typecheck: diagnose malformed @_spi_available

Author: nkcsgexi

include/swift/AST/DiagnosticsSema.def

@@ -1542,6 +1542,9 @@ ERROR(optional_attribute_initializer,none,
 ERROR(unavailable_method_non_objc_protocol,none,
       "protocol members can only be marked unavailable in an @objc protocol",
       ())
+ERROR(spi_available_malformed,none,
+      "SPI available only supports introducing version on specific platform",
+      ())
 ERROR(missing_in_class_init_1,none,
       "stored property %0 requires an initial value%select{| or should be "
       "@NSManaged}1", (Identifier, bool))

lib/Sema/TypeCheckAttr.cpp

@@ -1611,6 +1611,13 @@ void AttributeChecker::visitAvailableAttr(AvailableAttr *attr) {
   if (Ctx.LangOpts.DisableAvailabilityChecking)
     return;
 
+  while (attr->IsSPI) {
+    if (attr->hasPlatform() && attr->Introduced.hasValue())
+      break;
+    diagnoseAndRemoveAttr(attr, diag::spi_available_malformed);
+    break;
+  }
+
   if (auto *PD = dyn_cast<ProtocolDecl>(D->getDeclContext())) {
     if (auto *VD = dyn_cast<ValueDecl>(D)) {
       if (VD->isProtocolRequirement()) {