dotnet/roslyn

IDE0059 generates ambiguity errors for overloads with `out`

Open

#38,053 opened on Aug 16, 2019

View on GitHub
 (4 comments) (1 reaction) (0 assignees)C# (20,414 stars) (4,257 forks)batch import
Area-IDEBugFeature - IDE0059IDE-CodeStylehelp wanted

Description

Version Used: 3.3.0-beta3-19413-06+ac06df1bffb7b7fd0e5bf63cc91921af76b21e03

Steps to Reproduce:

class Program
{
    static void Main()
    {
        string s;
        Foo(out s);

        int i;
        Foo(out i);
    }

    static void Foo(out int blah) => blah = 0;
    static void Foo(out string blah) => blah = "";
}

Fix All for IDE0059.

Expected Behavior: IDE0059 shouldn't be offered, or if it is, the generated code should employ some disambiguation mechanism.

Actual Behavior:

class Program
{
    static void Main()
    {
        Foo(out _);
        Foo(out _);
    }

    static void Foo(out int blah) => blah = 0;
    static void Foo(out string blah) => blah = "";
}

which fails to compile:

1>Program.cs(5,9,5,12): error CS0121: The call is ambiguous between the following methods or properties: 'Program.Foo(out int)' and 'Program.Foo(out string)'
1>Program.cs(6,9,6,12): error CS0121: The call is ambiguous between the following methods or properties: 'Program.Foo(out int)' and 'Program.Foo(out string)'

Contributor guide

Tech stack
csharp
Domain
developer experiencetooling
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.
1-2 days
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
C#Roslyn analyzer basicsIDE0059 understanding
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30
Research direction
Investigate the IDE0059 analyzer and its code fix provider in the Roslyn repo. The issue is that when there are multiple overloads with 'out' parameters of different types, replacing the variable with a discard (' ') creates ambiguity. The fix must either avoid offering the fix in such cases or generate code that explicitly specifies the type, e.g., 'out int ' or 'out string '. Look at the code for 'UseDiscardDiagnosticAnalyzer' and 'UseDiscardCodeFixProvider' to understand the current logic and modify it to check for overload ambiguity before applying the fix.