[cfe-dev] discriminating explicit boolean expressions from implicit boolean expressions in the AST

[Richard]

In article <CAENS6EuerX1Fvf0NEo7fYvn3pR1RJHQxahGA+-C8ztZkgRi5yQ at mail.gmail.com>,
    David Blaikie <dblaikie at gmail.com> writes:

> On Tue, Mar 31, 2015 at 10:06 AM, Richard <legalize at xmission.com> wrote:
> > In the case of a return statement, you don't need to do anything.
> >
> > If they wrote 'if (sp != nullptr) return true; else return false;',
> > then the tool would replace this with 'return sp != nullptr;'.
> >
> > If they wrote 'if (sp) return true; else return false;', the tool
> > would replace this with 'return sp;'.
> >
> 
> This ^ transformation is most likely not valid for any modern user defined
> type (because they'll have an explicit operator bool, not an implicit one):
> 
> $ cat sp.cpp
> #include <memory>
> 
> bool f1(std::unique_ptr<int> u) {
>   if (u)
>     return true;
>   return false;
> }
> 
> bool f2(std::unique_ptr<int> u) {
>   return u;
> }
> $ clang++-tot sp.cpp -fsyntax-only  -std=c++11
> sp.cpp:10:10: error: no viable conversion from 'std::unique_ptr<int>' to
> 'bool'
>   return u;
>          ^
> 1 error generated.

Good point.  So I guess I need to handle these cases and it looks like
I can't avoid some type introspection in order to figure out what to
do.

> > At this point I am only concerned about correctness.
> 
> Sure, but your transformation is stylistic in nature - so it's not just
> important to preserve semantics, but also to improve readability. Perhaps
> it's worth not offering a fixit hint in these cases, until good quality
> ones can be provided? I'm not sure.

I think some type introspection can be done to deal with builtin types
and standard library types.

Yes, the transformation is stylistic, but it is also human invoked.
The user has asked for this transformation to be performed, so I'm ok
with transforming it in such a way that it leaves correctness intact
albeit possibly with an ugly static_cast<> lurking in there if I can't
figure out something better.  (Personally I don't know why people are
against the 'functional cast' bool(e), particularly when that's the
wording in the standard.)
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