ilasm doesn't preserve "where T : struct", instead produces "where T : ValueType"

[dagood]

Description

I'm observing in source-build that our 5.0.0 ilasm has a regression when given this IL method signature:

.method /*06000757*/ public hidebysig static 
        int32  Compare<valuetype .ctor (System.ValueType/*020000C7*/) T>(
          valuetype System.Nullable`1/*02000089*/<!!T> n1,
          valuetype System.Nullable`1/*02000089*/<!!T> n2) cil managed

Viewing the result in ILSpy shows me these IL results. Red is "bad", with 5.0.0 ilasm. Green is "good", with old 3.1 ilasm.

 	.method public hidebysig static 
-		int32 Compare<(System.ValueType) T> (
+		int32 Compare<valuetype .ctor (System.ValueType) T> (
 			valuetype System.Nullable`1<!!T> n1,
 			valuetype System.Nullable`1<!!T> n2
 		) cil managed 

This issue looks to me like it's blocking the source-build 5.0.0 GA release.

The FSharp compiler seems to rely on the signature being correct, or else the build fails. With the C# decompilation result, it's easier to see why (for me 😄):

-		public static int Compare<T>(T? n1, T? n2) where T : ValueType
+		public static int Compare<T>(T? n1, T? n2) where T : struct

More details and more examples at dotnet/source-build#1920 (comment).

Full original IL file that may be able to repro: https://raw.githubusercontent.com/dotnet/source-build-reference-packages/ddfd9f5918bdaefc0d02db8ea1ae8a8b1ff11b23/src/targetPacks/ILsrc/netstandard.library/2.0.3/build/netstandard2.0/ref/netstandard.il (signature at line 28420)

(Edited to add in context:) Self-contained repro to run on linux-x64: https://github.com/dagood/repro/tree/ilasm-struct-vs-valuetype

@JulieLeeMSFT @briansull can you help with this?

/cc @dleeapho @dseefeld

Regression?

Seems like a regression from 3.1.0-ish ilasm to a 5.0.0 ilasm. This happened when we upgraded from 3.1.0 to 5.0.0 in dotnet/source-build-reference-packages: dotnet/source-build-reference-packages@fe6afa2...c52443b.

The 5.0.0 ilasm includes the recent patch to fix a different regression: #44850.

[briansull]

I will take a look

[briansull]

It is helpful to have a complete repro that is ready to try.

[dagood]

Here's a self-contained repro to run on linux-x64: https://github.com/dagood/repro/tree/ilasm-struct-vs-valuetype

[briansull]

I have a fix for this now

[dseefeld]

I added a patch for source-build and tested it locally. It fixes the issue. Patch included in source-build here: dotnet/source-build#1933