Ensure GetCorElementType returns underlying enum type on Mono runtime

[mathieubourgeois]

Type.GetTypeCode on Mono was calling GetCorElementType, which, when invoked on an enum nested in a generic class, would return TypeCode.Object instead of TypeCode.Int32. Ensuring we grab the underlying type of the enum fixes the issue. This would provoke Enum parsing to trigger an assert in a debug runtime and go through a slower, rare path for parsing as well.

Fixes #72543

[ghost]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[mathieubourgeois]

hmmm ok it failed some tests. Looks like IsPrimitive is not matching because it looks like an enum is not considered to be a primitive even though the underlying type actually is, but the documentation doesn't really mention anything about enum's underlying types...

Based on further analysis, I think we may have been wrong in our earlier assumptions : GetTypeCodeImpl calls GetCorElementType, and has the following case :

                case CorElementType.ELEMENT_TYPE_VALUETYPE:
                    if (ReferenceEquals(this, typeof(decimal)))
                        typeCode = TypeCode.Decimal;
                    else if (ReferenceEquals(this, typeof(DateTime)))
                        typeCode = TypeCode.DateTime;
                    else if (IsActualEnum)
                        typeCode = GetTypeCode(Enum.InternalGetUnderlyingType(this));
                    else
                        typeCode = TypeCode.Object;
                    break;

So, it looks like GetCorElementType is not supposed to return the underlying type for the enum, but is actually supposed to return VALUETYPE. I can try and change this to just do that directly in the case of an enum instead of returning the underlying type.

[lambdageek]

thanks!

[mathieubourgeois]

Pushed an adapted version of the code which is more limited and should (hopefully) pass testing this time around :) GetCorElementType will now return VALUETYPE instead of the underlying enum type, which seems to track more with what CoreClr is doing on their end.

Also moved the tests in System.Runtime.Tests since it's a more logical location for it in my opinion.

[mathieubourgeois]

I suppose the test failures (System.Net.Http.Functional.Tests.WorkItemExecution) are "expected", since they happen only on net7.0-Linux-Debug-x64-CoreCLR_release-Ubuntu.1804.Amd64.Open and net7.0-windows-Debug-x64-CoreCLR_release-Windows.81.Amd64.Open, and I only modified the Mono runtime. Those seem to be the only test failures afaik.

[lambdageek]

Yea, the test failures are unrelated.