From 194de0418ca545462716e3226c586030fe5942b8 Mon Sep 17 00:00:00 2001
From: Jason Zaugg <jzaugg@gmail.com>
Date: Thu, 25 Jul 2019 15:45:25 +1000
Subject: [PATCH] Improve static elision of outer tests in pattern matcher

When the binder's info is a bounded type, look at the prefix of the
upper bound.

This stops an annoying unchecked warning that appeared since
a recent refactoring made `TermName` a final class. It also avoids
a needless runtime check.
---
 .../tools/nsc/transform/patmat/MatchTreeMaking.scala | 12 +++++++++++-
 test/files/pos/t11534.flags                          |  1 +
 test/files/pos/t11534.scala                          |  7 +++++++
 3 files changed, 19 insertions(+), 1 deletion(-)
 create mode 100644 test/files/pos/t11534.flags
 create mode 100644 test/files/pos/t11534.scala

diff --git a/src/compiler/scala/tools/nsc/transform/patmat/MatchTreeMaking.scala b/src/compiler/scala/tools/nsc/transform/patmat/MatchTreeMaking.scala
index 4a6731744dc..c960d702823 100644
--- a/src/compiler/scala/tools/nsc/transform/patmat/MatchTreeMaking.scala
+++ b/src/compiler/scala/tools/nsc/transform/patmat/MatchTreeMaking.scala
@@ -349,7 +349,17 @@ trait MatchTreeMaking extends MatchCodeGen with Debugging {
 
         override def withOuterTest(orig: Tree)(testedBinder: Symbol, expectedTp: Type): Tree = {
           val expectedPrefix = expectedTp.prefix
-          val testedPrefix = testedBinder.info.prefix
+          def prefixOf(tp: Type): Type = tp.normalize match {
+            case RefinedType(parents, _) =>
+              val temp1 = parents.filterNot(_.typeSymbol.isStatic)
+              val temp2 = temp1.map(sym => prefixOf(sym))
+              temp2.distinct match {
+                case prefix :: Nil => prefix
+                case _ => NoType
+              }
+            case tp => tp.prefix
+          }
+          val testedPrefix = prefixOf(abstractTypesToBounds(testedBinder.info))
 
           // Check if a type is defined in a static location. Unlike `tp.isStatic` before `flatten`,
           // this also includes methods and (possibly nested) objects inside of methods.
diff --git a/test/files/pos/t11534.flags b/test/files/pos/t11534.flags
new file mode 100644
index 00000000000..b5a87486527
--- /dev/null
+++ b/test/files/pos/t11534.flags
@@ -0,0 +1 @@
+-Xfatal-warnings -unchecked
diff --git a/test/files/pos/t11534.scala b/test/files/pos/t11534.scala
new file mode 100644
index 00000000000..2e36dfec5b7
--- /dev/null
+++ b/test/files/pos/t11534.scala
@@ -0,0 +1,7 @@
+object Test1 {
+    val g: scala.tools.nsc.Global = ???
+    import g._
+    def test(sym: Symbol) = sym.name match {
+        case _: TermName  =>
+    }
+}