dotnet/roslyn

ref local gives bad error when inited with ternary initializer

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#22,048 opened on Sep 11, 2017

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Area-CompilersConcept-Diagnostic ClarityFeature - Readonly ReferencesLanguage-C#Testhelp wanted

Description

Version Used: 2.3.1.61919 (57c81319) - on OSX with mono 5.4.0.135

Steps to Reproduce:

Compile this:

using System;

class Driver {
	static int X;
	static void Main () {
		ref int x = X == 0 ? ref X : ref X;
	}
}

Expected Behavior:

It should either compile or give a clear error message that what I'm doing is wrong

Actual Behavior:

Microsoft (R) Visual C# Compiler version 2.3.1.61919 (57c81319)
Copyright (C) Microsoft Corporation. All rights reserved.

ref2.cs(6,24): error CS1525: Invalid expression term 'ref'
ref2.cs(6,24): error CS1003: Syntax error, ':' expected
ref2.cs(6,24): error CS1002: ; expected
ref2.cs(6,30): error CS1001: Identifier expected
ref2.cs(6,30): error CS1002: ; expected
ref2.cs(6,30): error CS1513: } expected
ref2.cs(6,37): error CS1001: Identifier expected

I tried all combinations of adding/removing ref and parens and nothing worked. The only one that gave me something good was ref int x = ref (X == 0 ? X : X); with: error CS1510: A ref or out value must be an assignable variable

Contributor guide

Tech stack
csharp
Domain
developer experience
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Familiarity with C# ref localsBasic understanding of Roslyn's syntax parserKnowledge of error generation in the compiler
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35
Research direction
Investigate how the Roslyn parser handles ternary conditional expressions involving ref locals. Start by looking at the syntax parser for conditional expressions (likely in SyntaxParser.cs) and the error generation logic for invalid expression terms. Check if there are any existing tests for similar cases. The issue reporter suggests that 'ref int x = ref (X == 0 ? X : X);' gives a better error, so the fix may involve improving the parser's recognition of 'ref' in ternary initializers.