[release/9.0] Fix Enum field type bug found when underlying type is set from assembly loaded with MLC

[github-actions]

Backport of #106375 to release/9.0

/cc @buyaa-n

Customer Impact

• Customer reported
• Found internally

This is found while fixing a customer reported bug. When MLC used for setting the core assembly and a type loaded from that assembly used as underlying type when creating EnumBuilder, defining a field for that EnumBuilder would create a field with wrong type (The field type will be generated with underlying type instead of the EnumBuilder type). Plus this PR removes the validation that causing issue for setting constants for such field.

Root cause:

The EnumBuilder.UnderlyingSystemType, currently returns GetEnumUnderlyingType() which returns _underlyingField.FieldType underneath. Because Type.Equals checks UnderlyingSystemType for equality when the EnumBuilder underlying type is set from MLC Core assembly it is causing issue for enum fields defined, for example:

Type intType = mlc.CoreAssembly.GetType("System.Int32");
EnumBuilder enumBuilder = mb.DefineEnum("TestEnum", TypeAttributes.Public, intType);
FieldBuilder field = enumBuilder.DefineLiteral("Default", 0);

In this case the field type should be TestEnum, but it is evaluated to be equal to System.Int32 when writing the field signature on Save. We should not return underlying field type for EnumBuilder.UnderlyingSystemType, it should return the Enum itself instead (runtime enums returns the enum type itself). Though the EnumBuilder.GetEnumUnderlyingType() method should keep returning the underlying field type. Related to #105903

Regression

• Yes
• No

It is a bug in a new PersistedAssemblyBuilder added in .NET 9

Testing

A unit test added that reproes the issue

Risk

Low, the fix is straight forward and clean, will not cause a regression

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[carlossanlop]

We have a very short window for merging RC1 backports. Please send the email to Tactics requesting approval ASAP.

[buyaa-n]

We have a very short window for merging RC1 backports. Please send the email to Tactics requesting approval ASAP.

@carlossanlop this is targeting to RC2, do we still need Tactics approval?

[carlossanlop]

Ah my bad, I didn't notice the target branch.

RC2 (release/9.0) PRs need M2 approval. @artl93 can you please take a look?

[artl93]

Approved for me.