On 2015.05.11 at 17:54 +0200, Marek Polacek wrote:
> On Mon, May 11, 2015 at 05:09:26PM +0200, Manuel López-Ibáñez wrote:
> > On 11/05/15 16:21, Marek Polacek wrote:
> > >The -Wshift-negative-value patch caused grief since it breaks building
> > >some programs. The following patch should alleviate the pain a bit: mark
> > >a left shift of a negative value as non-const only if pedantic.
> >
> > Either this is not correct according to the guidelines ("the flag pedantic
> > should not cause generated code differences or errors",
> > https://gcc.gnu.org/wiki/DiagnosticsGuidelines) or the guidelines need
> > updating.
> [...]
>
> The problem here isn't in the -Wshift-negative-value warning itself; the
> problem is with marking -1 << 0 as a non-constant: later on, we warn in
> a context where a constant expression is needed ("initializer element is
> not a constant expression"), and for e.g. int foo = -1 << 0 | 9; there's
> an error ("initializer element is not constant").
I find both the warning and the error confusing. Instead of "is not a
constant", it should rather say "invokes undefined behavior".
Because it is not obvious at first sight why (-1 << 0 | 9) shouldn't be a
constant expression.
--
Markus
