From 7a28abdf2bedf90d8e43f891c39ad7ceafe0e7bb Mon Sep 17 00:00:00 2001
From: Liu Fengyun <liu@fengy.me>
Date: Tue, 11 May 2021 18:02:16 +0200
Subject: [PATCH] Fix #12408: Parameterized classes should be applied for
 prefix inference

---
 .../src/dotty/tools/dotc/core/TypeOps.scala   |  3 ++-
 tests/patmat/i12408.check                     |  2 ++
 tests/patmat/i12408.scala                     | 24 +++++++++++++++++++
 3 files changed, 28 insertions(+), 1 deletion(-)
 create mode 100644 tests/patmat/i12408.check
 create mode 100644 tests/patmat/i12408.scala

diff --git a/compiler/src/dotty/tools/dotc/core/TypeOps.scala b/compiler/src/dotty/tools/dotc/core/TypeOps.scala
index ac0b52b51972..a842c5e58261 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeOps.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeOps.scala
@@ -769,7 +769,8 @@ object TypeOps:
             this(tref)
           else {
             prefixTVar = WildcardType  // prevent recursive call from assigning it
-            prefixTVar = newTypeVar(TypeBounds.upper(this(tref)))
+            val tref2 = this(tref.applyIfParameterized(tref.typeParams.map(_ => TypeBounds.empty)))
+            prefixTVar = newTypeVar(TypeBounds.upper(tref2))
             prefixTVar
           }
         case tp => mapOver(tp)
diff --git a/tests/patmat/i12408.check b/tests/patmat/i12408.check
new file mode 100644
index 000000000000..60acc2cba84e
--- /dev/null
+++ b/tests/patmat/i12408.check
@@ -0,0 +1,2 @@
+13: Pattern Match Exhaustivity: A(_), C(_)
+21: Pattern Match
diff --git a/tests/patmat/i12408.scala b/tests/patmat/i12408.scala
new file mode 100644
index 000000000000..442e723cb195
--- /dev/null
+++ b/tests/patmat/i12408.scala
@@ -0,0 +1,24 @@
+class X[T] {
+  sealed trait P
+
+  case class A(id: Long)          extends P
+  case class B(id: Long, x: Long) extends P
+  case class C(id: Long)          extends P
+
+  def m(p: P): Unit = p match {
+    case B(_, x) =>
+    case _       =>
+  }
+
+  def n(p: P): Unit = p match {
+    case B(_, x) =>
+  }
+
+  def o(p: P): Unit = p match {
+    case A(_)    =>
+    case B(_, x) =>
+    case C(_)    =>
+    case _       =>
+  }
+
+}