Assert failing in LocalRewriter.CollectAndVisitConcatArguments

[jjonescz]

Version Used: de9d72e

Steps to Reproduce:

.NET Lab

[Fact]
public void CompoundAssignment()
{
    var source = """
        var c = new C();
        c.P += "a" + c.P;
        System.Console.WriteLine(c.P);

        class C
        {
            public string P { get; set; } = "x";
        }
        """;
    CompileAndVerify(source, expectedOutput: "xax").VerifyDiagnostics();
}

This assert fails:

roslyn/src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_StringConcat.cs

Lines 221 to 223 in d92fd99

	Debug.Assert(visitedCompoundAssignmentLeftRead is
	not (BoundCall or BoundConversion { ConversionKind: ConversionKind.Boxing, Type.SpecialType: SpecialType.System_Object, Operand.Type.SpecialType: SpecialType.System_Char })
	and { ConstantValueOpt: null });

The assert was introduced in #76955.

[jcouv]

FWIW, I hit this while building the runtime repo as well. Details in #80616 but the repro in OP seems better.

[jjonescz]

Building runtime is how I've discovered this too iirc