c++: Add make_temp_override generator functions

A common pattern before C++17 is the generator function, used to avoid
having to specify the type of a container element by using a function call
to get type deduction; for example, std::make_pair.  C++17 added class type
argument deduction, making generator functions unnecessary for many uses,
but GCC won't be written in C++17 for years yet.

gcc/cp/ChangeLog:

	* cp-tree.h (struct type_identity): New.
	(make_temp_override): New.
	* decl.c (grokdeclarator): Use it.
	* except.c (maybe_noexcept_warning): Use it.
	* parser.c (cp_parser_enum_specifier): Use it.
	(cp_parser_parameter_declaration_clause): Use it.
	(cp_parser_gnu_attributes_opt): Use it.
	(cp_parser_std_attribute): Use it.

1 files changed, 32 insertions, 0 deletions

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 5304f6b..5cd2999 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -1932,6 +1932,38 @@ public:
   ~temp_override() { overridden_variable = saved_value; }
 };
 
+/* Wrapping a template parameter in type_identity_t hides it from template
+   argument deduction.  */
+#if __cpp_lib_type_identity
+using std::type_identity_t;
+#else
+template <typename T>
+struct type_identity { typedef T type; };
+template <typename T>
+using type_identity_t = typename type_identity<T>::type;
+#endif
+
+/* Object generator function for temp_override, so you don't need to write the
+   type of the object as a template argument.
+
+   Use as auto x = make_temp_override (flag); */
+
+template <typename T>
+inline temp_override<T>
+make_temp_override (T& var)
+{
+  return { var };
+}
+
+/* Likewise, but use as auto x = make_temp_override (flag, value); */
+
+template <typename T>
+inline temp_override<T>
+make_temp_override (T& var, type_identity_t<T> overrider)
+{
+  return { var, overrider };
+}
+
 /* The cached class binding level, from the most recently exited
    class, or NULL if none.  */