Re: error: jump skips variable initialization

[Bruno Haible]

Hi Jim, Paul,

> parse-datetime.y: In function 'parse_datetime2':
> parse-datetime.y:1791:19: error: jump skips variable initialization \
>   [-Werror=jump-misses-init]

First this occurred in af_alg.c with pretty standard Linux kernel style
with gotos. [1] Now here.

What is the point of this warning?

I've read [2][3].

Test case:
===========================================================
extern void bar (int);
int foo (int a)
{
  if (a == 1) goto one;
  int b = 4;
  bar (b);
 one:
  return a;
}
===========================================================

$ gcc-version 8.1.0 -Wall -Wjump-misses-init -c foo.c
foo.c: In function 'foo':
foo.c:4:15: warning: jump skips variable initialization [-Wjump-misses-init]
   if (a == 1) goto one;
               ^~~~
foo.c:7:2: note: label 'one' defined here
  one:
  ^~~
foo.c:5:7: note: 'b' declared here
   int b = 4;
       ^

In C++ such a warning makes sense, for types that have a destructor,
because the destructor of 'b' would run even through its constructor
has not run.

But here
  1. The variable 'b' is of type 'int', which has a trivial destructor.
  2. We are in C, no C++, where ISO C types don't have destructors anyway.
     (And AFAICS there are no GCC extensions to C that would provide
     destructors either.)

Is the point to help people reduce the scope of the variables?
===========================================================
extern void bar (int);
int foo (int a)
{
  if (a == 1) goto one;
  {
    int b = 4;
    bar (b);
  }
 one:
  return a;
}
===========================================================

Is the point to force people to hoist the variable declarations
at the top of the function (back to the BSD style of the 1980ies),
like Paul did in [1]?
===========================================================
extern void bar (int);
int foo (int a)
{
  int b;
  if (a == 1) goto one;
  b = 4;
  bar (b);
 one:
  return a;
}
===========================================================

Is the point to help people write code that can be compiled with a C++ compiler?
If so, then there's no reason for it to be enabled on gnulib code.

Is the point merely "the standard says that a warning is possible, so let's
implement it"?

Can someone answer this?

The next question then is: Why don't we silence this warning in the
'manywarnings' module?

Bruno

[1] https://lists.gnu.org/archive/html/bug-gnulib/2018-05/msg00064.html
[2] 
https://stackoverflow.com/questions/37371794/using-goto-to-jump-to-inner-or-sibling-scope
[3] 
https://stackoverflow.com/questions/20998054/why-does-gcc-give-me-the-wjump-misses-init-warning