Possible patch for PR fortran/67806

Possible patch for PR fortran/67806

[Louis Krupp]

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The problem involves a derived type with a character component declared CHARACTER(NULL()) or CHARACTER(NULL(n)), where mold argument n is an integer pointer.

I might be missing something, but I'm not sure there's a point to having a character variable whose length is the target of a null pointer.  This program, for example,
crashes with a SEGV reported at line 10 (with line 11 deleted, the program runs to completion):

  1 program z
  2 implicit none
  3    integer, target :: k = 0
  4    integer, pointer :: p => k
  5    nullify(p)
  6    call s(p)
  7 contains
  8    subroutine s(n)
  9       integer, pointer :: n
 10       character (len=n) q
 11       q = 'a'
 12    end subroutine
 13 end program

What to do with CHARACTER(NULL([mold])), besides fix the ICE?  It might have been possible to generate code to define a null pointer and generate code to dereference it and get the expected SEGV, but it seemed easier and possibly more productive to treat CHARACTER(NULL(..)) as an error.  I don't know how what the standard has to say about this.  It might have been one of those things its authors never thought about.

Since this problem is detected at different places in the code, the attached test case gives the following errors with the attached patch:

! { dg-do compile }
! PR 67806
! 1. Initialize a variable of derived type with a string component having
!    a length that is the target of the NULL intrinsic.
! 2. Declare a derived type with a string component having a length that is
!    the target of the NULL intrinsic with an integer mold argument.
subroutine s1
   type t
      character(null()) :: c ! { dg-error "is target of NULL pointer" }
                1
Error: Character length of component ‘c’ is target of NULL pointer at (1)
   end type
   type(t) :: x = t('a')
end subroutine

subroutine s2
   integer, pointer :: n
   type t
      character(null(n)) :: c ! { dg-error "is target of NULL pointer" }
                1
Error: Character length is target of NULL pointer at (1)
   end type
end subroutine

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! { dg-do compile }
! PR 67806
! 1. Initialize a variable of derived type with a string component having
!    a length that is the target of the NULL intrinsic.
! 2. Declare a derived type with a string component having a length that is
!    the target of the NULL intrinsic with an integer mold argument.
subroutine s1
   type t
      character(null()) :: c ! { dg-error "is target of NULL pointer" }
   end type
   type(t) :: x = t('a')
end subroutine

subroutine s2
   integer, pointer :: n
   type t
      character(null(n)) :: c ! { dg-error "is target of NULL pointer" }
   end type
end subroutine

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Index: gcc/fortran/resolve.c
===================================================================
--- gcc/fortran/resolve.c	(revision 228700)
+++ gcc/fortran/resolve.c	(working copy)
@@ -1134,7 +1134,8 @@ resolve_structure_cons (gfc_expr *expr, int init)
   t = true;
 
   if (expr->ts.type == BT_DERIVED)
-    resolve_fl_derived0 (expr->ts.u.derived);
+    if (!resolve_fl_derived0 (expr->ts.u.derived))
+      return false;
 
   cons = gfc_constructor_first (expr->value.constructor);
 
@@ -10882,6 +10883,13 @@ resolve_charlen (gfc_charlen *cl)
 	}
     }
 
+  if (cl->length && cl->length->expr_type == EXPR_NULL)
+    {
+      gfc_error ("Character length is target of NULL pointer at %L",
+		 &cl->length->where);
+      return false;
+    }
+
   /* "If the character length parameter value evaluates to a negative
      value, the length of character entities declared is zero."  */
   if (cl->length && !gfc_extract_int (cl->length, &i) && i < 0)
@@ -13090,10 +13098,16 @@ resolve_fl_derived0 (gfc_symbol *sym)
 	     || (!resolve_charlen(c->ts.u.cl))
 	     || !gfc_is_constant_expr (c->ts.u.cl->length))
 	   {
-	     gfc_error ("Character length of component %qs needs to "
-			"be a constant specification expression at %L",
+	     gfc_error (c->ts.u.cl->length &&
+			c->ts.u.cl->length->expr_type == EXPR_NULL ?
+			  "Character length of component %qs is target of "
+			  "NULL pointer at %L"
+			:
+			  "Character length of component %qs needs to "
+			  "be a constant specification expression at %L",
 			c->name,
-			c->ts.u.cl->length ? &c->ts.u.cl->length->where : &c->loc);
+			c->ts.u.cl->length ?
+			  &c->ts.u.cl->length->where : &c->loc);
 	     return false;
 	   }
 	}

Re: Possible patch for PR fortran/67806

[Steve Kargl]

On Sun, Oct 11, 2015 at 10:18:48PM -0700, Louis Krupp wrote:
> The problem involves a derived type with a character component declared CHARACTER(NULL()) or CHARACTER(NULL(n)), where mold argument n is an integer pointer.
> 

I was looking at 67805 this weekend, which is somewhat
related to this PR.  AFAICT, gfortran does no checking
for n in CHARACTER(LEN=n). n should be a scalar-int-expr
(that is scalar INTEGER expression).  NULL() is not
an integer, and NULL(n) is a disassociated pointer.  So,
I believe neither can appear in an scalar-int-expr.

Note, also that there is a table in 13.7.125 on where
NULL() can appear.

My patch for 67805 leads to one regression that I've been
unable to resolve.

-- 
Steve

Re: Possible patch for PR fortran/67806

[Louis Krupp]

---- On Mon, 12 Oct 2015 08:41:43 -0700 Steve Kargl<sgk@troutmask.apl.washington.edu> wrote ---- 
 > On Sun, Oct 11, 2015 at 10:18:48PM -0700, Louis Krupp wrote: 
 > > The problem involves a derived type with a character component declared CHARACTER(NULL()) or CHARACTER(NULL(n)), where mold argument n is an integer pointer. 
 > >  
 >  
 > I was looking at 67805 this weekend, which is somewhat 
 > related to this PR.  AFAICT, gfortran does no checking 
 > for n in CHARACTER(LEN=n). n should be a scalar-int-expr 
 > (that is scalar INTEGER expression).  NULL() is not 
 > an integer, and NULL(n) is a disassociated pointer.  So, 
 > I believe neither can appear in an scalar-int-expr. 
 >  
 > Note, also that there is a table in 13.7.125 on where 
 > NULL() can appear. 
 >  
 > My patch for 67805 leads to one regression that I've been 
 > unable to resolve. 

For what it's worth, my patch does absolutely nothing for 67805.

As to my error message, should I fold this misuse of NULL() into the existing message saying "Character length needs to be a constant specification expression" and not mention NULL()?

There are times I wish I knew the story behind the code in some of these bug reports.  Were they written by someone looking for edge cases that might cause trouble, or was someone actually trying to do something?

Louis