From d76a553073d1083fbf650140d597e3df4adaf469 Mon Sep 17 00:00:00 2001
From: Eugene Flesselle <eugene@flesselle.net>
Date: Mon, 11 Mar 2024 18:36:48 +0100
Subject: [PATCH] Cover patterns using `reflet.TypeTest` in isMatchTypeShaped

[Cherry-picked 0942eef54c6c2eb31118175d9777a344c79275f7]
---
 compiler/src/dotty/tools/dotc/typer/Typer.scala | 4 +++-
 tests/pos/i19692.scala                          | 8 ++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)
 create mode 100644 tests/pos/i19692.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 7637bb3d5e2a..2ec42604ad08 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -1738,7 +1738,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
                 // check `pat` here and throw away the result.
                 val gadtCtx: Context = ctx.fresh.setFreshGADTBounds
                 val pat1 = typedPattern(pat, selType)(using gadtCtx)
-                val Typed(_, tpt) = tpd.unbind(tpd.unsplice(pat1)): @unchecked
+                val tpt = tpd.unbind(tpd.unsplice(pat1)) match
+                  case Typed(_, tpt) => tpt
+                  case UnApply(fun, _, p1 :: _) if fun.symbol == defn.TypeTest_unapply => p1
                 instantiateMatchTypeProto(pat1, pt) match {
                   case defn.MatchCase(patternTp, _) => tpt.tpe frozen_=:= patternTp
                   case _ => false
diff --git a/tests/pos/i19692.scala b/tests/pos/i19692.scala
new file mode 100644
index 000000000000..7cc64e59ff07
--- /dev/null
+++ b/tests/pos/i19692.scala
@@ -0,0 +1,8 @@
+
+trait UsingTypeTest[B](using reflect.TypeTest[Int, B]):
+
+  type M[U <: Int] = U match
+    case B => String
+
+  def m(t: Int): M[Int] = t match
+    case _: B => "hello"
\ No newline at end of file