diff --git a/ets2panda/checker/ets/helpers.cpp b/ets2panda/checker/ets/helpers.cpp
index f084c61fc6b92fcfdba32c2d4ea7ca0d415841c2..534263417da1220f3c982677ed61c22d6c4e2f7b 100644
--- a/ets2panda/checker/ets/helpers.cpp
+++ b/ets2panda/checker/ets/helpers.cpp
@@ -1011,9 +1011,15 @@ checker::Type *ETSChecker::GetExtensionAccessorReturnType(ir::MemberExpression *
 // Smart cast support
 //==============================================================================//
 
-static checker::Type *MaybeReadonlyType(ETSChecker *checker, checker::Type *sourceType, checker::Type const *targetType)
+static checker::Type *MaybeReadonlyType(ETSChecker *checker, checker::Type *sourceType, checker::Type *targetType)
 {
-    if (targetType->HasTypeFlag(TypeFlag::READONLY)) {
+    //  For left-hand variable of builtin type leave it as is.
+    if (targetType->IsBuiltinNumeric()) {
+        return targetType;
+    }
+
+    // Preserve 'Readonly' type flag in smart type if it exists in declared type
+    if (targetType->HasTypeFlag(TypeFlag::READONLY) && !sourceType->HasTypeFlag(TypeFlag::READONLY)) {
         sourceType = sourceType->Clone(checker);
         sourceType->AddTypeFlag(TypeFlag::READONLY);
     }
@@ -1036,11 +1042,6 @@ checker::Type *ETSChecker::ResolveSmartType(checker::Type *sourceType, checker::
         return sourceType;
     }
 
-    //  For left-hand variable of builtin type leave it as is.
-    if (targetType->IsETSObjectType() && targetType->AsETSObjectType()->HasObjectFlag(ETSObjectFlags::BUILTIN_TYPE)) {
-        return targetType;
-    }
-
     //  In case of Union left-hand type we try to select the proper type from the Union
     if (targetType->IsETSUnionType() && !sourceType->IsUnionType()) {
         auto *constituentType = targetType->AsETSUnionType()->GetAssignableType(this, sourceType, value);
diff --git a/ets2panda/checker/types/ets/etsUnionType.cpp b/ets2panda/checker/types/ets/etsUnionType.cpp
index 7e1743576091523ad929e9da1a0494758e5bc800..cb112121c82df4692d0fcbafdbee36da4bc05c13 100644
--- a/ets2panda/checker/types/ets/etsUnionType.cpp
+++ b/ets2panda/checker/types/ets/etsUnionType.cpp
@@ -307,19 +307,15 @@ checker::Type *ETSUnionType::GetAssignableType(checker::ETSChecker *checker, che
         }
     }
 
-    if (!sourceType->IsETSObjectType()) {
+    if (!sourceType->IsBuiltinNumeric()) {
         return nullptr;
     }
-    auto *objectType = checker->GetNonConstantType(sourceType)->AsETSObjectType();
-
-    if (!objectType->IsBuiltinNumeric()) {
-        return sourceType;
-    }
 
     // NOTE (DZ): we still keep 'numericTypes` collection for possible processing cases like 'let x: short|double = 1`
     //            Waiting for complete clearness in spec - now return the highest type in such a case or type itself.
     //            Maybe 'value' will be used for this purpose
     std::map<std::uint32_t, checker::ETSObjectType *> numericTypes {};
+    auto *objectType = sourceType->AsETSObjectType();
     if (auto *assignableType = GetAssignableBuiltinType(checker, objectType, numericTypes); assignableType != nullptr) {
         return assignableType;
     }
diff --git a/ets2panda/test/ast/parser/ets/differentTypeCompare.ets b/ets2panda/test/ast/parser/ets/differentTypeCompare.ets
index bc6decc66b7e172aae0973d508da4e90b5cf8900..a753a52689177eb37fdcdd4c679734e8b7c4e50f 100644
--- a/ets2panda/test/ast/parser/ets/differentTypeCompare.ets
+++ b/ets2panda/test/ast/parser/ets/differentTypeCompare.ets
@@ -23,4 +23,4 @@ function main(): void {
     foo(/* @@ label */a == b);
 }
 
-/* @@@ label Error TypeError: Operator '==' cannot be applied to types 'String' and 'Int'.  */
+/* @@@ label Error TypeError: Operator '==' cannot be applied to types '"alma"' and 'Int'.  */