Bad error message on const ref fields · dotnet/roslyn#17125

2017-02-14T00:03:46.000Z

**Valid code:** ```csharp static void Test(int x) { ref int y = ref x; } ``` **Invalid code:** ```csharp static void Test(int x) { const ref int y = ref x; } ``` **Expected:** an error message on `const` specifying that you cannot use `ref` and `const` modifiers together. **Found:** parser fails with the following errors: ![image](https://cloud.githubusercontent.com/assets/15987992/22908885/be566078-f205-11e6-9d7c-ce71740ecac7.png) 1. CS1525 Invalid expression term 'ref' 2. CS1002 ; expected 3. CS1001 Identifier expected 4. CS8172 Cannot initialize a by-reference variable with a value 5. CS0118 'x' is a variable but is used like a type 6. CS8174 A declaration of a by-reference variable must have an initializer

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Area-CompilersBugConcept-Diagnostic ClarityFeature - Ref Locals and Returnshelp wanted

Description

Valid code:

static void Test(int x)
{
    ref int y = ref x;
}

Invalid code:

static void Test(int x)
{
    const ref int y = ref x;
}

Expected: an error message on const specifying that you cannot use ref and const modifiers together.

Found: parser fails with the following errors:

CS1525 Invalid expression term 'ref'
CS1002 ; expected
CS1001 Identifier expected
CS8172 Cannot initialize a by-reference variable with a value
CS0118 'x' is a variable but is used like a type
CS8174 A declaration of a by-reference variable must have an initializer

Contributor guide

Tech stack: csharp
Domain: toolingdeveloper experience
Issue type: bug
Difficulty: 3
Estimated time: half day
Activity status: stale
Clarity: clear
Prerequisites: familiarity with C# compiler's parser
Newbie friendliness: 40
Research direction: The issue is about improving the error message when `const` and `ref` are used together on a local variable. Currently, the parser produces multiple misleading errors. The fix likely involves modifying the parser in the Roslyn repository to detect the invalid combination early and emit a single clear error. Examine the parser's handling of local variable declarations in files like `src/Compilers/CSharp/Portable/Parser/SyntaxParser.cs` or similar. Look for where `ref` and `const` modifiers are parsed and add a check for their mutual exclusivity.