From 101ce3afca2a0a6c41783fa7cbf1fdf1be2281c6 Mon Sep 17 00:00:00 2001
From: Jan Chyb <jan.chyb.99@gmail.com>
Date: Tue, 25 Jul 2023 12:56:09 +0200
Subject: [PATCH] Fix regression with Overloaded methods returning Functions

Before the regression, FunctionOf unapply would not try dealiasing,
meaning that an aliased function type would be handled by a general
case.
To fix that, instead of handling Function types separately when
filtering overloaded methods in `resolveOverloaded1`, we allow to
fallback to the general case if the previous one returns nothing.
Along with fixing the regression, this also improves other cases,
one of which was added to the test.

Readd a separate FunctionOf case, but with a fallback
---
 .../dotty/tools/dotc/typer/Applications.scala | 52 ++++++++++---------
 tests/pos/i17245.scala                        | 20 +++++++
 2 files changed, 48 insertions(+), 24 deletions(-)
 create mode 100644 tests/pos/i17245.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Applications.scala b/compiler/src/dotty/tools/dotc/typer/Applications.scala
index d2d36ce9e242..81d7ad23913b 100644
--- a/compiler/src/dotty/tools/dotc/typer/Applications.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Applications.scala
@@ -2059,31 +2059,35 @@ trait Applications extends Compatibility {
           if isDetermined(alts2) then alts2
           else resolveMapped(alts1, _.widen.appliedTo(targs1.tpes), pt1)
 
-      case defn.FunctionOf(args, resultType, _) =>
-        narrowByTypes(alts, args, resultType)
-
       case pt =>
-        val compat = alts.filterConserve(normalizedCompatible(_, pt, keepConstraint = false))
-        if (compat.isEmpty)
-          /*
-           * the case should not be moved to the enclosing match
-           * since SAM type must be considered only if there are no candidates
-           * For example, the second f should be chosen for the following code:
-           *   def f(x: String): Unit = ???
-           *   def f: java.io.OutputStream = ???
-           *   new java.io.ObjectOutputStream(f)
-           */
-          pt match {
-            case SAMType(mtp, _) =>
-              narrowByTypes(alts, mtp.paramInfos, mtp.resultType)
-            case _ =>
-              // pick any alternatives that are not methods since these might be convertible
-              // to the expected type, or be used as extension method arguments.
-              val convertible = alts.filterNot(alt =>
-                  normalize(alt, IgnoredProto(pt)).widenSingleton.isInstanceOf[MethodType])
-              if convertible.length == 1 then convertible else compat
-          }
-        else compat
+        val compat0 = pt match
+          case defn.FunctionOf(args, resType, _) =>
+            narrowByTypes(alts, args, resType)
+          case _ =>
+            Nil
+        if (compat0.isEmpty) then
+          val compat = alts.filterConserve(normalizedCompatible(_, pt, keepConstraint = false))
+          if (compat.isEmpty)
+            /*
+            * the case should not be moved to the enclosing match
+            * since SAM type must be considered only if there are no candidates
+            * For example, the second f should be chosen for the following code:
+            *   def f(x: String): Unit = ???
+            *   def f: java.io.OutputStream = ???
+            *   new java.io.ObjectOutputStream(f)
+            */
+            pt match {
+              case SAMType(mtp, _) =>
+                narrowByTypes(alts, mtp.paramInfos, mtp.resultType)
+              case _ =>
+                // pick any alternatives that are not methods since these might be convertible
+                // to the expected type, or be used as extension method arguments.
+                val convertible = alts.filterNot(alt =>
+                    normalize(alt, IgnoredProto(pt)).widenSingleton.isInstanceOf[MethodType])
+                if convertible.length == 1 then convertible else compat
+            }
+          else compat
+        else compat0
     }
 
     /** The type of alternative `alt` after instantiating its first parameter
diff --git a/tests/pos/i17245.scala b/tests/pos/i17245.scala
new file mode 100644
index 000000000000..3b5b3a74108d
--- /dev/null
+++ b/tests/pos/i17245.scala
@@ -0,0 +1,20 @@
+import scala.reflect.ClassTag
+
+trait MockSettings
+
+object Mockito {
+  def mock[T : ClassTag]: T = ???
+  def mock[T : ClassTag](settings: MockSettings): T = ???
+}
+
+trait Channel
+type OnChannel = Channel => Any
+
+@main def Test =
+  val case1: OnChannel = Mockito.mock[OnChannel]
+  val case2: OnChannel = Mockito.mock
+  val case3 = Mockito.mock[OnChannel]
+  val case4: OnChannel =  Mockito.mock[OnChannel](summon[ClassTag[OnChannel]])
+
+  // not a regressive case, but an added improvement with the fix for the above
+  val case5: Channel => Any = Mockito.mock[Channel => Any]