Fix lookahead for effects specifiers in function types

Author: DougGregor

Sources/SwiftParser/Types.swift

@@ -815,7 +815,6 @@ extension Parser.Lookahead {
       if peek(isAtAnyIn: EffectSpecifier.self) != nil {
         var backtrack = self.lookahead()
         backtrack.consumeAnyToken()
-        backtrack.consumeAnyToken()
         return backtrack.atFunctionTypeArrow()
       }
 

Tests/SwiftParserTest/DeclarationTests.swift

@@ -1048,6 +1048,15 @@ final class DeclarationTests: ParserTestCase {
       ],
       fixedSource: "func test() async throws(MyError){}"
     )
+
+    assertParse(
+      """
+      struct S<Element, Failure: Error> {
+        init(produce: @escaping () async throws(Failure) -> Element?) {
+        }
+      }
+      """
+    )
   }
 
   func testExtraneousRightBraceRecovery() {