[Concurrency] Only suggest adding 'async' to functions.

Author: DougGregor

lib/Sema/TypeCheckDecl.cpp

@@ -2156,9 +2156,10 @@ static Type validateParameterType(ParamDecl *decl) {
       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());
+        if (auto func = dyn_cast<FuncDecl>(dc)) {
+          func->diagnose(diag::note_add_async_to_function, func->getName());
+        }
       }
     }
   }

test/expr/unary/async_await.swift

@@ -40,6 +40,11 @@ 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}}
 
+struct HasAsyncBad {
+  init(_: @autoclosure () async -> Int) { }
+  // expected-error@-1{{'async' autoclosure parameter in a non-'async' function}}
+}
+
 func testAutoclosure() async {
   __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}}