From 6d7aa994ac37207a4a34d2d66fc862db66761a86 Mon Sep 17 00:00:00 2001
From: Nicolas Stucki <nicolas.stucki@gmail.com>
Date: Thu, 9 Nov 2023 16:46:44 +0100
Subject: [PATCH] Do not show deprecation waning for `_` in type match case

Fixes #18808
---
 compiler/src/dotty/tools/dotc/parsing/Parsers.scala | 12 ++++++++++--
 compiler/test-resources/repl/i13208.scala           |  5 -----
 tests/pos/i18808.scala                              |  9 +++++++++
 3 files changed, 19 insertions(+), 7 deletions(-)
 create mode 100644 tests/pos/i18808.scala

diff --git a/compiler/src/dotty/tools/dotc/parsing/Parsers.scala b/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
index 327f97a3cc9d..5642443eae73 100644
--- a/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
+++ b/compiler/src/dotty/tools/dotc/parsing/Parsers.scala
@@ -413,6 +413,14 @@ object Parsers {
       finally inEnum = saved
     }
 
+    private var inTypeMatchPattern = false
+    private def withinTypeMatchPattern[T](body: => T): T = {
+      val saved = inTypeMatchPattern
+      inTypeMatchPattern = true
+      try body
+      finally inTypeMatchPattern = saved
+    }
+
     private var staged = StageKind.None
     def withinStaged[T](kind: StageKind)(op: => T): T = {
       val saved = staged
@@ -1862,7 +1870,7 @@ object Parsers {
           val start = in.skipToken()
           Ident(tpnme.USCOREkw).withSpan(Span(start, in.lastOffset, start))
         else
-          if sourceVersion.isAtLeast(future) then
+          if !inTypeMatchPattern && sourceVersion.isAtLeast(future) then
             deprecationWarning(em"`_` is deprecated for wildcard arguments of types: use `?` instead")
             patch(source, Span(in.offset, in.offset + 1), "?")
           val start = in.skipToken()
@@ -2898,7 +2906,7 @@ object Parsers {
             val start = in.skipToken()
             Ident(tpnme.WILDCARD).withSpan(Span(start, in.lastOffset, start))
           case _ =>
-            rejectWildcardType(infixType())
+            withinTypeMatchPattern(rejectWildcardType(infixType()))
         }
       }
       CaseDef(pat, EmptyTree, atSpan(accept(ARROW)) {
diff --git a/compiler/test-resources/repl/i13208.scala b/compiler/test-resources/repl/i13208.scala
index 61ace43c732d..07cc67d3bf0b 100644
--- a/compiler/test-resources/repl/i13208.scala
+++ b/compiler/test-resources/repl/i13208.scala
@@ -1,8 +1,3 @@
 //> using options -source:future -deprecation
 scala> type M[X] = X match { case Int => String case _ => Int }
 scala> type N[X] = X match { case List[_] => Int }
-1 warning found
--- Deprecation Warning: --------------------------------------------------------
-1 | type N[X] = X match { case List[_] => Int }
-  |                                 ^
-  |        `_` is deprecated for wildcard arguments of types: use `?` instead
diff --git a/tests/pos/i18808.scala b/tests/pos/i18808.scala
new file mode 100644
index 000000000000..0556b3285d00
--- /dev/null
+++ b/tests/pos/i18808.scala
@@ -0,0 +1,9 @@
+//> using options -Werror
+
+import language.future
+
+type F[X] = X match
+  case List[_] => Int
+
+type G[X] = X match
+  case List[?] => Int