Better error messages when writing to expressions that cannot be written to.

libsolidity/ast/AST.cpp

@@ -432,8 +432,13 @@ string Scopable::sourceUnitName() const
 
 bool VariableDeclaration::isLValue() const
 {
-	// External function parameters and constant declared variables are Read-Only
-	return !isExternalCallableParameter() && !m_isConstant;
+	// Constant declared variables are Read-Only
+	if (m_isConstant)
+		return false;
+	// External function arguments of reference type are Read-Only
+	if (isExternalCallableParameter() && dynamic_cast<ReferenceType const*>(type()))
+		return false;
+	return true;
 }
 
 bool VariableDeclaration::isLocalVariable() const