From 822a29c5098082fc225e6f64343244fe0290fd27 Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Sun, 19 Jan 2025 20:03:10 +0100
Subject: [PATCH] Fix mapping of annotations

We previously did not do an (expensive) TreeTypeMap on an annotation if the mapped versions
of all types of subtrees of the annotation tree were =:= to the original types. But it turns
out this is too coarse. In the test we have capture set variables where we intend to map a
TypeRef to a TypeParamRef but the two were considered as =:= because of the bounds they had.
So no mapping took place.

We now use `eql` instead of =:=, which is structural comparison with `eq` for references to
corresponding binders.

[Cherry-picked 6d6da118ac15b16dc58e35ce85127f468e7c4674]
---
 compiler/src/dotty/tools/dotc/core/Annotations.scala | 2 +-
 compiler/src/dotty/tools/dotc/core/Types.scala       | 2 ++
 tests/pos-custom-args/captures/setup/a_1.scala       | 6 ++++++
 tests/pos-custom-args/captures/setup/b_1.scala       | 5 +++++
 tests/pos-custom-args/captures/setup/b_2.scala       | 5 +++++
 5 files changed, 19 insertions(+), 1 deletion(-)
 create mode 100644 tests/pos-custom-args/captures/setup/a_1.scala
 create mode 100644 tests/pos-custom-args/captures/setup/b_1.scala
 create mode 100644 tests/pos-custom-args/captures/setup/b_2.scala

diff --git a/compiler/src/dotty/tools/dotc/core/Annotations.scala b/compiler/src/dotty/tools/dotc/core/Annotations.scala
index f5825df84a8b..0706003f533e 100644
--- a/compiler/src/dotty/tools/dotc/core/Annotations.scala
+++ b/compiler/src/dotty/tools/dotc/core/Annotations.scala
@@ -65,7 +65,7 @@ object Annotations {
             if tm.isRange(x) then x
             else
               val tp1 = tm(tree.tpe)
-              foldOver(if !tp1.exists || (tp1 frozen_=:= tree.tpe) then x else tp1, tree)
+              foldOver(if !tp1.exists || tp1.eql(tree.tpe) then x else tp1, tree)
         val diff = findDiff(NoType, args)
         if tm.isRange(diff) then EmptyAnnotation
         else if diff.exists then derivedAnnotation(tm.mapOver(tree))
diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index 5fe6c3174c04..ac6089516571 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -5967,6 +5967,8 @@ object Types extends TypeUtils {
       override def stopAt = thisMap.stopAt
       def apply(tp: Type) = f(thisMap(tp))
     }
+
+    override def toString = s"${getClass.getSimpleName}@$hashCode" // otherwise would print as <function1>
   }
 
   /** A type map that maps also parents and self type of a ClassInfo */
diff --git a/tests/pos-custom-args/captures/setup/a_1.scala b/tests/pos-custom-args/captures/setup/a_1.scala
new file mode 100644
index 000000000000..21afde8be3ea
--- /dev/null
+++ b/tests/pos-custom-args/captures/setup/a_1.scala
@@ -0,0 +1,6 @@
+// a.scala
+import language.experimental.captureChecking
+import scala.caps.CapSet
+
+trait A:
+  def f[C^](x: AnyRef^{C^}): Unit
diff --git a/tests/pos-custom-args/captures/setup/b_1.scala b/tests/pos-custom-args/captures/setup/b_1.scala
new file mode 100644
index 000000000000..d5ba925970ba
--- /dev/null
+++ b/tests/pos-custom-args/captures/setup/b_1.scala
@@ -0,0 +1,5 @@
+import language.experimental.captureChecking
+import scala.caps.CapSet
+
+class B extends A:
+  def f[C^](x: AnyRef^{C^}): Unit = ???
diff --git a/tests/pos-custom-args/captures/setup/b_2.scala b/tests/pos-custom-args/captures/setup/b_2.scala
new file mode 100644
index 000000000000..d5ba925970ba
--- /dev/null
+++ b/tests/pos-custom-args/captures/setup/b_2.scala
@@ -0,0 +1,5 @@
+import language.experimental.captureChecking
+import scala.caps.CapSet
+
+class B extends A:
+  def f[C^](x: AnyRef^{C^}): Unit = ???