From c73c47aabc7130098fc1712a7c7732f2d5392718 Mon Sep 17 00:00:00 2001
From: Dale Wijnand <dale.wijnand@gmail.com>
Date: Sun, 22 Jan 2023 18:21:23 +0000
Subject: [PATCH] Fix upper bound constraints, that are higher-kinded

When recording an upper bound, when it's a some higher-kinded type
variable applied to some type arguments, TypeComparer was re-applying
the type arguments to the type parameter (the type variable's origin).
Meaning that the instantiation of the type variable won't reflect in the
upper bound.

See the error message: `F$1[Int]` correctly appears as the lower bound,
but `F$1[Int]` isn't the upper bound, `F[Int]` is, which is the original
type parameter, in `Foo.Bar.apply[F](..)`.

    -- [E007] Type Mismatch Error: i12478.scala:8:13 -------------------------------
    8 |      Foo.Bar(fu1)
      |      ^^^^^^^^^^^^
      |Found:    Foo.Bar[F$1]
      |Required: Foo[T1]
      |
      |where:    F$1 is a type in method id1 with bounds <: [_] =>> Any
      |          T1  is a type in method id1 with bounds >: F$1[Int] and <: F[Int]
      |
      | longer explanation available when compiling with `-explain`
    -- [E007] Type Mismatch Error: i12478.scala:18:13 ------------------------------
    18 |      Foo.Bar(fu3)
       |      ^^^^^^^^^^^^
       |Found:    Foo.Bar[F$2]
       |Required: Foo[T3]
       |
       |where:    F$2 is a type in method id3 with bounds <: [_] =>> Any
       |          T3  is a type in method id3 with bounds >: F$2[Int] and <: F[Int]
       |
       | longer explanation available when compiling with `-explain`
---
 .../dotty/tools/dotc/core/TypeComparer.scala  |  7 +++++--
 tests/pos/i12478.scala                        | 19 +++++++++++++++++++
 2 files changed, 24 insertions(+), 2 deletions(-)
 create mode 100644 tests/pos/i12478.scala

diff --git a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
index a0eb5139eb07..cf2507aa1724 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -1338,8 +1338,11 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
             }
           } || tryLiftedToThis2
 
-        case _: TypeVar =>
-          recur(tp1, tp2.superType)
+        case tv: TypeVar =>
+          if tv.isInstantiated then
+            recur(tp1, tp2.superType)
+          else
+            compareAppliedType2(tp2, tv.origin, args2)
         case tycon2: AnnotatedType if !tycon2.isRefining =>
           recur(tp1, tp2.superType)
         case tycon2: AppliedType =>
diff --git a/tests/pos/i12478.scala b/tests/pos/i12478.scala
new file mode 100644
index 000000000000..d1e247ae4e68
--- /dev/null
+++ b/tests/pos/i12478.scala
@@ -0,0 +1,19 @@
+sealed trait Foo[T]
+
+object Foo:
+  case class Bar[F[_]](fu: List[F[Unit]]) extends Foo[F[Unit]]
+
+class Test:
+  def id1[T1](foo1: Foo[T1]): Foo[T1] = foo1 match
+    case Foo.Bar(fu) =>
+      Foo.Bar(fu)
+
+  def id2[T2](foo2: Foo[T2]): Foo[T2] = foo2 match
+    case bar2 @ (_: Foo.Bar[f]) =>
+      val fu2 = bar2.fu
+      Foo.Bar(fu2)
+
+  def id3[T3](foo3: Foo[T3]): Foo[T3] = foo3 match
+    case bar3 @ Foo.Bar(_) =>
+      val fu3 = bar3.fu
+      Foo.Bar(fu3)