[ResultBuilders] Split diagnostic about return statements into error and warning versions

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -5460,7 +5460,10 @@ NOTE(previous_result_builder_here, none,
      "previous result builder specified here", ())
 ERROR(result_builder_arguments, none,
       "result builder attributes cannot have arguments", ())
-WARNING(result_builder_disabled_by_return, none,
+ERROR(result_builder_disabled_by_return, none,
+      "application of result builder %0 disabled by explicit 'return' "
+      "statement", (Type))
+WARNING(result_builder_disabled_by_return_warn, none,
         "application of result builder %0 disabled by explicit 'return' "
         "statement", (Type))
 NOTE(result_builder_remove_attr, none,

lib/Sema/BuilderTransform.cpp

@@ -1554,7 +1554,7 @@ Optional<BraceStmt *> TypeChecker::applyResultBuilderBodyTransform(
 
     ctx.Diags.diagnose(
         returnStmts.front()->getReturnLoc(),
-        diag::result_builder_disabled_by_return, builderType);
+        diag::result_builder_disabled_by_return_warn, builderType);
 
     // Note that one can remove the result builder attribute.
     auto attr = func->getAttachedResultBuilder();