Re: [boost] [qvm] deduce_xx traits wouldn't introduce ODR issues.

"Need" is too strong, specifying the return type for binary operations is optional. The deduction templates for binary operations must use two parameters because they define the return type specifically for a given pair of argument types. On Thursday, December 31, 2015, Vicente J. Botet Escriba < vicente.botet@wanadoo.fr> wrote:

Le 31/12/2015 10:24, Sergey Mitsyn a écrit :

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On 31.12.2015 12:00, Rob Stewart wrote:

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On December 31, 2015 2:42:55 AM EST, Sergey Mitsyn wrote:

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Correct me if I'm wrong, but it to me seems that the root of evil here is a specialization by third party library, by which I mean the case when template from library X is specialized with type(s) exclusively from library (libraries) Y (Y might be the same as X) by library Z (which is neither X nor Y, third party). If it's true, an ODR would be caused for a user even if _unary_ deduce_m (or any other deduce_X) will be specialized by two unrelated third party libraries A and B imported by user.

Your point is valid except that one-type customization points are likely to be provided by the library providing the type X or Y, or by the library providing the customization point using a separate header (opt in).

But then until they don't it's still an issue, because eventually they may provide, so a specialization made by a library user is a ticking bomb.

Right. We should expect that the library document the need of such a customization so that the end user can know if s/he can combine it with other libraries.

Customization of a class X are expected to be given by the library providing class X, or a friend library.

I don't remember if I've already suggested to replace the deduce_xx customizations by a customization that depends only on the first parameter. I don't think this will dismiss a lot the functionality of the library.

Vicente

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Bold warnings in the documentation may be the only real solution to the issue.

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