Slava Pestov f17baefd7a IRGen: Fix miscompile when a generic parameter is fixed to a tuple containing 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 https://github.com/swiftlang/swift/issues/78191.
- Fixes rdar://problem/135325886.

2025-05-16 17:34:11 -04:00

180 KiB

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