[Patch] Disable -Wtaulological-compare for substituted template types

[Arthur O'Dwyer]

On Thu, Oct 31, 2013 at 5:52 PM, Richard Trieu <rtrieu at google.com> wrote:
> On Thu, Oct 31, 2013 at 4:25 PM, David Blaikie <dblaikie at gmail.com> wrote:
>>
>> Could we just restrict these warnings to only be done on template
>> patterns, not template specializations? (I know Ted's had some concern about
>> doing work on template patterns in the past because he believed that might
>> create too much work analyzing templates that are never instantiated,
>> though)
>
> I think that would be possible and a cleaner way of approaching this.
> Currently, Clang warns in both template patters and specializations.  That
> means if you have :
>
> template<int Num>
> bool greater(unsigned Val) {
>   return Val >= 0;
> }
>
> ... greater<0>(42); ...
> ... greater<1>(42); ...
> ... greater<2>(42); ...
>
> There would be four warnings on the return line.  Ignoring the three
> warnings in the specializations and only showing the one from the template
> pattern would be best on cutting down the noise.

Only for -Wtautological-compare, though, right?

I imagine that there are plenty of warnings that would be useful to
produce during template instantiation, but impossible to detect by
just looking at the pattern (without checking dependent
types/expressions). A trivial example would be
-Wuninitialized for this code:

    template<typename T> T newval() { T t; return t; }

Here, newval<int>() has undefined behavior, but newval<std::string>()
is perfectly correct.

So whereas -Wtautological-compare empirically gives almost 100% false
positives on dependent expressions, -Wuninitialized probably gives
100% *true* positives.

–Arthur