dotnet/roslyn

Confusing error message for overloaded generic delegate-accepting method

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#8,603 opened on Feb 11, 2016

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 (1 comment) (1 reaction) (1 assignee)C# (20,414 stars) (4,257 forks)batch import
Area-CompilersBugConcept-Diagnostic Clarityhelp wanted

Description

I have the following code:

using System;

class Program
{
    static void Main()
    {
        M(x => { });
    }

    static void M(string s)
    {
    }

    static void M<T>(Action<T> a)
    {
    }
}

It produces the following error:

CS1660 Cannot convert lambda expression to type 'string' because it is not a delegate type

I think that a human can clearly see that the code is attempting to call the delegate-accepting overload, but is missing a type argument (M<int>(x => { });) or explicit parameter type (M((int x) => { });).

But the compiler shows error message that's related to the first (string-accepting) overload, which I think is confusing.

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.
blocked
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C# overload resolutionRoslyn diagnostics infrastructure
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60
Research direction
The issue describes a confusing error message CS1660 when calling an overloaded method with a lambda. The fix likely involves improving the overload resolution diagnostics in the Roslyn binder (e.g., Semantics/OverloadResolution). The issue is assigned (assignee not visible), so check the assignee's status before starting. Look at linked files in the compiler's error reporting for CS1660.