allow BindingPattern in FunctionRestParameter

also add downlevel emit for the destructured rest param

Part of #6275

Author: ajafff

src/compiler/checker.ts

@@ -28570,10 +28570,6 @@ namespace ts {
                         checkGrammarForDisallowedTrailingComma(parameters, Diagnostics.A_rest_parameter_or_binding_pattern_may_not_have_a_trailing_comma);
                     }
 
-                    if (isBindingPattern(parameter.name)) {
-                        return grammarErrorOnNode(parameter.name, Diagnostics.A_rest_element_cannot_contain_a_binding_pattern);
-                    }
-
                     if (parameter.questionToken) {
                         return grammarErrorOnNode(parameter.questionToken, Diagnostics.A_rest_parameter_cannot_be_optional);
                     }

src/compiler/transformers/es2015.ts

@@ -1340,8 +1340,8 @@ namespace ts {
          *                                          part of a constructor declaration with a
          *                                          synthesized call to `super`
          */
-        function shouldAddRestParameter(node: ParameterDeclaration | undefined, inConstructorWithSynthesizedSuper: boolean) {
-            return node && node.dotDotDotToken && node.name.kind === SyntaxKind.Identifier && !inConstructorWithSynthesizedSuper;
+        function shouldAddRestParameter(node: ParameterDeclaration | undefined, inConstructorWithSynthesizedSuper: boolean): node is ParameterDeclaration {
+            return !!(node && node.dotDotDotToken && !inConstructorWithSynthesizedSuper);
         }
 
         /**
@@ -1360,11 +1360,11 @@ namespace ts {
             }
 
             // `declarationName` is the name of the local declaration for the parameter.
-            const declarationName = getMutableClone(<Identifier>parameter!.name);
+            const declarationName = parameter.name.kind === SyntaxKind.Identifier ? getMutableClone(parameter.name) : createTempVariable(/*recordTempVariable*/ undefined);
             setEmitFlags(declarationName, EmitFlags.NoSourceMap);
 
             // `expressionName` is the name of the parameter used in expressions.
-            const expressionName = getSynthesizedClone(<Identifier>parameter!.name);
+            const expressionName = parameter.name.kind === SyntaxKind.Identifier ? getSynthesizedClone(parameter.name) : declarationName;
             const restIndex = node.parameters.length - 1;
             const temp = createLoopVariable();
 
@@ -1429,6 +1429,24 @@ namespace ts {
             setEmitFlags(forStatement, EmitFlags.CustomPrologue);
             startOnNewLine(forStatement);
             statements.push(forStatement);
+
+            if (parameter.name.kind !== SyntaxKind.Identifier) {
+                // do the actual destructuring of the rest parameter if necessary
+                statements.push(
+                    setEmitFlags(
+                        setTextRange(
+                            createVariableStatement(
+                                /*modifiers*/ undefined,
+                                createVariableDeclarationList(
+                                    flattenDestructuringBinding(parameter, visitor, context, FlattenLevel.All, expressionName),
+                                )
+                            ),
+                            parameter
+                        ),
+                        EmitFlags.CustomPrologue
+                    )
+                );
+            }
         }
 
         /**

tests/baselines/reference/iterableArrayPattern14.errors.txt

@@ -1,13 +1,10 @@
-tests/cases/conformance/es6/destructuring/iterableArrayPattern16.ts(1,17): error TS2501: A rest element cannot contain a binding pattern.
 tests/cases/conformance/es6/destructuring/iterableArrayPattern16.ts(2,5): error TS2345: Argument of type 'FooIterator' is not assignable to parameter of type '[Bar, Bar]'.
   Property '0' is missing in type 'FooIterator'.
 tests/cases/conformance/es6/destructuring/iterableArrayPattern16.ts(2,12): error TS2449: Class 'FooIteratorIterator' used before its declaration.
 
 
-==== tests/cases/conformance/es6/destructuring/iterableArrayPattern16.ts (3 errors) ====
+==== tests/cases/conformance/es6/destructuring/iterableArrayPattern16.ts (2 errors) ====
     function fun(...[a, b]: [Bar, Bar][]) { }
-                    ~~~~~~
-!!! error TS2501: A rest element cannot contain a binding pattern.
     fun(...new FooIteratorIterator);
         ~~~~~~~~~~~~~~~~~~~~~~~~~~
 !!! error TS2345: Argument of type 'FooIterator' is not assignable to parameter of type '[Bar, Bar]'.

tests/baselines/reference/iterableArrayPattern15.errors.txt

@@ -1,9 +1,8 @@
-tests/cases/conformance/es6/destructuring/iterableArrayPattern17.ts(16,17): error TS2501: A rest element cannot contain a binding pattern.
 tests/cases/conformance/es6/destructuring/iterableArrayPattern17.ts(17,5): error TS2345: Argument of type 'FooIterator' is not assignable to parameter of type 'Bar'.
   Property 'x' is missing in type 'FooIterator'.
 
 
-==== tests/cases/conformance/es6/destructuring/iterableArrayPattern17.ts (2 errors) ====
+==== tests/cases/conformance/es6/destructuring/iterableArrayPattern17.ts (1 errors) ====
     class Bar { x }
     class Foo extends Bar { y }
     class FooIterator {
@@ -20,8 +19,6 @@ tests/cases/conformance/es6/destructuring/iterableArrayPattern17.ts(17,5): error
     }
 
     function fun(...[a, b]: Bar[]) { }
-                    ~~~~~~
-!!! error TS2501: A rest element cannot contain a binding pattern.
     fun(new FooIterator);
         ~~~~~~~~~~~~~~~
 !!! error TS2345: Argument of type 'FooIterator' is not assignable to parameter of type 'Bar'.

tests/baselines/reference/iterableArrayPattern16.errors.txt

@@ -1,11 +1,8 @@
-tests/cases/conformance/es6/destructuring/iterableArrayPattern25.ts(1,33): error TS2501: A rest element cannot contain a binding pattern.
 tests/cases/conformance/es6/destructuring/iterableArrayPattern25.ts(2,1): error TS2554: Expected 2 arguments, but got 1.
 
 
-==== tests/cases/conformance/es6/destructuring/iterableArrayPattern25.ts (2 errors) ====
+==== tests/cases/conformance/es6/destructuring/iterableArrayPattern25.ts (1 errors) ====
     function takeFirstTwoEntries(...[[k1, v1], [k2, v2]]) { }
-                                    ~~~~~~~~~~~~~~~~~~~~
-!!! error TS2501: A rest element cannot contain a binding pattern.
     takeFirstTwoEntries(new Map([["", 0], ["hello", 1]]));
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 !!! error TS2554: Expected 2 arguments, but got 1.

tests/baselines/reference/iterableArrayPattern17.errors.txt

@@ -1,12 +1,9 @@
-tests/cases/conformance/es6/destructuring/iterableArrayPattern26.ts(1,33): error TS2501: A rest element cannot contain a binding pattern.
 tests/cases/conformance/es6/destructuring/iterableArrayPattern26.ts(2,21): error TS2345: Argument of type 'Map<string, number>' is not assignable to parameter of type '[string, number]'.
   Property '0' is missing in type 'Map<string, number>'.
 
 
-==== tests/cases/conformance/es6/destructuring/iterableArrayPattern26.ts (2 errors) ====
+==== tests/cases/conformance/es6/destructuring/iterableArrayPattern26.ts (1 errors) ====
     function takeFirstTwoEntries(...[[k1, v1], [k2, v2]]: [string, number][]) { }
-                                    ~~~~~~~~~~~~~~~~~~~~
-!!! error TS2501: A rest element cannot contain a binding pattern.
     takeFirstTwoEntries(new Map([["", 0], ["hello", 1]]));
                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 !!! error TS2345: Argument of type 'Map<string, number>' is not assignable to parameter of type '[string, number]'.