From e236cdd7766263da8e8633beaf46bb24bc85229d Mon Sep 17 00:00:00 2001
From: i10416 <ito.yo16uh90616@gmail.com>
Date: Mon, 22 Jan 2024 00:04:52 +0900
Subject: [PATCH] fix(#15784): ident rule for pat match  was too strict

close https://github.com/lampepfl/dotty/issues/15784

Scala 2 allows backticked identifier and capital identifier in pattern
match, but Scala 3 mistakenly prohibited them.

For example, the following code is valid in Scala 2,

```scala
List(42) match {
  case List(_, Rest @ _*) => Rest
  case List(_, `Rest` @ _*) => `Rest`
  _ => ???
}
```
whereas it resulted in `Not Found Rest` error in Scala 3.

This is because the condition to detect wildcard pattern was so strict
that it chose the wrong match arm; `case _ => ifExpr`.
---
 compiler/src/dotty/tools/dotc/typer/Typer.scala |  2 +-
 tests/neg/i15784.check                          | 12 ++++++++++++
 tests/neg/i15784.scala                          |  4 ++++
 tests/pos/i15784.scala                          |  8 ++++++++
 4 files changed, 25 insertions(+), 1 deletion(-)
 create mode 100644 tests/neg/i15784.check
 create mode 100644 tests/neg/i15784.scala
 create mode 100644 tests/pos/i15784.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 222640124db3..9b804d24b7e6 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -944,7 +944,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
      *                   (x: T) to (x @ (w: T)). This is either `_` or `_*`.
      */
     def cases(ifPat: => Tree, ifExpr: => Tree, wildName: TermName) = tree.expr match {
-      case id: untpd.Ident if (ctx.mode is Mode.Pattern) && untpd.isVarPattern(id) =>
+      case id: untpd.Ident if (ctx.mode is Mode.Pattern) =>
         if (id.name == nme.WILDCARD || id.name == nme.WILDCARD_STAR) ifPat
         else {
           import untpd.*
diff --git a/tests/neg/i15784.check b/tests/neg/i15784.check
new file mode 100644
index 000000000000..fea28b92814c
--- /dev/null
+++ b/tests/neg/i15784.check
@@ -0,0 +1,12 @@
+-- [E006] Not Found Error: tests/neg/i15784.scala:2:26 -----------------------------------------------------------------
+2 |      case List(_, Rest @ `a`) => Rest // error
+  |                          ^^^
+  |                          Not found: a
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E006] Not Found Error: tests/neg/i15784.scala:3:26 -----------------------------------------------------------------
+3 |      case List(_, Rest @ A) => Rest // error 
+  |                          ^
+  |                          Not found: A
+  |
+  | longer explanation available when compiling with `-explain`
diff --git a/tests/neg/i15784.scala b/tests/neg/i15784.scala
new file mode 100644
index 000000000000..aacd6a2b7837
--- /dev/null
+++ b/tests/neg/i15784.scala
@@ -0,0 +1,4 @@
+def i15784 = List(42) match
+      case List(_, Rest @ `a`) => Rest // error
+      case List(_, Rest @ A) => Rest // error 
+      case _ => ???
\ No newline at end of file
diff --git a/tests/pos/i15784.scala b/tests/pos/i15784.scala
new file mode 100644
index 000000000000..281e54ef4da5
--- /dev/null
+++ b/tests/pos/i15784.scala
@@ -0,0 +1,8 @@
+def i15784 = List(42) match
+      case List(_, rest @ _*) => rest
+      case List(_, Rest @ _*) => Rest
+      case List(_, `Rest` @ _*) => Rest
+      case _ => ???
+
+def i15784_auxiliary = 42 match
+      case `type` : Int => `type`