Fix #19746: Do not follow param term refs in isConcrete.

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

@@ -3360,37 +3360,44 @@ class MatchReducer(initctx: Context) extends TypeComparer(initctx) {
        *
        * See notably neg/wildcard-match.scala for examples of this.
        *
-       * See neg/i13780.scala and neg/i13780-1.scala for ClassCastException
-       * reproducers if we disable this check.
+       * See neg/i13780.scala, neg/i13780-1.scala and neg/i19746.scala for
+       * ClassCastException reproducers if we disable this check.
        */
 
-      def followEverythingConcrete(tp: Type): Type =
-        val widenedTp = tp.widenDealias
-        val tp1 = widenedTp.normalized
-
-        def followTp1: Type =
-          // If both widenDealias and normalized did something, start again
-          if (tp1 ne widenedTp) && (widenedTp ne tp) then followEverythingConcrete(tp1)
-          else tp1
+      def isConcrete(tp: Type): Boolean =
+        val tp1 = tp.normalized
 
         tp1 match
           case tp1: TypeRef =>
-            tp1.info match
-              case TypeAlias(tl: HKTypeLambda)  => tl
-              case MatchAlias(tl: HKTypeLambda) => tl
-              case _                            => followTp1
-          case tp1 @ AppliedType(tycon, args) =>
-            val concreteTycon = followEverythingConcrete(tycon)
-            if concreteTycon eq tycon then followTp1
-            else followEverythingConcrete(concreteTycon.applyIfParameterized(args))
+            if tp1.symbol.isClass then true
+            else
+              tp1.info match
+                case info: AliasingBounds => isConcrete(info.alias)
+                case _                    => false
+          case tp1: AppliedType =>
+            isConcrete(tp1.tycon) && isConcrete(tp1.superType)
+          case tp1: HKTypeLambda =>
+            true
+          case tp1: TermRef =>
+            !tp1.symbol.is(Param) && isConcrete(tp1.underlying)
+          case tp1: TermParamRef =>
+            false
+          case tp1: SingletonType =>
+            isConcrete(tp1.underlying)
+          case tp1: ExprType =>
+            isConcrete(tp1.underlying)
+          case tp1: AnnotatedType =>
+            isConcrete(tp1.parent)
+          case tp1: RefinedType =>
+            isConcrete(tp1.underlying)
+          case tp1: RecType =>
+            isConcrete(tp1.underlying)
+          case tp1: AndOrType =>
+            isConcrete(tp1.tp1) && isConcrete(tp1.tp2)
           case _ =>
-            followTp1
-      end followEverythingConcrete
-
-      def isConcrete(tp: Type): Boolean =
-        followEverythingConcrete(tp) match
-          case tp1: AndOrType => isConcrete(tp1.tp1) && isConcrete(tp1.tp2)
-          case tp1            => tp1.underlyingClassRef(refinementOK = true).exists
+            val tp2 = tp1.stripped.stripLazyRef
+            (tp2 ne tp) && isConcrete(tp2)
+      end isConcrete
 
       // Actual matching logic
 

tests/neg/i19746.check

@@ -0,0 +1,7 @@
+-- [E007] Type Mismatch Error: tests/neg/i19746.scala:9:30 -------------------------------------------------------------
+9 |    def asX(w: W[Any]): w.X = self // error: Type Mismatch
+  |                              ^^^^
+  |                              Found:    (self : Any)
+  |                              Required: w.X
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/i19746.scala

@@ -0,0 +1,15 @@
+trait V:
+  type X = this.type match
+    case W[x] => x
+
+trait W[+Y] extends V
+
+object Test:
+  extension (self: Any) def as[T]: T =
+    def asX(w: W[Any]): w.X = self // error: Type Mismatch
+    asX(new W[T] {})
+
+  def main(args: Array[String]): Unit =
+    val b = 0.as[Boolean] // java.lang.ClassCastException if the code is allowed to compile
+    println(b)
+end Test

tests/pos/TupleReverse.scala

@@ -12,5 +12,6 @@ def test[T1, T2, T3, T4] =
 def test2[Tup <: Tuple] =
   summon[Reverse[Tup] =:= Reverse[Tup]]
 
-def test3[T1, T2, T3, T4](tup1: (T1, T2, T3, T4)) =
-  summon[Reverse[tup1.type] =:= (T4, T3, T2, T1)]
+def test3[T1, T2, T3, T4](tup1: (T1, T2, T3, T4)): Unit =
+  val tup11: (T1, T2, T3, T4) = tup1
+  summon[Reverse[tup11.type] =:= (T4, T3, T2, T1)]

tests/pos/TupleReverseOnto.scala

@@ -13,6 +13,7 @@ def test2[Tup1 <: Tuple, Tup2 <: Tuple] =
   summon[ReverseOnto[EmptyTuple, Tup1] =:= Tup1]
   summon[ReverseOnto[Tup1, EmptyTuple] =:= Reverse[Tup1]]
 
-def test3[T1, T2, T3, T4](tup1: (T1, T2), tup2: (T3, T4)) =
-  summon[ReverseOnto[tup1.type, tup2.type] <:< (T2, T1, T3, T4)]
-  summon[ReverseOnto[tup1.type, tup2.type] =:= T2 *: T1 *: tup2.type]
+def test3[T1, T2, T3, T4](tup1: (T1, T2), tup2: (T3, T4)): Unit =
+  val tup11: (T1, T2) = tup1
+  summon[ReverseOnto[tup11.type, tup2.type] <:< (T2, T1, T3, T4)]
+  summon[ReverseOnto[tup11.type, tup2.type] =:= T2 *: T1 *: tup2.type]