Fix Issue #68212: Preserve type specialization in typealias extensions #85380
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…extensions When extending a partially specialized generic typealias (e.g., 'typealias IntField<Tag> = Field<Tag, Int>'), the compiler was losing type specialization information and returning only the base nominal type (Field<Tag, Value>), causing type checker errors when trying to use specialized type information in the extension body. This fix ensures that we return the typealias type itself, which properly preserves the partial specialization constraints. The previous logic used isPassThroughTypealias() to decide between returning extendedType or extendedNominal->getDeclaredType(). When the latter was returned, it lost the specialization information (e.g., that Value == Int in IntField<Tag>). By always returning extendedType for typealias extensions, we preserve the full type information and allow the type checker to correctly resolve types in the extension body. Fixes: swiftlang#68212
DanilKoshnarev
requested review from
hborla,
slavapestov and
xedin
as code owners
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When extending a partially specialized generic typealias (e.g., 'typealias IntField = Field<Tag, Int>'), the compiler was losing type specialization information and returning only the base nominal type (Field<Tag, Value>), causing type checker errors when trying to use specialized type information in the extension body.
This fix ensures that we return the typealias type itself, which properly preserves the partial specialization constraints.
The previous logic used isPassThroughTypealias() to decide between returning extendedType or extendedNominal->getDeclaredType(). When the latter was returned, it lost the specialization information (e.g., that Value == Int in IntField).
By always returning extendedType for typealias extensions, we preserve the full type information and allow the type checker to correctly resolve types in the extension body.
Fixes: #68212