Sema: Diagnose @available attibutes that cannot be noasync.

Instead of asserting.

Author: tshortli

lib/Sema/TypeCheckAttr.cpp

@@ -8298,18 +8298,25 @@ SemanticAvailableAttrRequest::evaluate(swift::Evaluator &evaluator,
   }
 
   if (domain->isSwiftLanguage() || domain->isPackageDescription()) {
-    if (attrKind == AvailableAttr::Kind::Deprecated) {
+    switch (attrKind) {
+    case AvailableAttr::Kind::Deprecated:
       diags.diagnose(attrLoc,
                      diag::attr_availability_expected_deprecated_version,
                      attrName, *string);
       return std::nullopt;
-    }
-    if (attrKind == AvailableAttr::Kind::Unavailable) {
+
+    case AvailableAttr::Kind::Unavailable:
       diags.diagnose(attrLoc, diag::attr_availability_cannot_be_used_for_domain,
                      "unavailable", attrName, *string);
       return std::nullopt;
+
+    case AvailableAttr::Kind::NoAsync:
+      diags.diagnose(attrLoc, diag::attr_availability_cannot_be_used_for_domain,
+                     "noasync", attrName, *string);
+      break;
+    case AvailableAttr::Kind::Default:
+      break;
     }
-    assert(attrKind == AvailableAttr::Kind::Default);
 
     bool hasVersionSpec = (attr->getRawIntroduced() ||
                            attr->getRawDeprecated() || attr->getRawObsoleted());

test/attr/attr_availability_noasync.swift

@@ -18,6 +18,16 @@ func asyncReplacement() async -> Int { }
 @available(*, noasync, renamed: "IOActor.readString()")
 func readStringFromIO() -> String {}
 
+// expected-warning@+2 {{'noasync' cannot be used in 'available' attribute for platform 'swift'}}
+// expected-warning@+1 {{expected 'introduced', 'deprecated', or 'obsoleted' in 'available' attribute for platform 'swift'}}
+@available(swift, noasync)
+func swiftNoAsync() { }
+
+// expected-warning@+2 {{'noasync' cannot be used in 'available' attribute for platform '_PackageDescription'}}
+// expected-warning@+1 {{expected 'introduced', 'deprecated', or 'obsoleted' in 'available' attribute for platform '_PackageDescription'}}
+@available(_PackageDescription, noasync)
+func packageDescriptionNoAsync() { }
+
 @available(SwiftStdlib 5.5, *)
 actor IOActor {
     func readString() -> String {