From 989ff1a08644b3163061d4a020561c68b98327a4 Mon Sep 17 00:00:00 2001
From: Dale Wijnand <dale.wijnand@gmail.com>
Date: Wed, 2 Aug 2023 11:33:25 +0100
Subject: [PATCH 1/2] Disallow overloading from breaking stable patterns

[Cherry-picked 85d656a81ad0fd3b0ea51f68b5d5a3059baad70e]
---
 compiler/src/dotty/tools/dotc/typer/Typer.scala |  4 +++-
 tests/pos/i18247.scala                          | 10 ++++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)
 create mode 100644 tests/pos/i18247.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 237f48d7bd96..9647bdf40978 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -668,7 +668,9 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     if ctx.mode.is(Mode.Pattern)
        && !tree.isType
        && !pt.isInstanceOf[ApplyingProto]
-       && !tree.tpe.isStable
+       && !tree.tpe.match
+        case tp: NamedType => tp.denot.hasAltWith(_.symbol.namedType.isStable)
+        case tp            => tp.isStable
        && !isWildcardArg(tree)
     then
       report.error(StableIdentPattern(tree, pt), tree.srcPos)
diff --git a/tests/pos/i18247.scala b/tests/pos/i18247.scala
new file mode 100644
index 000000000000..b3805f5a7d02
--- /dev/null
+++ b/tests/pos/i18247.scala
@@ -0,0 +1,10 @@
+sealed trait Op
+object Op {
+  case object `==` extends Op
+}
+
+def t1(a: Op): true = {
+  a match {
+    case Op.`==` => true // was: won't compile
+  }
+}

From c43d3877501c458c79fbe20dbed47631b67ff593 Mon Sep 17 00:00:00 2001
From: Dale Wijnand <dale.wijnand@gmail.com>
Date: Wed, 2 Aug 2023 16:33:16 +0100
Subject: [PATCH 2/2] Refine how we check for stability

In particular, must continue to use the type's prefix, rather than the
denotation's prefix.  That way, e.g. in tests/neg/i3812b.scala,
referencing value class'\''s value, will fail on the non-stable value
class prefix, rather than the denotation'\''s prefix, which is NoSymbol.

[Cherry-picked e6012a0801c9ce864232249c7ce2d7acd66596f5]
---
 compiler/src/dotty/tools/dotc/typer/Typer.scala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 9647bdf40978..34940d2f5277 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -669,7 +669,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
        && !tree.isType
        && !pt.isInstanceOf[ApplyingProto]
        && !tree.tpe.match
-        case tp: NamedType => tp.denot.hasAltWith(_.symbol.namedType.isStable)
+        case tp: NamedType => tp.denot.hasAltWith(_.symbol.isStableMember && tp.prefix.isStable || tp.info.isStable)
         case tp            => tp.isStable
        && !isWildcardArg(tree)
     then