Warn about sending on Sendable type in function parameters (#74616)

When a function parameter is Sendable, adding 'sending' annotation does nothing, as the value is safe to be sent across isolation boundaries. SE-0430 seems to imply that 'sending' is intended to be used for non-Sendable types only.

Author: Androp0v

include/swift/AST/DiagnosticsSema.def

@@ -7993,6 +7993,9 @@ ERROR(sending_unsupported_param_specifier, none,
       "'%0' cannot be applied to a 'sending' parameter", (StringRef))
 ERROR(sending_only_on_parameters_and_results, none,
       "'sending' may only be used on parameters and results", ())
+WARNING(sending_applied_to_sendable, none,
+        "'sending' has no effect on Sendable parameter %0",
+        (Type))
 ERROR(sending_cannot_be_applied_to_tuple_elt, none,
       "'sending' cannot be applied to tuple elements", ())
 ERROR(sending_function_wrong_sending,none,

lib/Sema/TypeCheckType.cpp

@@ -4975,8 +4975,29 @@ TypeResolver::resolveSendingTypeRepr(SendingTypeRepr *repr,
     return ErrorType::get(getASTContext());
   }
 
+  NeverNullType resolvedType = resolveType(repr->getBase(), options);
+  
+  // If resolved type is Sendable, warn about unnecessary 'sending' annotation.
+  if (options.is(TypeResolverContext::FunctionInput)) {
+    if(!resolvedType->hasTypeParameter()) {
+      if (resolvedType->isSendableType()) {
+        diagnose(repr->getSpecifierLoc(),
+                 diag::sending_applied_to_sendable,
+                 resolvedType.get())
+        .fixItRemove(repr->getSpecifierLoc());
+      }
+    } else {
+      if (resolvedType->getASTContext().getProtocol(KnownProtocolKind::Sendable)) {
+        diagnose(repr->getSpecifierLoc(),
+                 diag::sending_applied_to_sendable,
+                 resolvedType.get())
+        .fixItRemove(repr->getSpecifierLoc());
+      };
+    }
+  }
+
   // Return the type.
-  return resolveType(repr->getBase(), options);
+  return resolvedType;
 }
 
 NeverNullType

test/type/function/sending_sendable_parameter.swift

@@ -0,0 +1,20 @@
+// RUN: %target-swift-frontend -module-name TestModule -typecheck -verify %s
+
+struct SendableType {}
+class NonSendableType {}
+
+func testSendingOnNonSendable(input: sending NonSendableType) {
+    // okay
+}
+
+func testSendingOnSendable(input: sending SendableType) {
+    // expected-warning@-1{{'sending' has no effect}}{{35-43=}}
+}
+
+func testSendingOnSendable(input: sending some Sendable) {
+    // expected-warning@-1{{'sending' has no effect}}{{35-43=}}
+}
+
+func testSendingOnSendable(input: sending any Sendable) {
+    // expected-warning@-1{{'sending' has no effect}}{{35-43=}}
+}