Allow async as parameter in arrowfunction (#8488)

* Allow async as a parameter name in simple arrow function

* Add tests

src/compiler/parser.ts

@@ -3002,9 +3002,14 @@ namespace ts {
         }
 
         function isUnParenthesizedAsyncArrowFunctionWorker(): Tristate {
+            // AsyncArrowFunctionExpression:
+            //      1) async[no LineTerminator here]AsyncArrowBindingIdentifier[?Yield][no LineTerminator here]=>AsyncConciseBody[?In]
+            //      2) CoverCallExpressionAndAsyncArrowHead[?Yield, ?Await][no LineTerminator here]=>AsyncConciseBody[?In]
             if (token === SyntaxKind.AsyncKeyword) {
                 nextToken();
-                if (scanner.hasPrecedingLineBreak()) {
+                // If the "async" is followed by "=>" token then it is not a begining of an async arrow-function
+                // but instead a simple arrow-function which will be parsed inside "parseAssignmentExpressionOrHigher"
+                if (scanner.hasPrecedingLineBreak() || token === SyntaxKind.EqualsGreaterThanToken) {
                     return Tristate.False;
                 }
                 // Check for un-parenthesized AsyncArrowFunction

tests/baselines/reference/arrowFunctionWithParameterNameAsync.js

@@ -0,0 +1,6 @@
+//// [arrowFunctionWithParameterNameAsync.ts]
+
+const x = async => async;
+
+//// [arrowFunctionWithParameterNameAsync.js]
+var x = function (async) { return async; };

tests/baselines/reference/arrowFunctionWithParameterNameAsync.symbols

@@ -0,0 +1,7 @@
+=== tests/cases/conformance/async/es6/asyncArrowFunction/arrowFunctionWithParameterNameAsync.ts ===
+
+const x = async => async;
+>x : Symbol(x, Decl(arrowFunctionWithParameterNameAsync.ts, 1, 5))
+>async : Symbol(async, Decl(arrowFunctionWithParameterNameAsync.ts, 1, 9))
+>async : Symbol(async, Decl(arrowFunctionWithParameterNameAsync.ts, 1, 9))
+

tests/baselines/reference/arrowFunctionWithParameterNameAsync.types

@@ -0,0 +1,8 @@
+=== tests/cases/conformance/async/es6/asyncArrowFunction/arrowFunctionWithParameterNameAsync.ts ===
+
+const x = async => async;
+>x : (async: any) => any
+>async => async : (async: any) => any
+>async : any
+>async : any
+

tests/cases/conformance/async/es6/asyncArrowFunction/arrowFunctionWithParameterNameAsync.ts

@@ -0,0 +1,4 @@
+// @target: ES5
+// @noEmitHelpers: true
+
+const x = async => async;