[ConstraintSystem] Remove an unsound optimization hack for generic functions.

I think there is something far more narrow we could do here, but I'm
not sure if there is real code that such a hack would benefit from. We
can leave that to consider another day.

Fixes rdar://problem/40819547 (aka https://bugs.swift.org/browse/SR-7884).

Author: rudkx

lib/Sema/ConstraintSystem.cpp

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift.org open source project
 //
-// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Copyright (c) 2014 - 2018 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See https://swift.org/LICENSE.txt for license information
@@ -1448,64 +1448,6 @@ ConstraintSystem::getTypeOfMemberReference(
   return { openedType, type };
 }
 
-// Performance hack: if there are two generic overloads, and one is
-// more specialized than the other, prefer the more-specialized one.
-static void tryOptimizeGenericDisjunction(ConstraintSystem &cs,
-                                          ArrayRef<OverloadChoice> choices,
-                                          OverloadChoice *&favoredChoice) {
-  if (favoredChoice || choices.size() != 2)
-    return;
-
-  const auto &choiceA = choices[0];
-  const auto &choiceB = choices[1];
-
-  if (!choiceA.isDecl() || !choiceB.isDecl())
-    return;
-
-  auto isViable = [](ValueDecl *decl) -> bool {
-    assert(decl);
-
-    auto *AFD = dyn_cast<AbstractFunctionDecl>(decl);
-    if (!AFD || !AFD->isGeneric())
-      return false;
-
-    auto funcType = AFD->getInterfaceType();
-    auto hasAny = funcType.findIf([](Type type) -> bool {
-      if (auto objType = type->getOptionalObjectType())
-        return objType->isAny();
-
-      return type->isAny();
-    });
-
-    // If function declaration references `Any` or `Any?` type
-    // let's not attempt it, because it's unclear
-    // without solving which overload is going to be better.
-    return !hasAny;
-  };
-
-  auto *declA = choiceA.getDecl();
-  auto *declB = choiceB.getDecl();
-
-  if (!isViable(declA) || !isViable(declB))
-    return;
-
-  auto &TC = cs.TC;
-  auto *DC = cs.DC;
-
-  switch (TC.compareDeclarations(DC, declA, declB)) {
-  case Comparison::Better:
-    favoredChoice = const_cast<OverloadChoice *>(&choiceA);
-    break;
-
-  case Comparison::Worse:
-    favoredChoice = const_cast<OverloadChoice *>(&choiceB);
-    break;
-
-  case Comparison::Unordered:
-    break;
-  }
-}
-
 void ConstraintSystem::addOverloadSet(Type boundType,
                                       ArrayRef<OverloadChoice> choices,
                                       DeclContext *useDC,
@@ -1520,8 +1462,6 @@ void ConstraintSystem::addOverloadSet(Type boundType,
     return;
   }
 
-  tryOptimizeGenericDisjunction(*this, choices, favoredChoice);
-
   SmallVector<Constraint *, 4> overloads;
 
   // As we do for other favored constraints, if a favored overload has been

test/Constraints/optional.swift

@@ -94,7 +94,7 @@ func test9(_ i: Int, io: Int?) {
   let _: Double = result2
 
   let result3 = test9_helper2(i)
-  var _: Double = result3
+  var _: Int = result3
   let result4 = test9_helper2(io)
   let _: Double = result4
 }

test/Constraints/sr7884.swift

@@ -0,0 +1,19 @@
+// RUN: %target-typecheck-verify-swift
+
+// SR-7884
+func f<T>(_ x: T) -> T {
+  return x
+}
+
+func f<T>(_ x: T?) -> T? {
+  return x
+}
+
+let r = f(1) // calls optional variant
+let _ = r! // expected-error {{cannot force unwrap value of non-optional type 'Int'}}
+
+// SR-7899
+let optSeq: LazySequence<[Int]>? = nil
+let lazySequence = optSeq?.lazy
+let value = lazySequence?.compactMap({ $0 as? Int }).first // expected-warning {{conditional cast from 'Int' to 'Int' always succeeds}}
+let _: Int = value!

validation-test/Sema/type_checker_perf/fast/more_specialized_generic_func.swift.gyb renamed to validation-test/Sema/type_checker_perf/slow/more_specialized_generic_func.swift.gyb

@@ -1,4 +1,4 @@
-// RUN: %scale-test --begin 1 --end 10 --step 1 --select incrementScopeCounter %s --expected-exit-code 0
+// RUN: %scale-test --invert-result --begin 1 --end 10 --step 1 --select incrementScopeCounter %s --expected-exit-code 0
 // REQUIRES: OS=macosx
 // REQUIRES: asserts