SI-5900 Fix pattern inference regression

This commit does not close SI-5900. It only addresses a regression
in 2.11 prereleases caused by SI-7886.

The fix for SI-7886 was incomplete (as shown by the last commit)
and incorrect (as shown by the regression in pos/t5900a.scala and
the fact it ended up inferring type parameters.)

I believe that the key to fixing this problem will be unifying
the inference of case class constructor patterns and extractor
patterns.

I've explored that idea:

  https://gist.github.com/retronym/7704153
  https://github.com/retronym/scala/compare/ticket/5900

But didn't quite get there.

src/compiler/scala/tools/nsc/typechecker/PatternTypers.scala

@@ -221,10 +221,12 @@ trait PatternTypers {
      * see test/files/../t5189*.scala
      */
     private def convertToCaseConstructor(tree: Tree, caseClass: Symbol, ptIn: Type): Tree = {
-      def untrustworthyPt = (
+      // TODO SI-7886 / SI-5900 This is well intentioned but doesn't quite hit the nail on the head.
+      //      For now, I've put it completely behind -Xstrict-inference.
+      val untrustworthyPt = settings.strictInference && (
            ptIn =:= AnyTpe
         || ptIn =:= NothingTpe
-        || settings.strictInference && ptIn.typeSymbol != caseClass
+        || ptIn.typeSymbol != caseClass
       )
       val variantToSkolem     = new VariantToSkolemMap
       val caseClassType       = tree.tpe.prefix memberType caseClass
@@ -371,4 +373,4 @@ trait PatternTypers {
       }
     }
   }
-}
+}

test/files/neg/t4818.check

@@ -1,6 +1,6 @@
 t4818.scala:4: error: type mismatch;
  found   : Int(5)
- required: A
+ required: Nothing
 	def f(x: Any) = x match { case Fn(f) => f(5) }
                                                   ^
 one error found

test/files/neg/t5189.check

@@ -1,5 +1,5 @@
 t5189.scala:3: error: type mismatch;
- found   : T => U
+ found   : Nothing => Any
  required: Any => Any
   def f(x: Any): Any => Any = x match { case Foo(bar) => bar }
                                                          ^

test/files/neg/t6680a.flags

@@ -0,0 +1 @@
+-Xstrict-inference

test/files/neg/t6829.check

@@ -16,32 +16,32 @@ t6829.scala:49: error: not found: value nextState
         val (s,a,s2) = (state,actions(agent),nextState)
                                              ^
 t6829.scala:50: error: type mismatch;
- found   : s.type (with underlying type T1)
+ found   : s.type (with underlying type Any)
  required: _53.State where val _53: G
         val r = rewards(agent).r(s,a,s2)
                                  ^
 t6829.scala:50: error: type mismatch;
- found   : a.type (with underlying type T2)
+ found   : a.type (with underlying type Any)
  required: _53.Action where val _53: G
         val r = rewards(agent).r(s,a,s2)
                                    ^
 t6829.scala:50: error: type mismatch;
- found   : s2.type (with underlying type T3)
+ found   : s2.type (with underlying type Any)
  required: _53.State where val _53: G
         val r = rewards(agent).r(s,a,s2)
                                      ^
 t6829.scala:51: error: type mismatch;
- found   : s.type (with underlying type T1)
+ found   : s.type (with underlying type Any)
  required: _50.State
         agent.learn(s,a,s2,r): G#Agent
                     ^
 t6829.scala:51: error: type mismatch;
- found   : a.type (with underlying type T2)
+ found   : a.type (with underlying type Any)
  required: _50.Action
         agent.learn(s,a,s2,r): G#Agent
                       ^
 t6829.scala:51: error: type mismatch;
- found   : s2.type (with underlying type T3)
+ found   : s2.type (with underlying type Any)
  required: _50.State
         agent.learn(s,a,s2,r): G#Agent
                         ^

test/files/pos/t5900a.scala

@@ -0,0 +1,9 @@
+case class Transition[S](x: S)
+
+object C
+
+object Test {
+  (??? : Any) match {
+    case Transition(C) =>
+  }
+}