IRGen: Workaround for type substitution limitation #76486
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If a constrained extension has fewer conformance requirements than the nominal type declaration, because some of the type parameters of the nominal type are fixed to concrete types by the extension, we would run into trouble because interface type substitution does not correctly handle this case. Applying an identity substitution map to an interface type does not look up concrete types in the output generic signature, so we get back a type parameter that is not valid. getReducedType() has a hack to deal with this. I'd like to get rid of the hack and fix interface type substitution to do this correctly, but until then, this will do. Fixes swiftlang#76479
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If a constrained extension has fewer conformance requirements than the nominal type declaration, because some of the type parameters of the nominal type are fixed to concrete types by the extension, we would run into trouble because interface type substitution does not correctly handle this case.
Applying an identity substitution map to an interface type does not look up concrete types in the output generic signature, so we get back a type parameter that is not valid.
getReducedType() has a hack to deal with this. I'd like to get rid of the hack and fix interface type substitution to do this correctly, but until then, this will do.
Fixes #76479