Fix #19607: Allow to instantiate *wildcard* type captures to TypeBounds. (#19627)

When matching in a match type, if we encounter a `TypeBounds` scrutinee
and we have a wildcard capture on the right, we used to pick the `hi`
bound "because anything between between `lo` and `hi` would work".

It turns out that *nothing* between `lo` and `hi` works when the type
constructor is invariant. Instead, we must be keep the type bounds, and
instantiate the wildcard capture to a wildcard type argument.

This is fine because a wildcard capture can never be referred to in the
body of the case. However, previously this could never happen in
successful cases, and we therefore used the presence of a `TypeBounds`
in the `instances` as the canonical signal for "fail as not specific".
We now use a separate `noInstances` list to be that signal.

This change departs from the letter of the spec but not from its spirit.
As evidenced by the wording, the spec always *intended* for "the pick"
to one that would always succeed. We wrongly assumed `hi` was always
working.

---

Companion PR to fix the spec/SIP:
scala/improvement-proposals#77

Author: odersky

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -3388,29 +3388,38 @@ class MatchReducer(initctx: Context) extends TypeComparer(initctx) {
       // Actual matching logic
 
       val instances = Array.fill[Type](spec.captureCount)(NoType)
+      val noInstances = mutable.ListBuffer.empty[(TypeName, TypeBounds)]
 
       def rec(pattern: MatchTypeCasePattern, scrut: Type, variance: Int, scrutIsWidenedAbstract: Boolean): Boolean =
         pattern match
-          case MatchTypeCasePattern.Capture(num, isWildcard) =>
+          case MatchTypeCasePattern.Capture(num, /* isWildcard = */ true) =>
+            // instantiate the wildcard in a way that the subtype test always succeeds
+            instances(num) = variance match
+              case 1  => scrut.hiBound // actually important if we are not in a class type constructor
+              case -1 => scrut.loBound
+              case 0  => scrut
+            !instances(num).isError
+
+          case MatchTypeCasePattern.Capture(num, /* isWildcard = */ false) =>
+            def failNotSpecific(bounds: TypeBounds): TypeBounds =
+              noInstances += spec.origMatchCase.paramNames(num) -> bounds
+              bounds
+
             instances(num) = scrut match
               case scrut: TypeBounds =>
-                if isWildcard then
-                  // anything will do, as long as it conforms to the bounds for the subsequent `scrut <:< instantiatedPat` test
-                  scrut.hi
-                else if scrutIsWidenedAbstract then
-                  // always keep the TypeBounds so that we can report the correct NoInstances
-                  scrut
+                if scrutIsWidenedAbstract then
+                  failNotSpecific(scrut)
                 else
                   variance match
                     case 1  => scrut.hi
                     case -1 => scrut.lo
-                    case 0  => scrut
+                    case 0  => failNotSpecific(scrut)
               case _ =>
-                if !isWildcard && scrutIsWidenedAbstract && variance != 0 then
-                  // force a TypeBounds to report the correct NoInstances
+                if scrutIsWidenedAbstract && variance != 0 then
+                  // fail as not specific
                   // the Nothing and Any bounds are used so that they are not displayed; not for themselves in particular
-                  if variance > 0 then TypeBounds(defn.NothingType, scrut)
-                  else TypeBounds(scrut, defn.AnyType)
+                  if variance > 0 then failNotSpecific(TypeBounds(defn.NothingType, scrut))
+                  else failNotSpecific(TypeBounds(scrut, defn.AnyType))
                 else
                   scrut
             !instances(num).isError
@@ -3486,12 +3495,8 @@ class MatchReducer(initctx: Context) extends TypeComparer(initctx) {
           MatchResult.Stuck
 
       if rec(spec.pattern, scrut, variance = 1, scrutIsWidenedAbstract = false) then
-        if instances.exists(_.isInstanceOf[TypeBounds]) then
-          MatchResult.NoInstance {
-            constrainedCaseLambda.paramNames.zip(instances).collect {
-              case (name, bounds: TypeBounds) => (name, bounds)
-            }
-          }
+        if noInstances.nonEmpty then
+          MatchResult.NoInstance(noInstances.toList)
         else
           val defn.MatchCase(instantiatedPat, reduced) =
             instantiateParamsSpec(instances, constrainedCaseLambda)(constrainedCaseLambda.resultType): @unchecked

tests/pos/i19607.scala

@@ -0,0 +1,12 @@
+trait Foo
+trait Bar[T]
+
+type MatchType[T] = T match
+  case Bar[?] => Nothing
+  case _ => T
+
+object Test:
+  def foo(b: Bar[? >: Foo]): Unit =
+    summon[MatchType[b.type] =:= Nothing]
+    summon[MatchType[Bar[? >: Foo]] =:= Nothing]
+end Test