From e918307912367ccd022b23bd0a034b2cdd89005b Mon Sep 17 00:00:00 2001
From: Lukas Rytz <lukas.rytz@gmail.com>
Date: Fri, 19 Apr 2024 15:13:29 +0200
Subject: [PATCH] Normalize reference to undet param in stabilizer

Details in the ticket.
---
 .../scala/tools/nsc/typechecker/Typers.scala  |  7 ++++-
 test/files/pos/t12987.scala                   | 30 +++++++++++++++++++
 2 files changed, 36 insertions(+), 1 deletion(-)
 create mode 100644 test/files/pos/t12987.scala

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index 474cf78e672d..e4e8128cb369 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -5524,7 +5524,12 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
               context.pendingStabilizers ::= vdef
               qual.changeOwner(context.owner -> vsym)
               val newQual = Ident(vsym) setType singleType(NoPrefix, vsym) setPos qual.pos.focus
-              return typedSelect(tree, newQual, name)
+              return typedSelect(tree, newQual, name).modifyType(_.map {
+                // very specific fix for scala/bug#12987 (details in the ticket)
+                case t: AliasTypeRef if t.pre.termSymbol == vsym && context.undetparams.contains(t.normalize.typeSymbol) =>
+                  t.normalize
+                case t => t
+              })
             }
 
             val tree1 = tree match {
diff --git a/test/files/pos/t12987.scala b/test/files/pos/t12987.scala
new file mode 100644
index 000000000000..e9a866f4abc9
--- /dev/null
+++ b/test/files/pos/t12987.scala
@@ -0,0 +1,30 @@
+object typeMember {
+  class Foo {
+    type FT
+    class I
+    def m(b: FT, o: Option[I]): Int = 0
+  }
+
+  object Test {
+    def f[T]: Foo { type FT = T } = ???
+    def t = {
+      val b: Any = ???
+      f.m(b, None)
+    }
+  }
+}
+
+object typeParam {
+  class Foo[FT] {
+    class I
+    def m(b: FT, o: Option[I]): Int = 0
+  }
+
+  object Test {
+    def f[T]: Foo[T] = ???
+    def t = {
+      val b: Any = ???
+      f.m(b, None)
+    }
+  }
+}