[CSOptimizer] Introduce _OptionalNilComparisonType as candidate for nil while ranking == and != operators

`==` and `!=` operators have special overloads that allow matching
`nil` literal on either side even if wrapped type on the other side
doesn't conform to `Equatable`.

Author: xedin

include/swift/AST/KnownStdlibTypes.def

@@ -70,6 +70,7 @@ KNOWN_STDLIB_TYPE_DECL(WritableKeyPath, NominalTypeDecl, 2)
 KNOWN_STDLIB_TYPE_DECL(ReferenceWritableKeyPath, NominalTypeDecl, 2)
 
 KNOWN_STDLIB_TYPE_DECL(Optional, EnumDecl, 1)
+KNOWN_STDLIB_TYPE_DECL(_OptionalNilComparisonType, NominalTypeDecl, 0)
 
 KNOWN_STDLIB_TYPE_DECL(OptionSet, NominalTypeDecl, 1)
 

lib/Sema/CSOptimizer.cpp

@@ -113,6 +113,13 @@ static bool isStandardComparisonOperator(ValueDecl *decl) {
          decl->getBaseIdentifier().isStandardComparisonOperator();
 }
 
+static bool isOperatorNamed(Constraint *disjunction, StringRef name) {
+  auto *choice = disjunction->getNestedConstraints()[0];
+  if (auto *decl = getOverloadChoiceDecl(choice))
+    return decl->isOperator() && decl->getBaseIdentifier().is(name);
+  return false;
+}
+
 static bool isArithmeticOperator(ValueDecl *decl) {
   return decl->isOperator() && decl->getBaseIdentifier().isArithmeticOperator();
 }
@@ -622,6 +629,19 @@ static void determineBestChoicesInContext(
                                            optionals.size());
             types.push_back({type,
                              /*fromLiteral=*/true});
+          } else if (literal.first ==
+                         cs.getASTContext().getProtocol(
+                             KnownProtocolKind::ExpressibleByNilLiteral) &&
+                     literal.second.IsDirectRequirement) {
+            // `==` and `!=` operators have special overloads that accept `nil`
+            // as `_OptionalNilComparisonType` which is preferred over a
+            // generic form `(T?, T?)`.
+            if (isOperatorNamed(disjunction, "==") ||
+                isOperatorNamed(disjunction, "!=")) {
+              auto nilComparisonTy =
+                  cs.getASTContext().get_OptionalNilComparisonTypeType();
+              types.push_back({nilComparisonTy, /*fromLiteral=*/true});
+            }
           }
         }