GDScript: add variable shadowing warning

(cherry picked from commit 14078fbb826811ca77e0eef3684f2e5d6f55a6b5)

modules/gdscript/gdscript.cpp

@@ -1945,6 +1945,10 @@ String GDScriptWarning::get_message() const {
 			CHECK_SYMBOLS(1);
 			return "The local variable '" + symbols[0] + "' is declared but never used in the block.";
 		} break;
+		case SHADOWED_VARIABLE: {
+			CHECK_SYMBOLS(2);
+			return "The local variable '" + symbols[0] + "' is shadowing an already defined variable at line " + symbols[1] + ".";
+		} break;
 		case UNUSED_CLASS_VARIABLE: {
 			CHECK_SYMBOLS(1);
 			return "The class variable '" + symbols[0] + "' is declared but never used in the script.";
@@ -2048,6 +2052,7 @@ String GDScriptWarning::get_name_from_code(Code p_code) {
 		"UNASSIGNED_VARIABLE",
 		"UNASSIGNED_VARIABLE_OP_ASSIGN",
 		"UNUSED_VARIABLE",
+		"SHADOWED_VARIABLE",
 		"UNUSED_CLASS_VARIABLE",
 		"UNUSED_ARGUMENT",
 		"UNREACHABLE_CODE",

modules/gdscript/gdscript.h

@@ -273,6 +273,7 @@ struct GDScriptWarning {
 		UNASSIGNED_VARIABLE, // Variable used but never assigned
 		UNASSIGNED_VARIABLE_OP_ASSIGN, // Variable never assigned but used in an assignment operation (+=, *=, etc)
 		UNUSED_VARIABLE, // Local variable is declared but never used
+		SHADOWED_VARIABLE, // Variable name shadowed by other variable
 		UNUSED_CLASS_VARIABLE, // Class variable is declared but never used in the file
 		UNUSED_ARGUMENT, // Function argument is never used
 		UNREACHABLE_CODE, // Code after a return statement

modules/gdscript/gdscript_parser.cpp

@@ -7661,6 +7661,11 @@ void GDScriptParser::_check_function_types(FunctionNode *p_function) {
 		if (p_function->arguments_usage[i] == 0 && !p_function->arguments[i].operator String().begins_with("_")) {
 			_add_warning(GDScriptWarning::UNUSED_ARGUMENT, p_function->line, p_function->name, p_function->arguments[i].operator String());
 		}
+		for (int j = 0; j < current_class->variables.size(); j++) {
+			if (current_class->variables[j].identifier == p_function->arguments[i]) {
+				_add_warning(GDScriptWarning::SHADOWED_VARIABLE, p_function->line, p_function->arguments[i], itos(current_class->variables[j].line));
+			}
+		}
 #endif // DEBUG_ENABLED
 	}
 
@@ -7734,6 +7739,17 @@ void GDScriptParser::_check_function_types(FunctionNode *p_function) {
 		p_function->return_type.has_type = false;
 		p_function->return_type.may_yield = true;
 	}
+
+#ifdef DEBUG_ENABLED
+	for (Map<StringName, LocalVarNode *>::Element *E = p_function->body->variables.front(); E; E = E->next()) {
+		LocalVarNode *lv = E->get();
+		for (int i = 0; i < current_class->variables.size(); i++) {
+			if (current_class->variables[i].identifier == lv->name) {
+				_add_warning(GDScriptWarning::SHADOWED_VARIABLE, lv->line, lv->name, itos(current_class->variables[i].line));
+			}
+		}
+	}
+#endif // DEBUG_ENABLED
 }
 
 void GDScriptParser::_check_class_blocks_types(ClassNode *p_class) {