fixup! Allow constants in custom storage layout expression

Author: matheusaaguiar

Diff for: libsolidity/analysis/PostTypeContractLevelChecker.cpp

@@ -118,6 +118,15 @@ void PostTypeContractLevelChecker::checkStorageLayoutSpecifier(ContractDefinitio
 	rational baseSlotRationalValue;
 	if (auto const integerType = dynamic_cast<IntegerType const*>(baseSlotExpressionType))
 	{
+		if (integerType->isSigned())
+		{
+			m_errorReporter.typeError(
+				1481_error,
+				baseSlotExpression.location(),
+				"The base slot expression must be a constant unsigned integer value."
+			);
+			return;
+		}
 		std::optional<ConstantEvaluator::TypedRational> typedRational = ConstantEvaluator::evaluate(m_errorReporter, baseSlotExpression);
 		if (!typedRational)
 		{