[Concurrency] async autoclosures are only legal on async functions.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -2174,9 +2174,6 @@ NOTE(protocol_witness_settable_conflict,none,
      "candidate is not settable, but protocol requires it", ())
 NOTE(protocol_witness_rethrows_conflict,none,
      "candidate is not 'rethrows', but protocol requires it", ())
-NOTE(protocol_witness_async_conflict,none,
-     "candidate is %select{not |}0'async', but protocol requirement is%select{| not}0",
-     (bool))
 NOTE(protocol_witness_throws_conflict,none,
      "candidate throws, but protocol does not allow it", ())
 NOTE(protocol_witness_not_objc,none,
@@ -4036,6 +4033,16 @@ NOTE(note_error_to_optional,none,
       "did you mean to handle error as optional value?", ())
 NOTE(note_disable_error_propagation,none,
       "did you mean to disable error propagation?", ())
+
+WARNING(no_throw_in_try,none,
+        "no calls to throwing functions occur within 'try' expression", ())
+
+WARNING(no_throw_in_do_with_catch,none,
+        "'catch' block is unreachable because no errors are thrown in 'do' block", ())
+
+//------------------------------------------------------------------------------
+// MARK: Concurrency
+//------------------------------------------------------------------------------
 ERROR(async_call_without_await,none,
       "call is 'async' but is not marked with 'await'", ())
 ERROR(async_call_without_await_in_autoclosure,none,
@@ -4056,12 +4063,15 @@ ERROR(async_in_nonasync_function,none,
       (bool, bool))
 NOTE(note_add_async_to_function,none,
      "add 'async' to function %0 to make it asynchronous", (DeclName))
-
-WARNING(no_throw_in_try,none,
-        "no calls to throwing functions occur within 'try' expression", ())
-
-WARNING(no_throw_in_do_with_catch,none,
-        "'catch' block is unreachable because no errors are thrown in 'do' block", ())
+ERROR(not_objc_function_async,none,
+      "'async' function cannot be represented in Objective-C", ())
+NOTE(not_objc_function_type_async,none,
+      "'async' function types cannot be represented in Objective-C", ())
+NOTE(protocol_witness_async_conflict,none,
+     "candidate is %select{not |}0'async', but protocol requirement is%select{| not}0",
+     (bool))
+ERROR(async_autoclosure_nonasync_function,none,
+      "'async' autoclosure parameter in a non-'async' function", ())
 
 //------------------------------------------------------------------------------
 // MARK: Type Check Types
@@ -4417,10 +4427,6 @@ NOTE(not_objc_generic_type_param,none,
 NOTE(not_objc_function_type_param,none,
      "function types cannot be represented in Objective-C unless their "
      "parameters and returns can be", ())
-ERROR(not_objc_function_async,none,
-      "'async' function cannot be represented in Objective-C", ())
-NOTE(not_objc_function_type_async,none,
-      "'async' function types cannot be represented in Objective-C", ())
 NOTE(not_objc_function_type_throwing,none,
       "throwing function types cannot be represented in Objective-C", ())
 NOTE(objc_inferring_on_objc_protocol_member,none,

lib/Sema/TypeCheckDecl.cpp

@@ -2148,9 +2148,21 @@ static Type validateParameterType(ParamDecl *decl) {
       decl->setInvalid();
       return ErrorType::get(ctx);
     }
+  }
 
-    return Ty;
+  // async autoclosures can only occur as parameters to async functions.
+  if (decl->isAutoClosure()) {
+    if (auto fnType = Ty->getAs<FunctionType>()) {
+      if (fnType->isAsync() &&
+          !(isa<AbstractFunctionDecl>(dc) &&
+            cast<AbstractFunctionDecl>(dc)->hasAsync())) {
+        auto func = cast<AbstractFunctionDecl>(dc);
+        decl->diagnose(diag::async_autoclosure_nonasync_function);
+        func->diagnose(diag::note_add_async_to_function, func->getName());
+      }
+    }
   }
+
   return Ty;
 }
 

test/expr/unary/async_await.swift

@@ -33,20 +33,22 @@ struct SomeStruct {
   static var y = __await getInt() // expected-error{{'async' call cannot occur in a global variable initializer}}
 }
 
-func acceptAutoclosureNonAsync(_: @autoclosure () -> Int) { }
-func acceptAutoclosureAsync(_: @autoclosure () async -> Int) { }
+func acceptAutoclosureNonAsync(_: @autoclosure () -> Int) async { }
+func acceptAutoclosureAsync(_: @autoclosure () async -> Int) async { }
+
+func acceptAutoclosureNonAsyncBad(_: @autoclosure () async -> Int) { }
+// expected-error@-1{{'async' autoclosure parameter in a non-'async' function}}
+// expected-note@-2{{add 'async' to function 'acceptAutoclosureNonAsyncBad' to make it asynchronous}}
 
 func testAutoclosure() async {
-  acceptAutoclosureAsync(getInt()) // expected-error{{call is 'async' in an autoclosure argument is not marked with 'await'}}
-  acceptAutoclosureNonAsync(getInt()) // expected-error{{'async' in an autoclosure that does not support concurrency}}
+  __await acceptAutoclosureAsync(getInt()) // expected-error{{call is 'async' in an autoclosure argument is not marked with 'await'}}
+  __await acceptAutoclosureNonAsync(getInt()) // expected-error{{'async' in an autoclosure that does not support concurrency}}
 
-  acceptAutoclosureAsync(__await getInt())
-  acceptAutoclosureNonAsync(__await getInt()) // expected-error{{'async' in an autoclosure that does not support concurrency}}
+  __await acceptAutoclosureAsync(__await getInt())
+  __await acceptAutoclosureNonAsync(__await getInt()) // expected-error{{'async' in an autoclosure that does not support concurrency}}
 
   __await acceptAutoclosureAsync(getInt()) // expected-error{{call is 'async' in an autoclosure argument is not marked with 'await'}}
-  // expected-warning@-1{{no calls to 'async' functions occur within 'await' expression}}
   __await acceptAutoclosureNonAsync(getInt()) // expected-error{{'async' in an autoclosure that does not support concurrency}}
-  // expected-warning@-1{{no calls to 'async' functions occur within 'await' expression}}
 }
 
 // Test inference of 'async' from the body of a closure.