Merge pull request #14521 from xedin/rdar-37290898

[CSSolver] Prioritize bindings with fewer default types

Author: xedin

lib/Sema/ConstraintSystem.h

@@ -2758,6 +2758,9 @@ class ConstraintSystem {
       if (!x.hasNonDefaultableBindings())
         return false;
 
+      if (x.FullyBound || x.SubtypeOfExistentialType)
+        return false;
+
       llvm::SmallPtrSet<Constraint *, 8> intersection(x.Sources);
       llvm::set_intersect(intersection, y.Sources);
 
@@ -2774,7 +2777,9 @@ class ConstraintSystem {
           return x.TypeVar == typeVar;
       }
 
-      return false;
+      // If the only difference is default types,
+      // prioritize bindings with fewer of them.
+      return x.NumDefaultableBindings < y.NumDefaultableBindings;
     }
 
     void foundLiteralBinding(ProtocolDecl *proto) {

test/Constraints/operator.swift

@@ -194,3 +194,10 @@ struct S_35740653 {
 func rdar35740653(val: S_35740653) {
   let _ = 0...Int(val / .value(1.0 / 42.0)) // Ok
 }
+
+protocol P_37290898 {}
+struct S_37290898: P_37290898 {}
+
+func rdar37290898(_ arr: inout [P_37290898], _ element: S_37290898?) {
+  arr += [element].compactMap { $0 } // Ok
+}