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.

8 files changed, 18 insertions, 14 deletions

diff --git a/test/files/neg/t4818.check b/test/files/neg/t4818.check
index 8a2c024b30..a5e15e456b 100644
--- a/test/files/neg/t4818.check
+++ b/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
diff --git a/test/files/neg/t5189.check b/test/files/neg/t5189.check
index aecc1d11c4..4885de99cd 100644
--- a/test/files/neg/t5189.check
+++ b/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 }
                                                          ^
diff --git a/test/files/neg/t6680a.flags b/test/files/neg/t6680a.flags
new file mode 100644
index 0000000000..19243266d1
--- /dev/null
+++ b/test/files/neg/t6680a.flags
@@ -0,0 +1 @@
+-Xstrict-inference
\ No newline at end of file
diff --git a/test/files/neg/t6829.check b/test/files/neg/t6829.check
index a0b43e3b52..914a1c9260 100644
--- a/test/files/neg/t6829.check
+++ b/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
                         ^
diff --git a/test/files/neg/t7886.check b/test/files/neg/t7886.check
deleted file mode 100644
index 338eee9708..0000000000
--- a/test/files/neg/t7886.check
+++ /dev/null
@@ -1,6 +0,0 @@
-t7886.scala:10: error: type mismatch;
- found   : Contra[A]
- required: Contra[Any]
-    case Unravel(m, msg) => g(m)
-                              ^
-one error found
diff --git a/test/files/pos/t5900a.scala b/test/files/pos/t5900a.scala
new file mode 100644
index 0000000000..cb02f67fb2
--- /dev/null
+++ b/test/files/pos/t5900a.scala
@@ -0,0 +1,9 @@
+case class Transition[S](x: S)
+
+object C
+
+object Test {
+  (??? : Any) match {
+    case Transition(C) =>
+  }
+}
diff --git a/test/files/neg/t7886.scala b/test/pending/neg/t7886.scala
index 55d80a0a43..55d80a0a43 100644
--- a/test/files/neg/t7886.scala
+++ b/test/pending/neg/t7886.scala
diff --git a/test/files/pos/pattern-typing.scala b/test/pending/pos/pattern-typing.scala
index 7286cc38af..7286cc38af 100644
--- a/test/files/pos/pattern-typing.scala
+++ b/test/pending/pos/pattern-typing.scala