GADTs: freeze when approximating LHS singleton union

Author: abgruszecki

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

@@ -434,7 +434,7 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         def widenOK =
           (tp2.widenSingletons eq tp2)
           && (tp1.widenSingletons ne tp1)
-          && recur(tp1.widenSingletons, tp2)
+          && inFrozenGadt { recur(tp1.widenSingletons, tp2) }
 
         def joinOK = tp2.dealiasKeepRefiningAnnots match {
           case tp2: AppliedType if !tp2.tycon.typeSymbol.isClass =>

tests/neg/gadt-lhs-union.scala

@@ -0,0 +1,33 @@
+object A {
+  sealed trait Expr[+T]
+  final case class FooExpr() extends Expr[1 | 2]
+
+  object Test {
+    def foo[T](x: Expr[T]): T = x match {
+      case x: FooExpr =>
+        3 // error
+    }
+
+    val x: 1 | 2 = foo(FooExpr())
+  }
+}
+
+object B {
+  trait C
+  trait D extends C
+  trait E extends C
+  trait F extends C
+  trait G extends C
+
+  sealed trait Expr[+T]
+  final case class FooExpr[+S >: (D & E) | F]() extends Expr[S]
+
+  object Test {
+    def foo[T](x: Expr[T]): T = x match {
+      case x: FooExpr[(D & E) | F] =>
+        new D with E
+    }
+
+    val x: (D & E) | F = foo(FooExpr[(D & E) | F]())
+  }
+}