Support generic type parameter when created with TypeBuilder

[steveharter]

Fixes #112155

There may be other related edge cases related to "signature types"; this PR only fixes the case mentioned.

[jkotas]

Move up before Type et = t.GenericTypeArguments[0]; line and change to Assert.False.

[steveharter]

In this case, I wanted to make sure the new code that was added for IsValueType\IsEnum throws when the generic parameter is a signature type.

If I compare t with t.IsValueType that is just testing normal reflection and will return true for Span<sigtype> and false for the List<sigtype>.

[jkotas]

Type.MakeGenericSignatureType(typeof(List<>), typeof(int)).GetGenericArguments()[0] is going to be typeof(int), so this test does not seem to be actually testing IsValueType property on signature types.

Should this rather be something like:

yield return new object[] { Type.MakeGenericSignatureType(typeof(List<>), typeof(int)), false };
yield return new object[] { Type.MakeGenericSignatureType(typeof(KeyValuePair<,>), typeof(string), typeof(object)), true };

?

[steveharter]

Correct this is not verifying signature types but is verifying that when a non-signature type is used as a generic parameter then IsValueType works as expected. The test with GetGenericArguments() is done in MakeSignatureConstructedGenericType() as mentioned in the comment.

I'm not sure if your feedback here came before or after I pushed a new commit that changed this and added the comment.

[jkotas]

verifying that when a non-signature type is used as a generic parameter then IsValueType works as expected.

Would it be better to verify that GetGenericArguments() returns the exact same Type instances as was passed into MakeGenericSignatureType?

If we do that, we do not need to worry about testing individual properties of the types returned by GetGenericArguments. We can depend on that testing done elsewhere (e.g. as part of runtime Type testing)

[jkotas]

My higher level concerns in both this and the other comment are:

• Missing test coverage for the code introduced in this PR. For example, if SignatureConstructedGenericType.IsEnum implementation is changed to return hardcoded false, no test is going to fail. These test gaps should be fixed. They are kind of hard to see because they are muddied by the unnecessary tests.
• Unnecessary test coverage for Types that are not signature types. It would be best to delete most of it and assume that the non-signature Types are tested elsewhere. This is pre-existing problem in these tests. I would not mind if you delete tests from this file that are not relevant to testing of signature types.

[jkotas]

Dtto