dotnet/roslyn

'Use pattern-matching' should be missing on writes from local function captures

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#29,512 opened on Aug 24, 2018

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Area-IDEBughelp wanted

Description

From https://github.com/dotnet/roslyn/pull/26097#discussion_r210728805

We should look for writes that are logically within the span, but syntactically are not.

var s = field as string;
WriteField();
if (s != null) { ... }

void WriteField () => field = null;

/cc @sharwell

Contributor guide

Tech stack
csharp
Domain
tooling
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
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.
needs investigation
Prerequisites
C# compiler internalspattern matching semanticslocal function scoping
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20
Research direction
The issue references a discussion in PR #26097. The example shows a local function WriteField that writes to a captured variable 'field', but the pattern matching analysis incorrectly does not mark the write. Investigate the data flow analysis for captured variables in local functions. Look at the compiler's implementation of pattern matching and how it handles writes that are syntactically inside local functions but logically within the enclosing method's scope. The fix likely requires updating the analysis to consider writes from local function captures.