Fix false exhaustivity for GADT under separate compilation (#23966)

fixes #23817

Author: hamzaremmal

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -537,7 +537,7 @@ object SpaceEngine {
         scrutineeTp match
         case AppliedType(tycon, targs)
             if unappSym.is(Synthetic)
-            && (pt.resultType.asInstanceOf[MethodType].paramInfos.head.typeConstructor eq tycon) =>
+            && (pt.resultType.asInstanceOf[MethodType].paramInfos.head.typeConstructor =:= tycon) =>
           // Special case synthetic unapply/unapplySeq's
           // Provided the shapes of the types match:
           // the scrutinee type being unapplied and

tests/pos/i23817/file_1.scala

@@ -0,0 +1,2 @@
+sealed trait N[T]
+case class MakeTuple[T <: Tuple](v: T) extends N[T]

tests/pos/i23817/file_2.scala

@@ -0,0 +1,6 @@
+object test {
+  def t[T](expr: N[T]): Any =
+    expr match {
+      case MakeTuple(_) => ???
+    }
+}

tests/pos/i23817_2/file_1.scala

@@ -0,0 +1,3 @@
+trait TC2[X]
+sealed trait TC1[T]
+case class Iterable[T, C <: scala.Iterable[T]](tag: TC1[C]) extends TC1[C]

tests/pos/i23817_2/file_2.scala

@@ -0,0 +1,6 @@
+object test {
+  def f[T](c: TC1[T]): Unit =
+    c match {
+      case Iterable(_) => ???
+    }
+}