Cannot read attribute arguments from metadata when the original definition parameter is of a type parameter type

[RikkiGibson]

Version Used: features/generic-attributes

Steps to Reproduce:

roslyn/src/Compilers/CSharp/Test/Emit/Attributes/AttributeTests.cs

Lines 10094 to 10140 in 513ed88

	[Fact, WorkItem(55190, "https://github.com/dotnet/roslyn/issues/55190")]
	public void GenericAttributeParameter_01()
	{
	var source = @"
	using System;
	class Attr<T> : Attribute { public Attr(T param) { } }
	[Attr<string>(""a"")]
	class Program { }
	";
	var verifier = CompileAndVerify(source, sourceSymbolValidator: verify, symbolValidator: verifyMetadata);
	verifier.VerifyTypeIL("Program", @"
	.class private auto ansi beforefieldinit Program
	extends [netstandard]System.Object
	{
	.custom instance void class Attr`1<string>::.ctor(!0) = (
	01 00 01 61 00 00
	)
	// Methods
	.method public hidebysig specialname rtspecialname
	instance void .ctor () cil managed
	{
	// Method begins at RVA 0x2058
	// Code size 7 (0x7)
	.maxstack 8
	IL_0000: ldarg.0
	IL_0001: call instance void [netstandard]System.Object::.ctor()
	IL_0006: ret
	} // end of method Program::.ctor
	} // end of class Program
	");
	void verify(ModuleSymbol module)
	{
	var program = module.GlobalNamespace.GetMember<TypeSymbol>("Program");
	var attrs = program.GetAttributes();
	Assert.Equal(new[] { "Attr<System.String>(\"a\")" }, GetAttributeStrings(attrs));
	}
	void verifyMetadata(ModuleSymbol module)
	{
	var program = module.GlobalNamespace.GetMember<TypeSymbol>("Program");
	var attrs = program.GetAttributes();
	Assert.Equal(new[] { "Attr<System.String>" }, GetAttributeStrings(attrs));
	}
	}

Expected Behavior: If we reference Program from metadata and get a symbol for it, we should be able to GetAttributes() and find an attribute with an argument "a" on it.

Actual Behavior: We get an attribute with no arguments.

Related to #36285

[RikkiGibson]

There are really two issues here:

1. (blocking stable release of the feature) reflection at runtime doesn't work in some cases. Fixed in Cannot reflect on generic attributes whose constructors original definition uses type parameters runtime#56492
2. (not blocking) reading attribute arguments from metadata in the compiler doesn't work.

For (2) I think it is fine to just fix down the line because obtaining arguments for user-defined attributes from metadata isn't that common of a scenario. For example, analyzers which are interested in specific attributes are only interested in usages of that attribute in source.

For (1) it seems like a severe enough issue to block stable release of the feature. However, if we can confirm both of the following, it may be reasonable to ship it under the Preview LangVersion:

1. The compiler is emitting correct metadata in this scenario
2. The underlying issue which prevents reflection and reading with MetadataReader will be addressed.

@jcouv

[RikkiGibson]

I tried running a relevant unit test under a nightly .NET 6 runtime and I didn't see issue (2) fixed. I suspect there is still either a problem in System.Reflection.Metadata or the way we are using it.