From 62a22561b0597af2eb53257c7f8148d507d0c3c1 Mon Sep 17 00:00:00 2001
From: Aleksander Boruch-Gruszecki <aleksander.boruch-gruszecki@epfl.ch>
Date: Mon, 10 May 2021 10:25:35 +0200
Subject: [PATCH] GADTs: consider singletons when checking inv. refinement

---
 .../dotty/tools/dotc/core/PatternTypeConstrainer.scala   | 1 +
 tests/pos/i12390-gadt.scala                              | 9 +++++++++
 2 files changed, 10 insertions(+)
 create mode 100644 tests/pos/i12390-gadt.scala

diff --git a/compiler/src/dotty/tools/dotc/core/PatternTypeConstrainer.scala b/compiler/src/dotty/tools/dotc/core/PatternTypeConstrainer.scala
index 72914ebfee11..4c33d6e3e933 100644
--- a/compiler/src/dotty/tools/dotc/core/PatternTypeConstrainer.scala
+++ b/compiler/src/dotty/tools/dotc/core/PatternTypeConstrainer.scala
@@ -196,6 +196,7 @@ trait PatternTypeConstrainer { self: TypeComparer =>
    */
   def constrainSimplePatternType(patternTp: Type, scrutineeTp: Type, widenParams: Boolean): Boolean = {
     def refinementIsInvariant(tp: Type): Boolean = tp match {
+      case tp: SingletonType => true
       case tp: ClassInfo => tp.cls.is(Final) || tp.cls.is(Case)
       case tp: TypeProxy => refinementIsInvariant(tp.underlying)
       case _ => false
diff --git a/tests/pos/i12390-gadt.scala b/tests/pos/i12390-gadt.scala
new file mode 100644
index 000000000000..1525350b56f7
--- /dev/null
+++ b/tests/pos/i12390-gadt.scala
@@ -0,0 +1,9 @@
+enum Func[-A, +B] {
+  case Double extends Func[Int, Int]
+  case ToString extends Func[Float, String]
+
+  def run: A => B = this match {
+    case Double => (x: Int) => x * 2
+    case ToString => (x: Float) => x.toString
+  }
+}