Merge pull request #2631 from retronym/ticket/5022

SI-5022 Retain precise existentials through pattern matching

4 files changed, 42 insertions, 4 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index 8d422f415d..27b03001ee 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -5111,7 +5111,7 @@ trait Typers extends Adaptations with Tags {
             if (mode.inPatternMode) {
               val uncheckedTypeExtractor = extractorForUncheckedType(tpt.pos, tptTyped.tpe)
               // make fully defined to avoid bounded wildcard types that may be in pt from calling dropExistential (SI-2038)
-              val ptDefined = ensureFullyDefined(pt)
+              val ptDefined = ensureFullyDefined(pt) // FIXME this is probably redundant now that we don't dropExistenial in pattern mode.
               val ownType = inferTypedPattern(tptTyped, tptTyped.tpe, ptDefined, canRemedy = uncheckedTypeExtractor.nonEmpty)
               treeTyped setType ownType
 
@@ -5345,7 +5345,12 @@ trait Typers extends Adaptations with Tags {
               "context.owner"    -> context.owner
             )
           )
-          typed1(tree, mode, dropExistential(ptPlugins))
+          val ptWild = if (mode.inPatternMode)
+            ptPlugins // SI-5022 don't widen pt for patterns as types flow from it to the case body.
+          else
+            dropExistential(ptPlugins) // FIXME: document why this is done.
+
+          typed1(tree, mode, ptWild)
         }
         // Can happen during erroneous compilation - error(s) have been
         // reported, but we need to avoid causing an NPE with this tree
diff --git a/test/files/neg/t4515.check b/test/files/neg/t4515.check
index a60d16295f..64e7cc1ca7 100644
--- a/test/files/neg/t4515.check
+++ b/test/files/neg/t4515.check
@@ -1,6 +1,6 @@
 t4515.scala:37: error: type mismatch;
- found   : _0(in value $anonfun) where type _0(in value $anonfun)
- required: (some other)_0(in value $anonfun)
+ found   : _$1 where type _$1
+ required: _$2
         handler.onEvent(target, ctx.getEvent, node, ctx)
                                     ^
 one error found
diff --git a/test/files/pos/t2613.scala b/test/files/pos/t2613.scala
new file mode 100644
index 0000000000..3a64dbc282
--- /dev/null
+++ b/test/files/pos/t2613.scala
@@ -0,0 +1,11 @@
+import language.existentials
+
+object Test {
+  class Row
+
+  abstract class MyRelation [R <: Row, +Relation <: MyRelation[R, Relation]]
+
+  type M = MyRelation[R, Relation] forSome {type R <: Row; type Relation <: MyRelation[R, Relation]}
+
+  var (x,y): (String, M) = null
+}
diff --git a/test/files/pos/t5022.scala b/test/files/pos/t5022.scala
new file mode 100644
index 0000000000..b9a085fb35
--- /dev/null
+++ b/test/files/pos/t5022.scala
@@ -0,0 +1,22 @@
+class ForSomeVsUnapply {
+  def test {
+    def makeWrap: Wrap = ???
+    def useRep[e](rep: (e, X[e])) = ()
+
+    val repUnapply = Wrap.unapply(makeWrap).get
+    useRep(repUnapply)  // okay
+
+    val Wrap(rep0) = makeWrap
+    useRep(rep0) // error
+
+    val rep = makeWrap match {
+      case Wrap(r) => r
+    };
+
+    useRep(rep) // error
+  }
+}
+
+class X[e]
+
+case class Wrap(rep: (e, X[e]) forSome { type e })