[6.2] IRGen: Fix miscompile when a generic parameter is fixed to a tuple containing a pack #81565
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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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AnthonyLatsis
approved these changes
May 20, 2025
hborla
approved these changes
May 20, 2025
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6.2 cherry-pick of #68130.
Description: Fixes a miscompile if you fix a generic parameter of a nominal type to a tuple containing a pack expansion:
If you then call f() with a one-element pack, we would crash at runtime.
Origination: Regression was introduced with this change in Swift 5.10: IRGen: Fulfill pack metadata from tuple metadata #68130
Risk: Low. This does change the calling convention, but not the mangling, in this specific scenario where the old calling convention passed the wrong information. This change reverts to the Swift 5.9 calling convention, since the specific pattern was only intended to kick in with tuple conformance witness thunks.
Reviewed by: TBD