dotnet/roslyn

EE: "Internal error in C# compiler" reported for typeof(T?)

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#47,648 opened on Sep 12, 2020

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Area-InteractiveBugFeature - Nullable Type ParametersInteractive-Debugginghelp wanted

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Description

Debug the following:

#nullable enable

class Program
{
    static void F<T, U>(T t, U u)
        where T : class
    {
        System.Diagnostics.Debugger.Break();
    }

    static void Main(string[] args)
    {
        F("", "");
    }
}

At the breakpoint, evaluate either typeof(T?) or typeof(U?) in the EE.

Result: Internal error in the C# compiler

Contributor guide

Research direction
Examine the Expression Evaluator (EE) code path for nullable type resolution. Look for the handling of typeof(T?) where T is a type parameter. Identify where the internal error is thrown and trace the logic to understand why the compiler fails.
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.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
C#compiler internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25