From 6fd2734e7fc8f473342c526c8e0616e2418f3935 Mon Sep 17 00:00:00 2001
From: Eugene Flesselle <eugene@flesselle.net>
Date: Thu, 11 Apr 2024 23:16:17 +0200
Subject: [PATCH] tryCompiletimeConstantFold in disjointnessBoundary

Fixes #20166
---
 .../dotty/tools/dotc/core/TypeComparer.scala  |  2 ++
 tests/pos/i20166.scala                        | 24 +++++++++++++++++++
 2 files changed, 26 insertions(+)
 create mode 100644 tests/pos/i20166.scala

diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
index cee1ec7fffa8..0890d5889fa7 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -2902,6 +2902,8 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         tp
       case tp: ConstantType =>
         tp
+      case tp: AppliedType if tp.tryCompiletimeConstantFold.exists =>
+        tp.tryCompiletimeConstantFold
       case tp: HKTypeLambda =>
         tp
       case tp: ParamRef =>
diff --git a/tests/pos/i20166.scala b/tests/pos/i20166.scala
new file mode 100644
index 000000000000..38b7556a82f4
--- /dev/null
+++ b/tests/pos/i20166.scala
@@ -0,0 +1,24 @@
+import scala.compiletime.ops.int.*
+
+// NOTE ops.int.S is documented as equivalent to MyS
+
+type MyS[X] = X match
+  case 0 => 1
+  case 1 => 2
+  case 2 => 3
+
+type M[I <: Int] = 4 match
+  case 1 - 1 => "0"
+  case MyS[I] => "2"
+  case S[I] => "2" // Not provablyDisjoint before changes
+  case 2 + I => "3"
+  case I + 3 => "4"
+
+val _: M[1] = "4"
+
+
+type M2[I <: Int, P] = I match
+  case P => "b"
+  case _ => "c"
+
+val _: M2[5, 2 + 3] = "b"