libstdc++: Add [[nodiscard]] to some std::locale functions

libstdc++-v3/ChangeLog:

	* include/bits/locale_classes.h (locale::combine)
	(locale::name, locale::operator==, locale::operator!=)
	(locale::operator(), locale::classic): Add nodiscard
	attribute.
	* include/bits/locale_classes.tcc (has_facet, use_facet):
	Likewise.
	* testsuite/22_locale/locale/cons/12438.cc: Add dg-warning for
	nodiscard diagnostic.
	* testsuite/22_locale/locale/cons/2.cc: Cast use_facet
	expression to void, to suppress diagnostic.
	* testsuite/22_locale/locale/cons/unicode.cc: Likewise.
	* testsuite/22_locale/locale/operations/2.cc: Add dg-warning.

6 files changed, 12 insertions, 5 deletions

diff --git a/libstdc++-v3/include/bits/locale_classes.h b/libstdc++-v3/include/bits/locale_classes.h
index a2e9421..50a7480 100644
--- a/libstdc++-v3/include/bits/locale_classes.h
+++ b/libstdc++-v3/include/bits/locale_classes.h
@@ -240,6 +240,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
      *  @throw  std::runtime_error if __other has no facet of type _Facet.
     */
     template<typename _Facet>
+      _GLIBCXX_NODISCARD
       locale
       combine(const locale& __other) const;
 
@@ -248,7 +249,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
      *  @brief  Return locale name.
      *  @return  Locale name or "*" if unnamed.
     */
-    _GLIBCXX_DEFAULT_ABI_TAG
+    _GLIBCXX_NODISCARD _GLIBCXX_DEFAULT_ABI_TAG
     string
     name() const;
 
@@ -269,6 +270,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
      *  @return  True if other and this refer to the same locale instance, are
      *		 copies, or have the same name.  False otherwise.
     */
+    _GLIBCXX_NODISCARD
     bool
     operator==(const locale& __other) const throw();
 
@@ -279,6 +281,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
      *  @param  __other  The locale to compare against.
      *  @return  ! (*this == __other)
     */
+    _GLIBCXX_NODISCARD
     bool
     operator!=(const locale& __other) const throw()
     { return !(this->operator==(__other)); }
@@ -300,6 +303,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
      *  @return  True if collate<_Char> facet compares __s1 < __s2, else false.
     */
     template<typename _Char, typename _Traits, typename _Alloc>
+      _GLIBCXX_NODISCARD
       bool
       operator()(const basic_string<_Char, _Traits, _Alloc>& __s1,
 		 const basic_string<_Char, _Traits, _Alloc>& __s2) const;
@@ -321,6 +325,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     /**
      *  @brief  Return reference to the C locale.
     */
+    _GLIBCXX_NODISCARD
     static const locale&
     classic();
 
diff --git a/libstdc++-v3/include/bits/locale_classes.tcc b/libstdc++-v3/include/bits/locale_classes.tcc
index 00eeb7d..c79574e 100644
--- a/libstdc++-v3/include/bits/locale_classes.tcc
+++ b/libstdc++-v3/include/bits/locale_classes.tcc
@@ -173,6 +173,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @return  true if @p __loc contains a facet of type _Facet, else false.
   */
   template<typename _Facet>
+    _GLIBCXX_NODISCARD
     inline bool
     has_facet(const locale& __loc) _GLIBCXX_USE_NOEXCEPT
     {
@@ -202,6 +203,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wdangling-reference"
   template<typename _Facet>
+    _GLIBCXX_NODISCARD
     inline const _Facet&
     use_facet(const locale& __loc)
     {
diff --git a/libstdc++-v3/testsuite/22_locale/locale/cons/12438.cc b/libstdc++-v3/testsuite/22_locale/locale/cons/12438.cc
index 7ff3a48..4838e1b 100644
--- a/libstdc++-v3/testsuite/22_locale/locale/cons/12438.cc
+++ b/libstdc++-v3/testsuite/22_locale/locale/cons/12438.cc
@@ -45,7 +45,7 @@ void test01(int iters)
 	  locale loc2 = locale("");
 	  VERIFY( !has_facet<MyFacet>(loc2) );
 	  
-	  loc1.combine<MyFacet>(loc2);
+	  loc1.combine<MyFacet>(loc2); // { dg-warning "nodiscard" "" { target c++17 } }
 	  VERIFY( false );
 	}
       catch (std::runtime_error&)
diff --git a/libstdc++-v3/testsuite/22_locale/locale/cons/2.cc b/libstdc++-v3/testsuite/22_locale/locale/cons/2.cc
index 12478db..dce150e 100644
--- a/libstdc++-v3/testsuite/22_locale/locale/cons/2.cc
+++ b/libstdc++-v3/testsuite/22_locale/locale/cons/2.cc
@@ -68,7 +68,7 @@ void test01()
     { VERIFY( false ); }
 
   try 
-    { use_facet<gnu_facet>(loc02); }
+    { (void) use_facet<gnu_facet>(loc02); }
   catch(bad_cast& obj)
     { VERIFY( true ); }
   catch(...)
diff --git a/libstdc++-v3/testsuite/22_locale/locale/cons/unicode.cc b/libstdc++-v3/testsuite/22_locale/locale/cons/unicode.cc
index 24af414..98d744d 100644
--- a/libstdc++-v3/testsuite/22_locale/locale/cons/unicode.cc
+++ b/libstdc++-v3/testsuite/22_locale/locale/cons/unicode.cc
@@ -79,7 +79,7 @@ void test01()
     { VERIFY( false ); }
 
   try 
-    { use_facet<gnu_facet>(loc13); }
+    { (void) use_facet<gnu_facet>(loc13); }
   catch(bad_cast& obj)
     { VERIFY( true ); }
   catch(...)
diff --git a/libstdc++-v3/testsuite/22_locale/locale/operations/2.cc b/libstdc++-v3/testsuite/22_locale/locale/operations/2.cc
index 8995351..917adec 100644
--- a/libstdc++-v3/testsuite/22_locale/locale/operations/2.cc
+++ b/libstdc++-v3/testsuite/22_locale/locale/operations/2.cc
@@ -48,7 +48,7 @@ void test02()
   // Derivation, MF check.
   locale loc_gnu(loc_c, new gnu_collate);
   gnu_count = 0;
-  loc_gnu(s01, s02);
+  loc_gnu(s01, s02); // { dg-warning "nodiscard" "" { target c++17 } }
   VERIFY( gnu_count == 1 );
 }