Cleanup inheritance clause parsing #11781
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We had two slightly different codepaths to diagnose ': class' in an inheritance clause where it is not supported. For generic parameters, we would fix the 'class' to 'AnyObject', but for associated types we didn't do this. Perform the fix in all cases where it makes sense and remove one of the two diagnostics.
We allowed them for generic parameter inheritance clauses but not anywhere else. While arguably this has stylistic benefits, the restriction was not enforced consistently and was mostly a result of implementation limitations. Lift the restriction and fix things up where needed to make them work. This brings us closer to allowing protocols to constrain the 'Self' type to a subclass of a class by listing the class in the protocol's inheritance clause, which was a feature from SE-0156, but this doesn't quite work. Fixes <https://bugs.swift.org/browse/SR-4678> and <rdar://problem/31785092>.
We can just use parseType() everywhere instead. We already check for non-identifier types in inheritance clauses elsewhere, and indeed we have to anyway because an identifier type might resolve to a type alias whose underlying type is a non-nominal type. It doesn't look like this change made any diagnostics worse, but if we find a case where it did, we could revert it.
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In order to fully implement SE-0156, we need to implement two additional language changes:
Also, we have some code duplication and recursive validation bugs in Sema's inheritance clause checking logic. Cleaning this up will make the code clearer and fix various crashes.
This PR doesn't fully address any of the above, but it cleans up the code in preparation for making those changes. I don't intend on working on this any time soon, but I wanted to finish this cleanup as a quick follow-up to #11762.