Partial indexer with an IndexerName attribute causes a crash in compiler

[AlekseyTs]

https://sharplab.io/#v2:EYLgtghglgdgNAFxFANnAJiA1AHwAIAMABHgIwB0ASgK4wJRgCm5AwgPZgAOqjATgMp8AblADGjAM4BuALAAoeZwi96EFEQkJe1UQiL9S8gN7yiZkgGYiSlVDVFYehAAsoEgNqOiADwC6RIwBzRgQAXgA+bykAX3lTc3cASRh0Rm8+ADkIJgAKACIAWQBPLKY8gEpfeLM8KxtVdS8XN086H38gkJj5WLkgA= :

using System.Runtime.CompilerServices;

partial struct S1
{
    public partial int this[int x] {get=>x;}

    [IndexerName("MyName")]
    public partial int this[int x] {get;}
}

Or https://sharplab.io/#v2:EYLgtghglgdgNAFxFANnAJiA1AHwAIAMABHgIwB0ASgK4wJRgCm5AwgPZgAOqjATgMp8AblADGjAM4BuALAAoeZwi96EFEQkJe1UQiL9S8gN7yiZogG0AkjHSMAHnwByEJgAoARAFkAni6YeAJQAuqbmeADMREoqUGpEsHoIABZQEhaJRPbBREYA5owIALwAfPZSAL7yYWaR0cqq6pkpaRl0WTn5hZXyVXJAA=== :

using System.Runtime.CompilerServices;

partial struct S1
{
    [IndexerName("MyName")]
    public partial int this[int x] {get=>x;}

    public partial int this[int x] {get;}
}

Observed:

System.ArgumentException: adding a duplicate (Parameter 'key')
   at Roslyn.Utilities.ConcurrentDictionaryExtensions.Add[K,V](ConcurrentDictionary`2 dict, K key, V value) in /_/src/Compilers/Core/Portable/InternalUtilities/ConcurrentDictionaryExtensions.cs:line 24
   at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileMethod(MethodSymbol methodSymbol, Int32 methodOrdinal, ProcessedFieldInitializers& processedInitializers, SynthesizedSubmissionFields previousSubmissionFields, TypeCompilationState compilationState) in /_/src/Compilers/CSharp/Portable/Compiler/MethodCompiler.cs:line 1343
   at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileNamedType(NamedTypeSymbol containingType) in /_/src/Compilers/CSharp/Portable/Compiler/MethodCompiler.cs:line 537
   at Microsoft.CodeAnalysis.CSharp.MethodCompiler.<>c__DisplayClass25_0.<CompileNamedTypeAsync>b__0() in /_/src/Compilers/CSharp/Portable/Compiler/MethodCompiler.cs:line 439
   at Roslyn.Utilities.UICultureUtilities.<>c__DisplayClass5_0.<WithCurrentUICulture>b__0() in /_/src/Compilers/Core/Portable/InternalUtilities/UICultureUtilities.cs:line 139
   at System.Threading.Tasks.Task.InnerInvoke()
   at System.Threading.Tasks.Task.<>c.<.cctor>b__292_0(Object obj)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location ---
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot, Thread threadPoolThread)
--- End of stack trace from previous location ---
   at Microsoft.CodeAnalysis.CSharp.MethodCompiler.CompileMethodBodies(CSharpCompilation compilation, PEModuleBuilder moduleBeingBuiltOpt, Boolean emittingPdb, Boolean hasDeclarationErrors, Boolean emitMethodBodies, BindingDiagnosticBag diagnostics, Predicate`1 filterOpt, CancellationToken cancellationToken) in /_/src/Compilers/CSharp/Portable/Compiler/MethodCompiler.cs:line 160
   at Microsoft.CodeAnalysis.Compilation.Emit(Stream peStream, Stream metadataPEStream, Stream pdbStream, Stream xmlDocumentationStream, Stream win32Resources, IEnumerable`1 manifestResources, EmitOptions options, IMethodSymbol debugEntryPoint, Stream sourceLinkStream, IEnumerable`1 embeddedTexts, RebuildData rebuildData, CompilationTestData testData, CancellationToken cancellationToken) in /_/src/Compilers/Core/Portable/Compilation/Compilation.cs:line 2970

[AlekseyTs]

#76871 fixes up the partial indexer merge process to a degree that the crash no longer occurs. However, I think a follow up (at least in the form of more targeted testing) is still necessary. For example we need to make sure both parts are consistent in terms of name related information, etc.

[AlekseyTs]

For example, the following test reflects the current behavior (wrong indexer name is emitted to metadata):

        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/76842")]
        public void IndexerWithName_02()
        {
            var source = """
                using System.Runtime.CompilerServices;

                partial struct S1
                {
                    [IndexerName("MyName")]
                    public partial int this[int x] {get=>x;}

                    public partial int this[int x] {get;}
                }
                """;

            var comp = CreateCompilation(source);

            // Note, the indexer name in metadata is "Item", expected "MyName"
            CompileAndVerify(comp).VerifyDiagnostics().VerifyTypeIL("S1",
"""
.class private sequential ansi sealed beforefieldinit S1
    extends [mscorlib]System.ValueType
{
    .custom instance void [mscorlib]System.Reflection.DefaultMemberAttribute::.ctor(string) = (
        01 00 04 49 74 65 6d 00 00
    )
    .pack 0
    .size 1
    // Methods
    .method public hidebysig specialname 
        instance int32 get_Item (
            int32 x
        ) cil managed 
    {
        // Method begins at RVA 0x2067
        // Code size 2 (0x2)
        .maxstack 8
        IL_0000: ldarg.1
        IL_0001: ret
    } // end of method S1::get_Item
    // Properties
    .property instance int32 Item(
        int32 x
    )
    {
        .get instance int32 S1::get_Item(int32)
    }
} // end of class S1
""".Replace("[mscorlib]", ExecutionConditionUtil.IsMonoOrCoreClr ? "[netstandard]" : "[mscorlib]"));
        }