[CSBindings] Don't try to rank bindings based on number of defaults if they are literal

Number of defaults is a fallback condition if there are no other
differences, but pontential bindings which are literal should always
be ranked lower than anything else because there is a higher chance
that such bindings are wrong because their full set might not have
been resolved yet.

Resolves: rdar://problem/39616039

Author: xedin

lib/Sema/ConstraintSystem.h

@@ -2790,10 +2790,7 @@ class ConstraintSystem {
       if (formBindingScore(x) < formBindingScore(y))
         return true;
 
-      if (!x.hasNonDefaultableBindings())
-        return false;
-
-      if (x.FullyBound || x.SubtypeOfExistentialType)
+      if (formBindingScore(y) < formBindingScore(x))
         return false;
 
       // If the only difference is default types,

test/Constraints/generics.swift

@@ -561,3 +561,20 @@ do {
 func rdar35890334(_ arr: inout [Int]) {
   _ = arr.popFirst() // expected-error {{'[Int]' requires the types '[Int]' and 'ArraySlice<Int>' be equivalent to use 'popFirst'}}
 }
+
+// rdar://problem/39616039
+
+func rdar39616039() {
+  func foo<V>(default: V, _ values: [String: V]) -> V {
+    return values["foo"] ?? `default`
+  }
+
+  var a = foo(default: 42, ["foo": 0])
+  a += 1 // ok
+
+  var b = foo(default: 42.0, ["foo": 0])
+  b += 1 // ok
+
+  var c = foo(default: 42.0, ["foo": Float(0)])
+  c += 1 // ok
+}