Match types: fix wildcard captures handling to match intent.

As demonstrated by scala/scala3#19607,
picking `H` does *not* in fact always work. It was always the
intent of the spec to pick an instantiation that would make
subtyping work, but the particular choice was wrong.

IMO this is therefore clearly a spec bug, and we can address it
irrespective of compatibility concerns.

Author: sjrd

content/match-types-spec.md

@@ -253,8 +253,9 @@ At the top level, `variance = 1` and `scrutIsWidenedAbstract = false`.
 * If `P` is a `TypeWithoutCapture`:
   * Do nothing (always succeed).
 * If `P` is a `WildcardCapture` `ti = _`:
-  * If `X` is of the form `_ >: L <: H`, instantiate `ti := H` (anything between `L` and `H` would work here),
-  * Otherwise, instantiate `ti := X`.
+  * Instantiate `ti` so that the subtype test in Step (2) above always succeeds:
+    * If `X` is of the form `_ >: L <: H`, instantiate `ti := H` (resp. `L`, `X`) if `variance = 1` (resp. `-1`, `0`).
+    * Otherwise, instantiate `ti := X`.
 * If `P` is a `TypeCapture` `ti`:
   * If `X` is of the form `_ >: L <: H`,
     * If `scrutIsWidenedAbstract` is `true`, fail as not specific.