Disallow overloading from breaking stable patterns

[Cherry-picked 85d656a]

Author: dwijnand

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)

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
+  }
+}