fix cases where error message was not specific when missing an 'await' in autoclosure argument.

when the call in an autoclosure is async and throwing, if the call is covered by a try! or try?,
the existing check that used Context::isAutoClosure was too pessimistic, saying it is not an
autoclosure because errors are handled within it. For the error message about a missing await,
we don't care about whether the throwing-ness was handled.

Author: kavon

lib/Sema/TypeCheckEffects.cpp

@@ -1275,8 +1275,12 @@ class Context {
       highlight = apply->getSourceRange();
 
     auto diag = diag::async_call_without_await;
-    if (isAutoClosure())
+    // To produce a better error message, check if it is an autoclosure.
+    // We do not use 'Context::isAutoClosure' b/c it gives conservative answers.
+    if (Function && llvm::isa_and_nonnull<AutoClosureExpr>(
+                                            Function->getAbstractClosureExpr()))
       diag = diag::async_call_without_await_in_autoclosure;
+
     ctx.Diags.diagnose(node.getStartLoc(), diag)
         .fixItInsert(node.getStartLoc(), "await ")
         .highlight(highlight);

test/Concurrency/async_throwing.swift

@@ -105,10 +105,8 @@ func asyncTest() async {
   let _ = await invokeAuto(await try! throwingTask())
   let _ = await invokeAuto(await try? throwingTask())
 
-  // FIXME: we currently emit a less-helpful error message here.
-  // it would be better if we said "call is 'async' in an autoclosure argument that is not marked with 'await'"
-  let _ = await invokeAuto(try! throwingTask()) // expected-error{{call is 'async' but is not marked with 'await'}}
-  let _ = await invokeAuto(try? throwingTask()) // expected-error{{call is 'async' but is not marked with 'await'}}
+  let _ = await invokeAuto(try! throwingTask()) // expected-error{{call is 'async' in an autoclosure argument that is not marked with 'await'}}
+  let _ = await invokeAuto(try? throwingTask()) // expected-error{{call is 'async' in an autoclosure argument that is not marked with 'await'}}
 
   let _ = await invokeAuto(await try! throwingTask())
   let _ = await invokeAuto(await try? throwingTask())