Update unreducible match types error reporting #19954

This reverts commit 9ae1598 Note that the changes in Typer: ``` val unsimplifiedType = result.tpe simplify(result, pt, locked) result.tpe.stripTypeVar match case e: ErrorType if !unsimplifiedType.isErroneous => errorTree(xtree, e.msg, xtree.srcPos) case _ => result ``` cannot be reverted yet since the MatchReducer now also reduces to an `ErrorType` for MatchTypeLegacyPatterns, introduced after 9ae1598.

i18488.scala was only passing because of the bug in the MatchReducer, as we can see in the subtyping trace: ``` ==> isSubType TableQuery[BaseCrudRepository.this.EntityTable] <:< Query[BaseCrudRepository.this.EntityTable, E[Option]]? ==> isSubType Query[BaseCrudRepository.this.EntityTable, Extract[BaseCrudRepository.this.EntityTable]] <:< Query[BaseCrudRepository.this.EntityTable, E[Option]] (left is approximated)? ==> isSubType E[Option] <:< Extract[BaseCrudRepository.this.EntityTable]? ==> isSubType [T[_$1]] =>> Any <:< Extract? ==> isSubType Any <:< Extract[T]? ==> isSubType Any <:< T match { case AbstractTable[t] => t } <: t (right is approximated)? ==> isSubType Any <:< <error Match type reduction failed since selector T matches none of the cases case AbstractTable[t] => t> (right is approximated)? <== isSubType Any <:< <error Match type reduction failed since selector T matches none of the cases case AbstractTable[t] => t> (right is approximated) = true <== isSubType Any <:< T match { case AbstractTable[t] => t } <: t (right is approximated) = true <== isSubType Any <:< Extract[T] = true <== isSubType [T[_$1]] =>> Any <:< Extract = true ... <== isSubType Extract[BaseCrudRepository.this.EntityTable] <:< E[Option] = true <== isSubType Query[BaseCrudRepository.this.EntityTable, Extract[BaseCrudRepository.this.EntityTable]] <:< Query[BaseCrudRepository.this.EntityTable, E[Option]] (left is approximated) = true <== isSubType TableQuery[BaseCrudRepository.this.EntityTable] <:< Query[BaseCrudRepository.this.EntityTable, E[Option]] = true ```

Modify the MatchReducer to return NoType in the case of no matches, rather than throwing a MatchTypeReductionError. This makes it consistent with the other match type reduction failures, where being stuck does not result in an error, but simply in an unreduced match type. We still get the explanations of the underlying error in the MatchTypeTrace, but in positions which need the reduction for conformance, rather than at application site of the match type.

The diff in neg/10349.scala is quite interesting. With a few intermediate values: ```scala type First[X] = X match case Map[_, v] => First[Option[v]] def first[X](x: X): First[X] = x match case x: Map[k, v] => val hdOpt: Option[v] = x.values.headOption first(hdOpt): First[Option[v]] // error only before changes ``` This now type-checks but will fail at runtime because of the in-exhaustivity of the match expression. Perhaps we should add some additional condition in `isMatchTypeShaped` to account for this, or at least emit a warning ?

`recurseWith` can be called with the same scrutinee (even if match type reduction is cached) if it is an applied match alias For example, `Tuple.Head[Tuple.Tail[T]]` will attempt to reduce `Tuple.Tail[T]` twice: - once as an argument of the match alias `Head`, and - once as a scrutinee in body of `Head` (after the substitution).

If a match type pattern is an opaque type, we use its bounds when checking the validity of the pattern. Following the ElimOpaque phase however, the pattern is beta-reduced (as normal applied type aliases), which may result in an illegal pattern.

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Update unreducible match types error reporting #19954

Update unreducible match types error reporting #19954

Commits on Mar 26, 2024