dotnet/roslyn

Unexpected diagnostic in `with` expression with indexer on a struct

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#81,666 opened on Dec 12, 2025

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Area-CompilersConcept-Diagnostic Clarityhelp wanted

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Description

Although this is an error scenario, the first diagnostic is unexpected.

    [Fact]
    public void TODO2()
    {
        var src = """
var c = new S() with { [0] = 1 };

public struct S
{
    public int this[int i] { set { } }
}
""";

        CreateCompilation(src).VerifyEmitDiagnostics(
            // (1,24): error CS0131: The left-hand side of an assignment must be a variable, property or indexer
            // var c = new S() with { [0] = 1 };
            Diagnostic(ErrorCode.ERR_AssgLvalueExpected, "[0]").WithLocation(1, 24),
            // (1,24): error CS0747: Invalid initializer member declarator
            // var c = new S() with { [0] = 1 };
            Diagnostic(ErrorCode.ERR_InvalidInitializerElementInitializer, "[0] = 1").WithLocation(1, 24));
    }

Contributor guide

Research direction
Inspect the failing test and trace the compiler's handling of `with` expressions with indexers to find where the extra diagnostic is emitted.
Tech stack
csharp
Domain
backend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
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
C#Roslyn compiler basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25