From 9afa629bc844340527361473e0326a64d9e6a5da Mon Sep 17 00:00:00 2001
From: Dale Wijnand <dale.wijnand@gmail.com>
Date: Wed, 8 Mar 2023 18:40:14 +0000
Subject: [PATCH] AppliedTypes returned by baseType shouldn't have type lambdas
 as constructors

In tests/pos/argDenot-alpakka.min.scala, we want

    `(([O] =>> Foo[O, S]) @uV)[Int]`.baseType(`Foo`)

to return `Foo[Int]` rather than an applied type lambda, so that we can rely on
the invariant that the type arguments of the type returned by baseType(cls)
correspond to the type parameter of cls which `NamedType#argDenot` is implicitly
relying on.

This could be achieved by removing the initial if branch from the AppliedType
case in baseTypeOf, since the recursive fallback will always work, but it makes
sense to keep a special case for performance, so we just explicitly add as a
condition to the fast-path that the type constructor of the AppliedType can't be
a lambda.
---
 .../tools/dotc/core/SymDenotations.scala      |  5 ++---
 tests/pos/argDenot-alpakka.min.scala          |  9 ++++++++
 tests/pos/argDenot-alpakka.scala              | 21 +++++++++++++++++++
 3 files changed, 32 insertions(+), 3 deletions(-)
 create mode 100644 tests/pos/argDenot-alpakka.min.scala
 create mode 100644 tests/pos/argDenot-alpakka.scala

diff --git a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
index 0f94d52f5be9..beeaa2ee922e 100644
--- a/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
+++ b/compiler/src/dotty/tools/dotc/core/SymDenotations.scala
@@ -2226,13 +2226,12 @@ object SymDenotations {
             def computeApplied = {
               btrCache(tp) = NoPrefix
               val baseTp =
-                if (tycon.typeSymbol eq symbol) tp
-                else (tycon.typeParams: @unchecked) match {
+                if (tycon.typeSymbol eq symbol) && !tycon.isLambdaSub then tp
+                else (tycon.typeParams: @unchecked) match
                   case LambdaParam(_, _) :: _ =>
                     recur(tp.superType)
                   case tparams: List[Symbol @unchecked] =>
                     recur(tycon).substApprox(tparams, args)
-                }
               record(tp, baseTp)
               baseTp
             }
diff --git a/tests/pos/argDenot-alpakka.min.scala b/tests/pos/argDenot-alpakka.min.scala
new file mode 100644
index 000000000000..0e509be59cfd
--- /dev/null
+++ b/tests/pos/argDenot-alpakka.min.scala
@@ -0,0 +1,9 @@
+import scala.annotation.unchecked.uncheckedVariance as uV
+
+trait Test:
+  def test[S] =
+    val a: (([O] =>> Foo[O, S]) @uV)[Int] = ???
+    a.m()
+
+class Foo[X, Y]:
+  def m(): Y = ???
diff --git a/tests/pos/argDenot-alpakka.scala b/tests/pos/argDenot-alpakka.scala
new file mode 100644
index 000000000000..41d6ad52ac97
--- /dev/null
+++ b/tests/pos/argDenot-alpakka.scala
@@ -0,0 +1,21 @@
+import scala.annotation.unchecked.uncheckedVariance as uV
+
+trait Test:
+  def split[I, M](in: Flow[I, Byte, M]): SubFlow[Byte, M, in.Repr]
+  def test =
+    split(new Flow[Int, Byte, Unit])
+      .via[Char]
+      .merge
+      .filter()
+
+trait FlowOps[+Out, +Mat]:
+  type Repr[+O] <: FlowOps[O, Mat] { type Repr[+O] = FlowOps.this.Repr[O] }
+  def via[O]:   Repr[O]   = ???
+  def filter(): Repr[Out] = ???
+
+class Flow[-In, +Out, +Mat] extends FlowOps[Out, Mat]:
+  type Repr[+O] = Flow[In @uV, O, Mat @uV]
+
+class SubFlow[+Out, +Mat, +F[+_]] extends FlowOps[Out, Mat]:
+  type Repr[+O] = SubFlow[O, Mat @uV, F @uV]
+  def merge: F[Out] = ???