IRGen: Fix miscompile when a generic parameter is fixed to a tuple containing a pack #81564
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…ntaining a pack
If you had something like:
struct G<T> {
func f<each U>(_: repeat each U) where T == (repeat each U) {}
}
We would fulfill 'each U' from the metadata for 'G<(repeat each U)>',
by taking apart the tuple metadata for `(repeat each U)` and forming
a pack.
However this code path was only intended to kick in for a tuple
conformance witness thunk. In the general case, this optimization
is not correct, because if 'each U' is substituted with a
one-element pack, the generic argument of `G<(repeat each U)>` is
just that one element's metadata, and not a tuple. In fact, we
cannot distinguish the one-element tuple case, because the wrapped
element may itself be a tuple.
The fix is to just split off FulfillmentMap::searchTupleTypeMetadata()
from searchTypeMetadata(), and only call the former when we're in
the specific situation that requires it.
- Fixes swiftlang#78191.
- Fixes rdar://problem/135325886.
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May 22, 2025
…-78191" This reverts commit 490edfa.
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If you had something like:
We would fulfill 'each U' from the metadata for 'G<(repeat each U)>', by taking apart the tuple metadata for
(repeat each U)and forming a pack.However this code path was only intended to kick in for a tuple conformance witness thunk. In the general case, this optimization is not correct, because if 'each U' is substituted with a one-element pack, the generic argument of
G<(repeat each U)>is just that one element's metadata, and not a tuple. In fact, we cannot distinguish the one-element tuple case, because the wrapped element may itself be a tuple.The fix is to just split off FulfillmentMap::searchTupleTypeMetadata() from searchTypeMetadata(), and only call the former when we're in the specific situation that requires it.