From 374cd4f7b5fd45c6d1ae96e7cbff7ca7f71010cf Mon Sep 17 00:00:00 2001
From: Matt Bovel <matthieu@bovel.net>
Date: Mon, 7 Oct 2024 19:36:37 +0200
Subject: [PATCH] Always treat underscores as type bounds inside patterns

Always treat underscores as type bounds inside patterns, even when `ctx.settings.XkindProjector.value == "underscores"`.

Fixes #14952 and #21400.
---
 compiler/src/dotty/tools/dotc/parsing/Parsers.scala |  2 +-
 tests/pos/14952.scala                               |  9 +++++++++
 tests/pos/21400.scala                               |  7 +++++++
 tests/pos/21400b.scala                              | 10 ++++++++++
 4 files changed, 27 insertions(+), 1 deletion(-)
 create mode 100644 tests/pos/14952.scala
 create mode 100644 tests/pos/21400.scala
 create mode 100644 tests/pos/21400b.scala

diff --git a/compiler/src/dotty/tools/dotc/parsing/Parsers.scala b/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
index d933e55a9823..5a3be6505715 100644
--- a/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
+++ b/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
@@ -1988,7 +1988,7 @@ object Parsers {
       if isSimpleLiteral then
         SingletonTypeTree(simpleLiteral())
       else if in.token == USCORE then
-        if ctx.settings.XkindProjector.value == "underscores" then
+        if ctx.settings.XkindProjector.value == "underscores" && !inMatchPattern then
           val start = in.skipToken()
           Ident(tpnme.USCOREkw).withSpan(Span(start, in.lastOffset, start))
         else
diff --git a/tests/pos/14952.scala b/tests/pos/14952.scala
new file mode 100644
index 000000000000..350607b7ef77
--- /dev/null
+++ b/tests/pos/14952.scala
@@ -0,0 +1,9 @@
+//> using options -Xkind-projector:underscores
+
+import Tuple.*
+
+type LiftP[F[_], T] <: Tuple =
+  T match {
+    case _ *: _ => F[Head[T]] *: LiftP[F, Tail[T]]
+    case _ => EmptyTuple
+  }
diff --git a/tests/pos/21400.scala b/tests/pos/21400.scala
new file mode 100644
index 000000000000..6d1534703a00
--- /dev/null
+++ b/tests/pos/21400.scala
@@ -0,0 +1,7 @@
+//> using options -Xkind-projector:underscores
+
+import scala.compiletime.ops.int.S
+
+type IndexOf[T <: Tuple, E] <: Int = T match
+  case E *: _  => 0
+  case _ *: es => 1 // S[IndexOf[es, E]]
diff --git a/tests/pos/21400b.scala b/tests/pos/21400b.scala
new file mode 100644
index 000000000000..9e8768d48261
--- /dev/null
+++ b/tests/pos/21400b.scala
@@ -0,0 +1,10 @@
+//> using options -Xkind-projector:underscores
+
+import scala.quoted.Type
+import scala.quoted.Quotes
+
+def x[A](t: Type[A])(using Quotes): Boolean = t match
+  case '[_ *: _] =>
+    true
+  case _ =>
+    false