Add constexpr to std::optional::value_or(U&&)&&

In C++1z drafts up to N4606 the constexpr keyword was missing from the
detailed description of this function, despite being shown in the class
synopsis.  That was fixed editorially for N4618, but our implementation
was not corrected to match.

	* include/std/optional (optional::value_or(U&&) &&): Add missing
	constexpr specifier.
	* testsuite/20_util/optional/constexpr/observers/4.cc: Check value_or
	for disengaged optionals and rvalue optionals.
	* testsuite/20_util/optional/observers/4.cc: Likewise.

From-SVN: r270409

libstdc++-v3/ChangeLog

+2019-04-17  Jonathan Wakely  <jwakely@redhat.com>
+
+	* include/std/optional (optional::value_or(U&&) &&): Add missing
+	constexpr specifier.
+	* testsuite/20_util/optional/constexpr/observers/4.cc: Check value_or
+	for disengaged optionals and rvalue optionals.
+	* testsuite/20_util/optional/observers/4.cc: Likewise.
+
 2019-04-12  Thomas Rodgers  <trodgers@redhat.com>
 	
 	* include/pstl/algorithm_impl.h: Uglify identfiers.

libstdc++-v3/include/std/optional

@@ -959,7 +959,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	}
 
       template<typename _Up>
-	_Tp
+	constexpr _Tp
 	value_or(_Up&& __u) &&
 	{
 	  static_assert(is_move_constructible_v<_Tp>);

libstdc++-v3/testsuite/20_util/optional/constexpr/observers/4.cc

@@ -25,10 +25,42 @@ struct value_type
   int i;
 };
 
-int main()
+void test01()
 {
   constexpr std::optional<value_type> o { value_type { 51 } };
   constexpr value_type fallback { 3 };
-  static_assert( o.value_or(fallback).i == 51, "" );
-  static_assert( o.value_or(fallback).i == (*o).i, "" );
+  static_assert( o.value_or(fallback).i == 51 );
+  static_assert( o.value_or(fallback).i == (*o).i );
+}
+
+void test02()
+{
+  constexpr std::optional<value_type> o;
+  constexpr value_type fallback { 3 };
+  static_assert( o.value_or(fallback).i == 3 );
+}
+
+template<typename T>
+  constexpr std::optional<value_type>
+  make_rvalue(T t)
+  { return std::optional<value_type>{t}; }
+
+void test03()
+{
+  constexpr value_type fallback { 3 };
+  static_assert( make_rvalue(value_type{51}).value_or(fallback).i == 51 );
+}
+
+void test04()
+{
+  constexpr value_type fallback { 3 };
+  static_assert( make_rvalue(std::nullopt).value_or(fallback).i == 3 );
+}
+
+int main()
+{
+  test01();
+  test02();
+  test03();
+  test04();
 }

libstdc++-v3/testsuite/20_util/optional/observers/4.cc

@@ -26,10 +26,42 @@ struct value_type
   int i;
 };
 
-int main()
+void test01()
 {
   std::optional<value_type> o { value_type { 51 } };
   value_type fallback { 3 };
   VERIFY( o.value_or(fallback).i == 51 );
   VERIFY( o.value_or(fallback).i == (*o).i );
 }
+
+void test02()
+{
+  std::optional<value_type> o;
+  value_type fallback { 3 };
+  VERIFY( o.value_or(fallback).i == 3 );
+}
+
+void test03()
+{
+  std::optional<value_type> o { value_type { 51 } };
+  value_type fallback { 3 };
+  VERIFY( std::move(o).value_or(fallback).i == 51 );
+  VERIFY( o.has_value() );
+  VERIFY( std::move(o).value_or(fallback).i == (*o).i );
+}
+
+void test04()
+{
+  std::optional<value_type> o;
+  value_type fallback { 3 };
+  VERIFY( std::move(o).value_or(fallback).i == 3 );
+  VERIFY( !o.has_value() );
+}
+
+int main()
+{
+  test01();
+  test02();
+  test03();
+  test04();
+}