[Diagnostics] Downgrade l-value resolves to an unused variable to warning

Author: LucianoPAlmeida

include/swift/AST/DiagnosticsSema.def

@@ -3786,8 +3786,8 @@ ERROR(expression_unused_closure,none,
       "closure expression is unused", ())
 ERROR(expression_unused_function,none,
       "expression resolves to an unused function", ())
-ERROR(expression_unused_lvalue,none,
-      "expression resolves to an unused %select{variable|property|subscript}0", (unsigned))
+WARNING(expression_unused_lvalue,none,
+        "expression resolves to an unused %select{variable|property|subscript}0", (unsigned))
 WARNING(expression_unused_result_call,none,
         "result of call to %0 is unused", (DeclName))
 WARNING(expression_unused_result_operator,none,

lib/SILOptimizer/Mandatory/DataflowDiagnostics.cpp

@@ -63,7 +63,7 @@ static void diagnoseMissingReturn(const UnreachableInst *UI,
   if (!BS->empty()) {
     auto element = BS->getLastElement();
     if (auto expr = element.dyn_cast<Expr *>()) {
-      if (expr->getType()->isEqual(ResTy)) {
+      if (expr->getType()->getRValueType()->isEqual(ResTy)) {
         Context.Diags.diagnose(
           expr->getStartLoc(),
           diag::missing_return_last_expr, ResTy,