add missing constexpr to optional::value_or

[Credit to cpplearner]

Differential Revision: https://reviews.llvm.org/D27850

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@304813 91177308-0d34-0410-b5e6-96231b3b80d8

include/optional

@@ -897,7 +897,7 @@ public:
 
     template <class _Up>
     _LIBCPP_INLINE_VISIBILITY
-    value_type value_or(_Up&& __v) &&
+    constexpr value_type value_or(_Up&& __v) &&
     {
         static_assert(is_move_constructible_v<value_type>,
                       "optional<T>::value_or: T must be move constructible");

test/std/utilities/optional/optional.object/optional.object.observe/value_or.pass.cpp

@@ -10,7 +10,7 @@
 // UNSUPPORTED: c++98, c++03, c++11, c++14
 // <optional>
 
-// template <class U> T optional<T>::value_or(U&& v) &&;
+// template <class U> constexpr T optional<T>::value_or(U&& v) &&;
 
 #include <optional>
 #include <type_traits>
@@ -26,22 +26,22 @@ struct Y
 {
     int i_;
 
-    Y(int i) : i_(i) {}
+    constexpr Y(int i) : i_(i) {}
 };
 
 struct X
 {
     int i_;
 
-    X(int i) : i_(i) {}
-    X(X&& x) : i_(x.i_) {x.i_ = 0;}
-    X(const Y& y) : i_(y.i_) {}
-    X(Y&& y) : i_(y.i_+1) {}
+    constexpr X(int i) : i_(i) {}
+    constexpr X(X&& x) : i_(x.i_) {x.i_ = 0;}
+    constexpr X(const Y& y) : i_(y.i_) {}
+    constexpr X(Y&& y) : i_(y.i_+1) {}
     friend constexpr bool operator==(const X& x, const X& y)
         {return x.i_ == y.i_;}
 };
 
-int main()
+constexpr int test()
 {
     {
         optional<X> opt(in_place, 2);
@@ -65,4 +65,10 @@ int main()
         assert(std::move(opt).value_or(Y(3)) == 4);
         assert(!opt);
     }
+    return 0;
+}
+
+int main()
+{
+    static_assert(test() == 0);
 }