[cfe-dev] Question regarding error

[Eli Friedman]

On Wed, May 5, 2010 at 4:40 PM, John Thompson
<john.thompson.jtsoftware at gmail.com> wrote:
> Note that in the actual code, the vector constructor is explicit:
>
> class test
> {
> public:
>  test( float x );
>  explicit test(__attribute__((vector_size(16))) float v );
> };
> Shouldn't that help the compiler decide that it's not a candidate?  (gcc
> doesn't seem to care either way.)
>
> I have one class like this that is used frequently.  I added a constructor
> taking an int, but that caused other problems.  Adding a (float) cast to the
> argument in the instanciations is the hack that seems to work.
>
> Also, because you can't cast scalars to vectors, I think the implicit
> conversion code should exclude the vector case.  If you agree, I'll file a
> bug.

Yes, please file.

-Eli

> On Fri, Apr 30, 2010 at 5:44 PM, Eli Friedman <eli.friedman at gmail.com>
> wrote:
>>
>> On Fri, Apr 30, 2010 at 5:11 PM, Douglas Gregor <dgregor at apple.com> wrote:
>> >
>> > On Apr 30, 2010, at 4:55 PM, John Thompson wrote:
>> >
>> >> This doesn't compile.  Should it?:
>> >>
>> >> class test
>> >> {
>> >> public:
>> >>  test( float x );
>> >>  test(__attribute__((vector_size(16))) float v );
>> >> };
>> >> int vi = 0;
>> >> void func()
>> >> {
>> >>  test object = test(vi);
>> >> }
>> >> C:\Sony\Clang\exp>clang -cc1 scalar.cpp
>> >> scalar.cpp:13:16: error: functional-style cast from 'int' to 'test' is
>> >> not allowed
>> >>         test object = test(vi);
>> >>                       ^~~~
>> >
>> > The logic for computing implicit conversion sequences in C++ does not
>> > know anything about vectors. What you're seeing here is a poor diagnostic
>> > due to the cast. Change it to a direct initialization and you'll see the
>> > ambiguity:
>> >
>> > t2.cpp:10:7: error: call to constructor of 'test' is ambiguous
>> >  test object(vi);
>> >      ^      ~~
>> > t2.cpp:4:2: note: candidate constructor
>> >  test( float x );
>> >  ^
>> > t2.cpp:5:2: note: candidate constructor
>> >  test(__attribute__((vector_size(16))) float v );
>> >  ^
>> > t2.cpp:1:7: note: candidate is the implicit copy constructor
>> > class test
>> >      ^
>> >
>> > Clang can't figure out which constructor is the better match.
>>
>> Right, it's treating it like a class which has constructors for, for
>> example, float and double. It does this because our routines for
>> identifying types do strange things for queries like
>> isRealFloatingType().  See also
>> http://llvm.org/bugs/show_bug.cgi?id=4208 .
>>
>> -Eli
>
>
>
> --
> John Thompson
> John.Thompson.JTSoftware at gmail.com
>
>
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