From 0a759cc40699d5191c9f7d334e21ec7887cbdb17 Mon Sep 17 00:00:00 2001
From: Eugene Flesselle <eugene@flesselle.net>
Date: Mon, 29 Jan 2024 11:07:10 +0100
Subject: [PATCH] Only cache baseTypes when gadtState is empty

---
 .../src/dotty/tools/dotc/core/SymDenotations.scala  |  2 +-
 tests/pos/i19521.scala                              | 13 +++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)
 create mode 100644 tests/pos/i19521.scala

diff --git a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
index 74eb99c1efd9..13eb5ce8b5ba 100644
--- a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
+++ b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
@@ -2192,7 +2192,7 @@ object SymDenotations {
           Stats.record("basetype cache entries")
           if (!baseTp.exists) Stats.record("basetype cache NoTypes")
         }
-        if (!tp.isProvisional && !CapturingType.isUncachable(tp))
+        if !(tp.isProvisional || CapturingType.isUncachable(tp) || ctx.gadt.isNarrowing) then
           btrCache(tp) = baseTp
         else
           btrCache.remove(tp) // Remove any potential sentinel value
diff --git a/tests/pos/i19521.scala b/tests/pos/i19521.scala
new file mode 100644
index 000000000000..19230fa4ed69
--- /dev/null
+++ b/tests/pos/i19521.scala
@@ -0,0 +1,13 @@
+
+final abstract class PLet
+
+sealed trait Expr[+P]
+case class ELet[+A](name: String, expr: Expr[A]) extends Expr[A | PLet]
+
+def go[P](e: Expr[P]): P = e match
+  case ELet(_, _) =>
+    val x: Expr[P] | ELet[P] = ???
+    val y: Expr[P] = x // conforms iff using gadt constraints
+    // error before changes: cast from gadt reasoning was not inserted because
+    // `Expr[P]` was erronously cached as a baseType of `Expr[P] | ELet[P]` (only true with gadt constraints)
+    ???