Take the IUO-ness of a parameter declaration into account in ranking.

lib/Sema/CSRanking.cpp

@@ -384,6 +384,21 @@ static Type getAdjustedParamType(const AnyFunctionType::Param &param) {
   return param.getType();
 }
 
+// Is a particular parameter of a function or subscript declaration
+// declared to be an IUO?
+static bool paramIsIUO(Decl *decl, int paramNum) {
+  if (auto *fn = dyn_cast<AbstractFunctionDecl>(decl)) {
+    auto *paramList =
+        fn->getParameterList(fn->getDeclContext()->isTypeContext());
+    auto *param = paramList->get(paramNum);
+    return param->getAttrs().hasAttribute<ImplicitlyUnwrappedOptionalAttr>();
+  }
+
+  auto *subscript = cast<SubscriptDecl>(decl);
+  auto *index = subscript->getIndices()->get(paramNum);
+  return index->getAttrs().hasAttribute<ImplicitlyUnwrappedOptionalAttr>();
+}
+
 /// \brief Determine whether the first declaration is as "specialized" as
 /// the second declaration.
 ///
@@ -676,6 +691,17 @@ static bool isDeclAsSpecializedAs(TypeChecker &tc, DeclContext *dc,
             continue;
           }
 
+          // Emulate behavior from when IUO was a type, where IUOs
+          // were considered subtypes of plain optionals, but not
+          // vice-versa.  This wouldn't normally happen, but there are
+          // cases where we can rename imported APIs so that we have a
+          // name collision, and where the parameter type(s) are the
+          // same except for details of the kind of optional declared.
+          auto param1IsIUO = paramIsIUO(decl1, param1);
+          auto param2IsIUO = paramIsIUO(decl2, param2);
+          if (param2IsIUO && !param1IsIUO)
+            return false;
+
           if (!maybeAddSubtypeConstraint(params1[param1], params2[param2]))
             return false;
 

test/Constraints/Inputs/disambiguate_iuo_param.h

@@ -0,0 +1,9 @@
+@import Foundation;
+
+@interface NSObject (NSMyTest)
+- (BOOL)isEqualTo:(nullable id)object;
+@end
+
+@interface Obj : NSObject
+- (BOOL)isEqualToObject:(id)value;
+@end

test/Constraints/disambiguate_iuo_param.swift

@@ -0,0 +1,11 @@
+// RUN: %empty-directory(%t)
+// RUN: %build-clang-importer-objc-overlays
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) -import-objc-header %S/Inputs/disambiguate_iuo_param.h %s -emit-ir | %FileCheck %s
+// -module-name objc_ir -I %S/Inputs/custom-modules -emit-ir -g -o - -primary-file %s | %FileCheck %s
+
+// REQUIRES: objc_interop
+
+// CHECK: define {{.*}} @main
+// CHECK: load {{.*}}, {{.*}}@"\01L_selector(isEqualToObject:)"
+let c = Obj()
+_ = c.isEqual(to: c)