[AST] Preserve type sugar for generic typealiases #15416
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DougGregor
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Introduce a new Type node, BoundNameAliasType, which describes a reference to a typealias that requires substitutions to produce the underlying type. This new type node is used both for references to generic typealiases and for references to (non-generic) typealiases that occur within generic contexts, e.g., Array<Int>.Element. At present, the new type node is mainly useful in preserving type sugar for diagnostics purposes, as well as being reflected in other tools (indexing, code completion, etc.). The intent is to completely replace NameAliasType in the future.
… contexts Currently, when we reference a (non-generic) typealias within a generic context, we would completely lose type sugar for the typealias, replacing it with the underlying type. Instead, use BoundNameAliasType for this purpose, which allows us to maintain all of the type sugar as well as storing complete substitutions for later use.
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Introduce a new
Typenode,BoundNameAliasType, which describes a reference to atypealiasthat requires substitutions to produce the underlying type. This new type node is used both for references to generic typealiases and for references to (non-generic) typealiases that occur within generic contexts, e.g.,Array<Int>.Element.At present, the new type node is mainly useful in preserving type sugar for diagnostics purposes, as well as being reflected in other tools (indexing, code completion, etc.). The intent is to completely replace
NameAliasTypein the future.