gdb: make_scoped_restore and types convertible to T

A following patch will want to do

   string_file str_file;

   scoped_restore save_stdout
    = make_scoped_restore (&gdb_stdout, &str_file);

where gdb_stdout is a ui_file *, and string_file is a type that
inherits from ui_file, but that doesn't compile today:

  src/gdb/top.c: In function ‘std::__cxx11::string execute_command_to_string(char*, int)’:
  src/gdb/top.c:710:50: error: no matching function for call to ‘make_scoped_restore(ui_file**, string_file*)’
       = make_scoped_restore (&gdb_stdout, &str_file);
						    ^
  [...]
  In file included from src/gdb/utils.h:25:0,
		   from src/gdb/defs.h:732,
		   from src/gdb/top.c:20:
  src/gdb/common/scoped_restore.h:94:24: note: candidate: template<class T> scoped_restore_tmpl<T> make_scoped_restore(T*, T)
   scoped_restore_tmpl<T> make_scoped_restore (T *var, T value)
			  ^
  src/gdb/common/scoped_restore.h:94:24: note:   template argument deduction/substitution failed:
  src/gdb/top.c:710:50: note:   deduced conflicting types for parameter ‘T’ (‘ui_file*’ and ‘string_file*’)
       = make_scoped_restore (&gdb_stdout, &str_file);
						    ^

This commit makes code such as the above possible.

gdb/ChangeLog:
2017-01-31  Pedro Alves  <palves@redhat.com>

	* common/scoped_restore.h
	(scoped_restore_tmpl::scoped_restore_tmpl): Template on T2, and
	change the value's parameter type to T2.
	(make_scoped_restore): Likewise.

1 files changed, 7 insertions, 4 deletions

diff --git a/gdb/common/scoped_restore.h b/gdb/common/scoped_restore.h
index 0c8c30d..ae7a49f 100644
--- a/gdb/common/scoped_restore.h
+++ b/gdb/common/scoped_restore.h
@@ -47,8 +47,11 @@ class scoped_restore_tmpl : public scoped_restore_base
 
   /* Create a new scoped_restore object that saves the current value
      of *VAR, and sets *VAR to VALUE.  *VAR will be restored when this
-     scoped_restore object is destroyed.  */
-  scoped_restore_tmpl (T *var, T value)
+     scoped_restore object is destroyed.  This is templated on T2 to
+     allow passing VALUEs of types convertible to T.
+     E.g.: T='base'; T2='derived'.  */
+  template <typename T2>
+  scoped_restore_tmpl (T *var, T2 value)
     : m_saved_var (var),
       m_saved_value (*var)
   {
@@ -90,8 +93,8 @@ scoped_restore_tmpl<T> make_scoped_restore (T *var)
 
 /* Make a scoped_restore.  This is useful because it lets template
    argument deduction work.  */
-template<typename T>
-scoped_restore_tmpl<T> make_scoped_restore (T *var, T value)
+template<typename T, typename T2>
+scoped_restore_tmpl<T> make_scoped_restore (T *var, T2 value)
 {
   return scoped_restore_tmpl<T> (var, value);
 }