From ae3b2716b6d00be325f5e107a00901c909d8967f Mon Sep 17 00:00:00 2001
From: Eugene Flesselle <eugene@flesselle.net>
Date: Wed, 24 Apr 2024 19:47:51 +0200
Subject: [PATCH] Normalize for underlyingClassRef

Related to #20258
Here too, I would be fine with not normalizing,
but in that case pos/i15827 should become a neg test.
---
 compiler/src/dotty/tools/dotc/core/Types.scala | 3 ++-
 tests/pos/i15827b/A_1.scala                    | 8 ++++++++
 tests/pos/i15827b/B_2.scala                    | 3 +++
 3 files changed, 13 insertions(+), 1 deletion(-)
 create mode 100644 tests/pos/i15827b/A_1.scala
 create mode 100644 tests/pos/i15827b/B_2.scala

diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index 870e985fd48e..a34a5206a011 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -1584,7 +1584,8 @@ object Types extends TypeUtils {
         else if (tp.symbol.isAliasType) tp.underlying.underlyingClassRef(refinementOK)
         else NoType
       case tp: AppliedType =>
-        if (tp.tycon.isLambdaSub) NoType
+        if tp.isMatchAlias then tp.tryNormalize.underlyingClassRef(refinementOK)
+        else if (tp.tycon.isLambdaSub) NoType
         else tp.superType.underlyingClassRef(refinementOK)
       case tp: AnnotatedType =>
         tp.parent.underlyingClassRef(refinementOK)
diff --git a/tests/pos/i15827b/A_1.scala b/tests/pos/i15827b/A_1.scala
new file mode 100644
index 000000000000..b8734fbdaa2f
--- /dev/null
+++ b/tests/pos/i15827b/A_1.scala
@@ -0,0 +1,8 @@
+
+case class Foo()
+
+type T = Tuple.Elem[(Foo, Any), 0]
+type U = Tuple.Elem[(Any, T), 1]
+
+val _ = new T() // ok
+val _ = new U() // ok
diff --git a/tests/pos/i15827b/B_2.scala b/tests/pos/i15827b/B_2.scala
new file mode 100644
index 000000000000..e39f0727b544
--- /dev/null
+++ b/tests/pos/i15827b/B_2.scala
@@ -0,0 +1,3 @@
+
+val _ = new T() // was error
+val _ = new U() // was error