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

Term refs that reference term parameters can be substituted later
by more precise ones, which can lead to different instantiations
of type captures. They must therefore be considered as non concrete
when following `baseType`s to captures in variant positions, like
we do for type param refs and other substitutable references.

We actually rewrite `isConcrete` in the process to be more based
on an "allow list" of things we know to be concrete, rather than
an "exclusion list" of things we know to be non-concrete. That
should make it more straightforward to evaluate the validity of
the algorithm.

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]