[ConstraintSystem] Increase the score when attempting overload choices

for unapplied references when the choice is a function declaration.

This will allow the solver to prune those overload choices when it
has already found a solultion with a property (all else equal in the
score). This is already done as an ambiguity tie-breaker in solution
ranking, but adding this bit to the score will prune a lot of search
space within the solver.

Author: hborla

include/swift/Sema/ConstraintSystem.h

@@ -768,8 +768,18 @@ enum ScoreKind {
   SK_ValueToPointerConversion,
   /// A closure/function conversion to an autoclosure parameter.
   SK_FunctionToAutoClosureConversion,
-
-  SK_LastScoreKind = SK_FunctionToAutoClosureConversion,
+  /// An unapplied reference to a function. The purpose of this
+  /// score bit is to prune overload choices that are functions
+  /// when a solution has already been found using property.
+  ///
+  /// \Note The solver only prefers variables over functions
+  /// to resolve ambiguities, so please be sure that any score
+  /// kind added after this is truly less impactful. Only other
+  /// ambiguity tie-breakers should go after this; anything else
+  /// should be added above.
+  SK_UnappliedFunction,
+
+  SK_LastScoreKind = SK_UnappliedFunction,
 };
 
 /// The number of score kinds.

lib/Sema/CSRanking.cpp

@@ -90,6 +90,9 @@ static StringRef getScoreKindName(ScoreKind kind) {
 
   case SK_ImplicitValueConversion:
     return "value-to-value conversion";
+
+  case SK_UnappliedFunction:
+    return "overloaded unapplied function";
   }
 }
 

lib/Sema/ConstraintSystem.cpp

@@ -2947,6 +2947,12 @@ void ConstraintSystem::resolveOverload(ConstraintLocator *locator,
       }
     }
 
+    if (isa<AbstractFunctionDecl>(decl) || isa<TypeDecl>(decl)) {
+      if (choice.getFunctionRefKind() == FunctionRefKind::Unapplied) {
+        increaseScore(SK_UnappliedFunction);
+      }
+    }
+
     if (auto *afd = dyn_cast<AbstractFunctionDecl>(decl)) {
       // Check whether applying this overload would result in invalid
       // partial function application e.g. partial application of