Make base::Optional constructor constexpr.

base/optional_unittest.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/optional.h"
 
 #include <set>
 
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ skipping 78 matching lines @@
 
 }  // anonymous namespace
 
 static_assert(is_trivially_destructible<Optional<int>>::value,
               "OptionalIsTriviallyDestructible");
 
 static_assert(
     !is_trivially_destructible<Optional<NonTriviallyDestructible>>::value,
     "OptionalIsTriviallyDestructible");
 
+static_assert(!base::Optional<bool>(), "OptionalHasConstexprConstructor");

[danakj, 2016/10/21 21:05:01]

Can you add tests for the other constructors as well that should be constexpr?
http://en.cppreference.com/w/cpp/utility/optional/optional

[alshabalin, 2016/10/21 22:36:05]

Done (partially). Reading http://en.cppreference.com/w/cpp/language/constexpr it
seems that until C++14 constructors of union-like classes cannot be constexpr.
This means that only Optional of types with trivial special member functions can
have constexpr constructors.

+
 TEST(OptionalTest, DefaultConstructor) {
   {
     Optional<float> o;
     EXPECT_FALSE(o);
   }
 
   {
     Optional<std::string> o;
     EXPECT_FALSE(o);
   }
@@ skipping 1228 matching lines @@
 
 TEST(OptionalTest, Reset_NoOp) {
   Optional<int> a;
   EXPECT_FALSE(a.has_value());
 
   a.reset();
   EXPECT_FALSE(a.has_value());
 }
 
 }  // namespace base