From 7eb4042d5b1462ce14e1ee542cdf564b06965d69 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Thu, 12 Mar 2020 16:51:34 +0100
Subject: [PATCH 1/2] Fix #8516: Keep correct parameter names when selecting in
 refined types

---
 compiler/src/dotty/tools/dotc/core/Types.scala | 8 +++++++-
 tests/pos/i8516.scala                          | 3 +++
 2 files changed, 10 insertions(+), 1 deletion(-)
 create mode 100644 tests/pos/i8516.scala

diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index 4f627c815073..d51210af0b73 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -673,10 +673,16 @@ object Types {
           pdenot.asSingleDenotation.derivedSingleDenotation(pdenot.symbol, jointInfo)
         }
         else
-          pdenot & (
+          val joint = pdenot & (
             new JointRefDenotation(NoSymbol, rinfo, Period.allInRun(ctx.runId), pre),
             pre,
             safeIntersection = ctx.base.pendingMemberSearches.contains(name))
+          joint match
+            case joint: SingleDenotation
+            if rinfo.isInstanceOf[MethodOrPoly] && rinfo <:< joint.info =>
+              joint.derivedSingleDenotation(joint.symbol, rinfo)
+            case _ =>
+              joint
       }
 
       def goApplied(tp: AppliedType, tycon: HKTypeLambda) =
diff --git a/tests/pos/i8516.scala b/tests/pos/i8516.scala
new file mode 100644
index 000000000000..9e95e3e4eb02
--- /dev/null
+++ b/tests/pos/i8516.scala
@@ -0,0 +1,3 @@
+val x: Function1[Int, Int] { def apply(arg: Int): Int } = x => x
+val x1 = x
+val y = x.apply(arg = 1)
\ No newline at end of file

From 465b81b2586e64c02d07841ca813122b7fbba1f2 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Thu, 12 Mar 2020 20:51:21 +0100
Subject: [PATCH 2/2] Add explanation comment

---
 compiler/src/dotty/tools/dotc/core/Types.scala | 1 +
 1 file changed, 1 insertion(+)

diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index d51210af0b73..55f6c6e06662 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -680,6 +680,7 @@ object Types {
           joint match
             case joint: SingleDenotation
             if rinfo.isInstanceOf[MethodOrPoly] && rinfo <:< joint.info =>
+              // use `rinfo` to keep the right parameter names for named args. See i8516.scala.
               joint.derivedSingleDenotation(joint.symbol, rinfo)
             case _ =>
               joint