Hi David,

  Attached is a revised version of the patch which I hope addresses all 
  of your (very helpful) comments on the v3 patch.

  OK to apply once the sources are back on stage 1 ?

Cheers
  Nick

gcc/ChangeLog
2018-02-09  Nick Clifton  <nickc@redhat.com>

	PR 84195
	* tree.c (escaped_string): New class.  Converts an unescaped
	string into its escaped equivalent.
	(warn_deprecated_use): Use the new class to convert the
	deprecation message, if present.
	(test_escaped_strings): New self test.
	(test_c_tests): Add test_escaped_strings.
	* doc/extend.texi (deprecated): Add a note that the
	deprecation message is affected by the -fmessage-length
	option, and that control characters will be escaped.
	(#pragma GCC error): Document this pragma.
	(#pragma GCC warning): Likewise.
	* doc/invoke.texi (-fmessage-length): Document this option's
	effect on the #warning and #error preprocessor directives and
	the deprecated attribute.
	
gcc/testsuite/ChangeLog
2018-02-09  Nick Clifton  <nickc@redhat.com>

	PR 84195
	* gcc.c-torture/compile/pr84195.c: New test.