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.
The subexpression of a MaterializePackExpr (which is always a tuple value
currently) is emitted while preparing to emit a pack expansion expr, and its
elements are projected from within the dynamic pack loop. This means that a
materialized pack is only evaluated once, rather than being evaluated on
every iteration over the pack elements.
