[PATCH] PR fortran/77978 -- STOP code fixes

[Steve Kargl]

The attach patch fixes a number of shortcomings with
STOP codes in gfortran.  The updated comment in the
code nicely summarizes the problem.

 /* Match a number or character constant after an (ERROR) STOP or PAUSE
-   statement.  */
+   statement.  The requirements for a stop-code differs in the standards.
+
+   Fortran 95 has
+
+   R840 stop-stmt  is STOP [ stop-code ]
+   R841 stop-code  is scalar-char-constant
+                   or digit [ digit [ digit [ digit [ digit ] ] ] ]
+
+   Fortran 2003 is the same as Fortran 95 except R840 and R841 are now
+   R849 and R850.
+
+   Fortran 2008 has
+
+   R855 stop-stmt     is STOP [ stop-code ]
+   R856 allstop-stmt  is ALL STOP [ stop-code ]
+   R857 stop-code     is scalar-default-char-constant-expr
+                      or scalar-int-constant-expr
+*/

So, the F95/2003 "digit [...]" is not a scalar-int-constant-expr.
It sort of looks like a statement label, but of course it is not
a statement label as the stop code does label anything.  Currently,
gfortran parses "digit [...]" as an expression.  I've added the
necessary checking that "digit [...]" is valid with one exception. 
For the code 

  program foo
    stop merge(667, 668, .true.)
  end 

gfortran with either -std=f95 or -std=f2003 should reject 
this code.  My patch does not fix this issue, because it
would (1) require a complete rewrite of gfc_match_stopcode
(which I am not willing to do) and (2) it simply is a vastly
unimportant corner case that gives the desired behavior.

A second issue raised by John in PR fortran/77978 is that
for F95/2003, the following is valid free-form source code:

  program foo
    stop666
  end

but is invalid F2008.  The patch fixes this bug, too.

OK to commit?

2016-10-XX  Steven G. Kargl  <kargl@gcc.gnu.org>

	PR fortran/77978
	* match.c (gfc_match_stopcode): Fix error reporting for several
	deficiencies in matching STOP codes.
 
2016-10-XX  Steven G. Kargl  <kargl@gcc.gnu.org>

	PR fortran/77978
	* gfortran.dg/pr77978_1.f90: New test.
	* gfortran.dg/pr77978_2.f90: Ditto.
	* gfortran.dg/pr77978_3.f90: Ditto.

-- 
Steve
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