Sema: Avoid diagnosing required availability in swiftinterface files

lib/Sema/TypeCheckAvailability.cpp

@@ -3456,6 +3456,12 @@ static bool declNeedsExplicitAvailability(const Decl *decl) {
   if (!ctx.supportsVersionedAvailability())
     return false;
 
+  // Don't enforce explicit availability requirements in .swiftinterface files.
+  // These diagnostics are only designed to be emitted when building from
+  // source.
+  if (decl->getDeclContext()->isInSwiftinterface())
+    return false;
+
   // Skip non-public decls.
   if (auto valueDecl = dyn_cast<const ValueDecl>(decl)) {
     AccessScope scope =

test/ModuleInterface/require_explicit_availability.swift

@@ -0,0 +1,12 @@
+// -require-explicit-availability is not printed in interfaces, so no warnings
+// should be emitted when verifying the interface.
+// RUN: %target-swift-emit-module-interface(%t_require.swiftinterface) -require-explicit-availability=warn %s
+// RUN: %target-swift-typecheck-module-from-interface(%t_require.swiftinterface) -verify
+
+// -library-level=api implies -require-explicit-availability=warn and it _is_
+// printed in the interface. Still, no diagnostics about required explicit
+// availability should be emitted when verifying the interface.
+// RUN: %target-swift-emit-module-interface(%t_api.swiftinterface) -library-level=api %s
+// RUN: %target-swift-typecheck-module-from-interface(%t_api.swiftinterface) -verify
+
+public struct NoAvailability { }