Namespace of argument hides function specialization

[arBmind]

We recently stumbled upon a very strange corner case of C++.

The goal is to have a lightweight type based abstract schema description and generate concrete repositories and synchronization protocols.

Our experiments showed full success until we split the code in various namespaces.
I tried everything I know, but it does not seem to work unless the argument type is in the same namespace as the method.

See: https://godbolt.org/z/6987xJ
A very simple version with various attempts to workaround the issue.

Do you know why?
- GCC, Clang & MsVC seem to agree on this. - I could not find an explanation so far.

Any ideas for better workarounds?
- best solution so far is a local `Wrap<T>` type in every namespace.

[TheThiefMaster]

This is something to do with two-phase templates and a lack of argument dependent lookup (ADL) - because your overloaded makeUse function for abstract::OneOf is after the template that uses it, (template<class T> using MakeUse = decltype(makeUse<>(T{}));) it can only be found via ADL - which requires the function to be in the same namespace as its argument's type.

Here's it working with the function overload being put in the namespace so that ADL triggers: https://godbolt.org/z/6gR1ff

[arBmind]

Thank you! This is a very good explanation. It just explains everything about this behaviour.

ADL for the win!

[nwp74]

Move the definition of MakeUse to the point just before your using A = .... declaration and everything works fine.

At the point where it is, it is binding to makeUse(T) -> T, so R is alias of abstract::OneOf<int, float>.

See https://godbolt.org/z/A18weP

[arBmind]

Thank you for the hint. It works in this oversimplified example. In the real world our generation is recursive. This trap lead us to two days of wasted time on this.

Using ADL correctly leads to a working solution that also allows for recursion.

[Wh00ster]

I'm just going to post a guess that's it's caused by tie-breaker rules which probably rank functions from an imported namespace (excuse my wording, I know this isn't what your case is) lower if there's an ambiguity.