dotnet/roslyn

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

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#81666 opened on Dec 12, 2025

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Area-CompilersConcept-Diagnostic Clarityhelp wanted

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

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.
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
C# programmingRoslyn compiler basics
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60
Research direction
The issue presents a test case where an unexpected diagnostic (CS0131) is emitted in a `with` expression with an indexer on a struct. The expected behavior is that only CS0747 should be reported. Investigate the Roslyn source code for handling `with` expressions, specifically in the semantic model or binding. Look at the error reporting for indexer initializers. The test file can be added to the existing test suite. The fix may involve modifying the code that emits ERR AssgLvalueExpected to suppress it when the construct is within a `with` expression initializer that already has an error. Check recent commits related to `with` expressions for context.