (not (Peek.is_line_terminator env)) && Peek.token env = T_LCURLY ->
-        Cover_expr (match_expression ~start_loc ~leading ~arg env)
-      (* It's actually a call expression of the form `match(...)` *)
-      | _ ->
+      if (not (Peek.is_line_terminator env)) && Peek.token env = T_LCURLY then (
+        match args with
+        | { Ast.Expression.ArgList.arguments = [Ast.Expression.Expression arg]; _ } ->
+          Cover_expr (match_expression ~start_loc ~leading ~arg env)
+        | _ ->
+          error_at env (args_loc, Parse_error.MatchNonSingleArgument);
+          let error_arg =
+            ( args_loc,
+              Ast.Expression.Sequence { Ast.Expression.Sequence.expressions = []; comments = None }
+            )
+          in
+          Cover_expr (match_expression ~start_loc ~leading ~arg:error_arg env)
+      ) else
+        (* It's actually a call expression of the form `match(...)` *)
         let callee = (fst id, Expression.Identifier id) in
         let loc = Loc.btwn start_loc args_loc in
         let comments = Flow_ast_utils.mk_comments_opt ~leading () in
@@ -1360,7 +1368,7 @@ module Expression
           ~in_optional_chain:false
           env
           start_loc
-          (Cover_expr (loc, call)))
+          (Cover_expr (loc, call))
     | T_IDENTIFIER { raw = "abstract"; _ } when Peek.ith_token ~i:1 env = T_CLASS ->
       Cover_expr (Parse.class_expression env)
     | _ when Peek.is_identifier env ->

src/parser/parse_error.ml

@@ -105,6 +105,7 @@ type t =
   | JSXAttributeValueEmptyExpression
   | LiteralShorthandProperty
   | MalformedUnicode
+  | MatchNonSingleArgument
   | MethodInDestructuring
   | MissingJSXClosingTag of string
   | MissingTypeParam
@@ -393,6 +394,7 @@ module PP = struct
       "JSX attributes must only be assigned a non-empty expression"
     | LiteralShorthandProperty -> "Literals cannot be used as shorthand properties."
     | MalformedUnicode -> "Malformed unicode"
+    | MatchNonSingleArgument -> "`match` only supports one argument"
     | MethodInDestructuring -> "Object pattern can't contain methods"
     | MissingJSXClosingTag name ->
       Printf.sprintf "JSX element %s has no corresponding closing tag." name

src/parser/statement_parser.ml

@@ -584,15 +584,23 @@ module Statement
     let start_loc = Peek.loc env in
     (* Consume `match` as an identifier, in case it's a call expression. *)
     let id = Parse.identifier env in
-    (* Allows trailing comma - ban? *)
+    (* Allows trailing comma. *)
     let (args_loc, args) = Expression.arguments env in
-    match args with
-    (* `match (<expr>) {` *)
-    | { Ast.Expression.ArgList.arguments = [Ast.Expression.Expression arg]; _ }
-      when (not (Peek.is_line_terminator env)) && Peek.token env = T_LCURLY ->
-      match_statement ~start_loc ~leading ~arg env
-    (* It's actually a call expression statement of the form `match(...)` *)
-    | _ ->
+    (* `match (<exprs>) {` *)
+    if (not (Peek.is_line_terminator env)) && Peek.token env = T_LCURLY then (
+      match args with
+      | { Ast.Expression.ArgList.arguments = [Ast.Expression.Expression arg]; _ } ->
+        match_statement ~start_loc ~leading ~arg env
+      | _ ->
+        error_at env (args_loc, Parse_error.MatchNonSingleArgument);
+        let error_arg =
+          ( args_loc,
+            Ast.Expression.Sequence { Ast.Expression.Sequence.expressions = []; comments = None }
+          )
+        in
+        match_statement ~start_loc ~leading ~arg:error_arg env
+    ) else
+      (* It's actually a call expression statement of the form `match(...)` *)
       let callee = (fst id, Ast.Expression.Identifier id) in
       let expr_loc = Loc.btwn start_loc args_loc in
       let comments = Flow_ast_utils.mk_comments_opt ~leading () in

src/parser/test/flow/match/expression-multiple-args.js

@@ -0,0 +1 @@
+const e = match (a, b) {};

src/parser/test/flow/match/expression-multiple-args.options.json

@@ -0,0 +1,3 @@
+{
+  "pattern_matching": true
+}

src/parser/test/flow/match/expression-multiple-args.tree.json

@@ -0,0 +1,47 @@
+{
+  "errors":[
+    {
+      "loc":{"source":null,"start":{"line":1,"column":16},"end":{"line":1,"column":22}},
+      "message":"`match` only supports one argument"
+    }
+  ],
+  "type":"Program",
+  "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":26}},
+  "range":[0,26],
+  "body":[
+    {
+      "type":"VariableDeclaration",
+      "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":26}},
+      "range":[0,26],
+      "declarations":[
+        {
+          "type":"VariableDeclarator",
+          "loc":{"source":null,"start":{"line":1,"column":6},"end":{"line":1,"column":25}},
+          "range":[6,25],
+          "id":{
+            "type":"Identifier",
+            "loc":{"source":null,"start":{"line":1,"column":6},"end":{"line":1,"column":7}},
+            "range":[6,7],
+            "name":"e",
+            "typeAnnotation":null,
+            "optional":false
+          },
+          "init":{
+            "type":"MatchExpression",
+            "loc":{"source":null,"start":{"line":1,"column":10},"end":{"line":1,"column":25}},
+            "range":[10,25],
+            "argument":{
+              "type":"SequenceExpression",
+              "loc":{"source":null,"start":{"line":1,"column":16},"end":{"line":1,"column":22}},
+              "range":[16,22],
+              "expressions":[]
+            },
+            "cases":[]
+          }
+        }
+      ],
+      "kind":"const"
+    }
+  ],
+  "comments":[]
+}

src/parser/test/flow/match/statement-multiple-args.js

@@ -0,0 +1 @@
+match (a, b) {}

src/parser/test/flow/match/statement-multiple-args.options.json

@@ -0,0 +1,3 @@
+{
+  "pattern_matching": true
+}

src/parser/test/flow/match/statement-multiple-args.tree.json

@@ -0,0 +1,26 @@
+{
+  "errors":[
+    {
+      "loc":{"source":null,"start":{"line":1,"column":6},"end":{"line":1,"column":12}},
+      "message":"`match` only supports one argument"
+    }
+  ],
+  "type":"Program",
+  "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":15}},
+  "range":[0,15],
+  "body":[
+    {
+      "type":"MatchStatement",
+      "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":15}},
+      "range":[0,15],
+      "argument":{
+        "type":"SequenceExpression",
+        "loc":{"source":null,"start":{"line":1,"column":6},"end":{"line":1,"column":12}},
+        "range":[6,12],
+        "expressions":[]
+      },
+      "cases":[]
+    }
+  ],
+  "comments":[]
+}