Disallow async IBAction methods

The IBAction signature does not allow completion handlers, so making
IBAction methods async doesn't make sense. Disallowing it statically so
it doesn't just break people when they try.

Author: etcwilde

include/swift/AST/DiagnosticsSema.def

@@ -1433,6 +1433,8 @@ ERROR(invalid_ibinspectable,none,
       "only instance properties can be declared @%0", (StringRef))
 ERROR(invalid_ibaction_decl,none,
       "only instance methods can be declared @%0", (StringRef))
+ERROR(invalid_ibaction_decl_async,none,
+      "@%0 instance methods cannot be async", (StringRef))
 ERROR(invalid_ibaction_result,none,
       "methods declared @%0 must %select{|not }1return a value", (StringRef, bool))
 ERROR(invalid_ibaction_argument_count,none,

lib/Sema/TypeCheckAttr.cpp

@@ -530,6 +530,12 @@ void AttributeChecker::visitIBActionAttr(IBActionAttr *attr) {
     return;
   }
 
+  if (FD->isAsyncContext()) {
+    diagnoseAndRemoveAttr(attr, diag::invalid_ibaction_decl_async,
+                          attr->getAttrName());
+    return;
+  }
+
   if (isRelaxedIBAction(Ctx))
     // iOS, tvOS, and watchOS allow 0-2 parameters to an @IBAction method.
     validateIBActionSignature(Ctx, attr, FD, /*minParams=*/0, /*maxParams=*/2);

test/attr/attr_ibaction.swift

@@ -41,6 +41,9 @@ class IBActionWrapperTy {
   func moreMagic(_: AnyObject) -> () {} // no-warning
   @objc @IBAction
   func evenMoreMagic(_: AnyObject) -> () {} // no-warning
+
+  @IBAction // expected-error {{@IBAction instance methods cannot be async}}
+  func asyncIBAction(_: AnyObject) async -> () {}
 }
 
 struct S { }