[binutils-gdb] gdb: make_scoped_restore and types convertible to T

[sergiodj+buildbot at sergiodj dot net]

*** TEST RESULTS FOR COMMIT 60adb36c08942fd4a2f8bf598864254c043668a1 ***

Author: Pedro Alves <palves@redhat.com>
Branch: master
Commit: 60adb36c08942fd4a2f8bf598864254c043668a1

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.