Fix #19445: Remove too-strict test in match type matching.

Previously, for a `BaseTypeTest`, we explicitly checked the prefix
of the type constructor for `=:=`. This is however too strict, as
shown by #19445. We remove the test entirely, as the correct
subprefixing test is already part of the overall
`scrut <:< instantiatedPat` done at the end.

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

@@ -3436,7 +3436,8 @@ class TrackingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
           case MatchTypeCasePattern.BaseTypeTest(classType, argPatterns, needsConcreteScrut) =>
             val cls = classType.classSymbol.asClass
             scrut.baseType(cls) match
-              case base @ AppliedType(baseTycon, baseArgs) if baseTycon =:= classType =>
+              case base @ AppliedType(baseTycon, baseArgs) =>
+                // #19445 Don't check the prefix of baseTycon here; it is handled by `scrut <:< instantiatedPat`.
                 val innerScrutIsWidenedAbstract =
                   scrutIsWidenedAbstract
                     || (needsConcreteScrut && !isConcrete(scrut)) // no point in checking concreteness if it does not need to be concrete

tests/neg/i19445.check

@@ -0,0 +1,36 @@
+-- [E007] Type Mismatch Error: tests/neg/i19445.scala:17:15 ------------------------------------------------------------
+17 |  val x: Int = ??? : UnwrapTypes[FooBar.Bar[Int]] // error
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |               Found:    Test.UnwrapTypes[FooBar.Bar[Int]]
+   |               Required: Int
+   |
+   |               Note: a match type could not be fully reduced:
+   |
+   |                 trying to reduce  Test.UnwrapTypes[FooBar.Bar[Int]]
+   |                 failed since selector FooBar.Bar[Int]
+   |                 does not match  case BarFoo.Bar[x] => Test.UnwrapTypes[x]
+   |                 and cannot be shown to be disjoint from it either.
+   |                 Therefore, reduction cannot advance to the remaining cases
+   |
+   |                   case String => String
+   |                   case Int => Int
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg/i19445.scala:18:28 ------------------------------------------------------------
+18 |  val tup: (Int, String)  = ??? : UnwrapTypes[(FooBar.Bar[Int], FooBar.Bar[String])] // error
+   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                            Found:    (Test.UnwrapTypes[FooBar.Bar[Int]], Test.UnwrapTypes[FooBar.Bar[String]])
+   |                            Required: (Int, String)
+   |
+   |                            Note: a match type could not be fully reduced:
+   |
+   |                              trying to reduce  Test.UnwrapTypes[FooBar.Bar[Int]]
+   |                              failed since selector FooBar.Bar[Int]
+   |                              does not match  case BarFoo.Bar[x] => Test.UnwrapTypes[x]
+   |                              and cannot be shown to be disjoint from it either.
+   |                              Therefore, reduction cannot advance to the remaining cases
+   |
+   |                                case String => String
+   |                                case Int => Int
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg/i19445.scala

@@ -0,0 +1,19 @@
+trait Foo:
+  case class Bar[A](value: A)
+
+object FooBar extends Foo
+
+object BarFoo extends Foo
+
+object Test:
+  type UnwrapTypes[Xs] =
+    Xs match
+      case EmptyTuple => Xs
+      case x *: xs => UnwrapTypes[x] *: UnwrapTypes[xs]
+      case BarFoo.Bar[x] => UnwrapTypes[x]
+      case String => String
+      case Int => Int
+
+  val x: Int = ??? : UnwrapTypes[FooBar.Bar[Int]] // error
+  val tup: (Int, String)  = ??? : UnwrapTypes[(FooBar.Bar[Int], FooBar.Bar[String])] // error
+end Test

tests/pos/i19445.scala

@@ -0,0 +1,17 @@
+trait Foo:
+  case class Bar[A](value: A)
+
+object FooBar extends Foo
+
+object Test:
+  type UnwrapTypes[Xs] =
+    Xs match
+      case EmptyTuple => Xs
+      case x *: xs => UnwrapTypes[x] *: UnwrapTypes[xs]
+      case Foo#Bar[x] => UnwrapTypes[x]
+      case String => String
+      case Int => Int
+
+  val x: Int = ??? : UnwrapTypes[FooBar.Bar[Int]]
+  val tup: (Int, String)  = ??? : UnwrapTypes[(FooBar.Bar[Int], FooBar.Bar[String])]
+end Test