Always freeze GADTs when comparing type member infos

Author: dwijnand

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

@@ -1849,8 +1849,6 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
           || symInfo.isInstanceOf[MethodType]
               && symInfo.signature.consistentParams(info2.signature)
 
-        def tp1IsSingleton: Boolean = tp1.isInstanceOf[SingletonType]
-
         // A relaxed version of isSubType, which compares method types
         // under the standard arrow rule which is contravarient in the parameter types,
         // but under the condition that signatures might have to match (see sigsOK)
@@ -1865,8 +1863,8 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
                   matchingMethodParams(info1, info2, precise = false)
                   && isSubInfo(info1.resultType, info2.resultType.subst(info2, info1), symInfo1.resultType)
                   && sigsOK(symInfo1, info2)
-                case _ => inFrozenGadtIf(tp1IsSingleton) { isSubType(info1, info2) }
-            case _ => inFrozenGadtIf(tp1IsSingleton) { isSubType(info1, info2) }
+                case _ => inFrozenGadt { isSubType(info1, info2) }
+            case _ => inFrozenGadt { isSubType(info1, info2) }
 
         val info1 = m.info.widenExpr
         isSubInfo(info1, tp2.refinedInfo.widenExpr, m.symbol.info.orElse(info1))

tests/neg/i15485.scala

@@ -0,0 +1,20 @@
+enum SUB[-A, +B]:
+  case Refl[C]() extends SUB[C, C]
+
+trait Tag { type T }
+
+def foo[A, B, X <: Tag { type T <: A } ](
+    e: SUB[X, Tag { type T <: B } ],
+    x: A,
+): B = e match {
+  case SUB.Refl() =>
+    // unsound GADT constr because of approx state resetting:
+    // A <: B
+    x // error: Found: (x: A) Required: B
+}
+
+def bad(x: Int): String =
+  foo[Int, String, Tag { type T = Nothing } ](SUB.Refl(), x)  // cast Int to String
+
+object Test:
+  def main(args: Array[String]): Unit = bad(1) // was: ClassCastException: class java.lang.Integer cannot be cast to class java.lang.String