[PATCH] integer overflow checking builtins in constant expressions

[Jakub Jelinek]

On Tue, Jun 07, 2016 at 03:35:28PM -0400, Jason Merrill wrote:
> >Indeed, confirming removal of the cp/tree.c hunk doesn't affect anything in
> >the testsuite, nor e.g.
> >enum A {
> >  B = 1,
> >  C = 2,
> >  D = __builtin_add_overflow_p (B, C, C)
> >};
> >int e[__builtin_add_overflow_p (B, C, C) + 1];
> >template <int N> int foo (int);
> >void
> >bar ()
> >{
> >  foo <__builtin_add_overflow_p (B, C, C) + 1> (0);
> >}
> >That said, builtin_valid_in_constant_expr_p is used in lots of spots, and in
> >various of them I see that without the change it actually sets
> >*non_integral_constant_expression_p = true;
> >or something similar, but I have no idea why it still works despite of that.
> 
> Ah, that's the C++98 constant expression handling, which is a lot more
> restricted.  C++11 and up don't care about that flag.

Oops, actually, it seems even the cp/tree.c hunks are significant:

I've only compiled the above testcase with the default -std=c++14, with
-std=c++98 without the cp/tree.c bits I get:

a.C:4:7: error: ‘bool __builtin_add_overflow_p(...)’ cannot appear in a
constant-expression
   D = __builtin_add_overflow_p (B, C, C)
       ^~~~~~~~~~~~~~~~~~~~~~~~
a.C:4:40: error: a function call cannot appear in a constant-expression
   D = __builtin_add_overflow_p (B, C, C)
                                        ^
a.C:6:45: error: array bound is not an integer constant before ‘]’ token
 int e[__builtin_add_overflow_p (B, C, C) + 1];
                                             ^
a.C: In function ‘void bar()’:
a.C:11:8: error: ‘bool __builtin_add_overflow_p(...)’ cannot appear in a
constant-expression
   foo <__builtin_add_overflow_p (B, C, C) + 1> (0);
        ^~~~~~~~~~~~~~~~~~~~~~~~
a.C:11:41: error: a function call cannot appear in a constant-expression
   foo <__builtin_add_overflow_p (B, C, C) + 1> (0);
                                         ^
a.C:11:50: error: no matching function for call to ‘foo(int)’
   foo <__builtin_add_overflow_p (B, C, C) + 1> (0);
                                                  ^
a.C:7:22: note: candidate: template<int N> int foo(int)
 template <int N> int foo (int);
                      ^~~
a.C:7:22: note:   template argument deduction/substitution failed:
a.C:11:50: error: template argument 1 is invalid
   foo <__builtin_add_overflow_p (B, C, C) + 1> (0);
                                                  ^

Though, maybe it is only worth supporting the __builtin_*_overflow_p
builtins for C++98 integer constant expression contexts and
thus only handle there the 3 builtins instead of all the others.

I guess I should add this testcase to the testsuite then.

	Jakub