dotnet/roslyn

Internal error in the C# compiler when using "is" operator in VS 2019 watch window

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#50,867 opened on Jan 28, 2021

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Area-InteractiveBugInteractive-Debugginghelp wanted

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Description

Version Used: .net 5.0 , VS 16.8.4

Steps to Reproduce: object ob = "Baseball"; var istype = ob is int dc ? "game" : "nothing";

  1. Hit the breakpoint at var istype
  2. Add "ob is int dc" to watch widow. It throws the following exception: ob is int dc | error CS0136: A local or parameter named 'dc' cannot be declared in this scope because that name is used in an enclosing local scope to define a local or parameter |   (I don't know why)
  3. In the same watch window, replace dc with any other token, say, marvel (ob is int marvel). This shows "Internal error in c# compiler" (you might have to hit enter twice)

Expected Behavior: show false. Actual Behavior: Shows above to errors.

Contributor guide

Research direction
Reproduce the issue in Visual Studio and trace the compiler's behavior during watch expression evaluation. Examine the 'is' pattern handling in the Roslyn Expression Evaluator (Microsoft.CodeAnalysis.CSharp.ExpressionEvaluator) to identify the source of the internal error.
Tech stack
csharp
Domain
compilers
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.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
GitC#
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
35