dotnet/roslyn

Shortcomings in computing diagnostics for extension methods

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#24,787 opened on Feb 12, 2018

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Area-CompilersBugConcept-Design DebtConcept-Diagnostic Clarityhelp wanted

Description

I was recently implementing support for dotnet/csharplang#98 and discovered a regression in the quality of diagnostic for the situation in test DelegatesFromInvalidOverloads.

It is caused by a combination of two shortcomings in the computation of diagnostics. First, in BindExtensionMethod when we fail to find an applicable extension method, we only report a diagnostic for the first extension method group that failed, even if some other extension method group contains a much better candidate. In the case of this test the first extension method group contains a method with the wrong number of parameters, while the second one has an extension method that fails only because of its return type mismatch.

Second, in OverloadResolutionResult<TMember>.ReportDiagnostics<T>, we do not report a diagnostic for the failure MemberResolutionKind.NoCorrespondingParameter, leaving it to the caller to notice that we failed to produce a diagnostic (the caller has to grub through the diagnostic bag to see that there is no error there) and then the caller has to produce a generic error message, which we see below. It does not appear that all callers have that test, though, suggesting there may be a latent bug of missing diagnostics.

Contributor guide

Tech stack
csharp
Domain
backendtooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
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.
needs investigation
Prerequisites
C# knowledgecompiler internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25
Research direction
Investigate the methods BindExtensionMethod and OverloadResolutionResult<TMember>.ReportDiagnostics<T> in the Roslyn source code. Understand how diagnostics are currently reported for extension method groups, especially the logic that selects which diagnostic to emit. The issue mentions that in BindExtensionMethod, only the first failing group is considered, and in ReportDiagnostics, NoCorrespondingParameter failure is not reported, causing missing diagnostics. Look for test cases like DelegatesFromInvalidOverloads to reproduce the regression. The fix should ensure all extension method groups are evaluated for the best candidate and that NoCorrespondingParameter yields a diagnostic. The maintainer's comment about callers having to check for missing diagnostics suggests a potential latent bug that should also be addressed.