GenAPI produces where T : ValueType given ref T return with no type constraint

[dagood]

• This issue is blocking
• This issue is causing unreasonable pain

GenAPI 1.0.0-beta.18619.2

In https://www.nuget.org/packages/System.Runtime.CompilerServices.Unsafe/5.0.0 lib/netcoreapp2.0/System.Runtime.CompilerServices.Unsafe.dll, we have this method (output from ILSpy):

// System.Runtime.CompilerServices.Unsafe
[MethodImpl(MethodImplOptions.AggressiveInlining)]
public static ref T Unbox<T>(object box)
{
	return ref (T)box;
}

.method public hidebysig static 
	!!T& Unbox<([System.Runtime]System.ValueType) T> (
		object 'box'
	) cil managed aggressiveinlining 
{
	.param constraint T, [System.Runtime]System.ValueType
		.custom instance void System.Runtime.Versioning.NonVersionableAttribute::.ctor() = (
			01 00 00 00
		)
	// Method begins at RVA 0x22b4
	// Code size 7 (0x7)
	.maxstack 1

	IL_0000: ldarg.0
	IL_0001: unbox !!T
	IL_0006: ret
} // end of method Unsafe::Unbox

GenAPI gives me this:

public static ref T Unbox<T>(object box) where T : System.ValueType { throw null; }
// ^ error CS0702: Constraint cannot be special class 'ValueType' 

In the package's ref/netstandard2.0/[...].dll, this API is actually generated like this:

// System.Runtime.CompilerServices.Unsafe
public static ref T Unbox<T>(object box) where T : struct
{
	throw null;
}

/cc @safern

[dagood]

Interesting that ilasm had a similar bug: dotnet/runtime#45183. Might not be any meaningful connection though.