[release/8.0] Handle binding to initialized abstract members #93120
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Backport of #93090 to release/8.0
/cc @ericstj
Customer Impact
Cannot bind to abstract types when using the ConfigurationBinder source generator.
When binding to types which contain pre-initialized abstract members the ConfigurationBinder source generator will throw an exception, whereas the reflection-based binder will not. The generator also emits a diagnostic in this case.
We keep the diagnostic because the source generator cannot guarantee that the member will be pre-initialized, but the generator will emit code that uses the pre-initialized value for binding if present.
Testing
Tested real world application to confirm the fix. Added unit tests to cover new behavior.
Risk
There is still a discrepancy with the runtime binder, which is able to see the runtime type. I've filed #93130 to track that discrepancy. TLDR: we bind against the reference type only and not runtime type.