Undefined class template is not instantiated to check for friend functions

Question

The following program compiles (see on godbolt), but it would not compile if we uncommented the definition of Buffer.

template <int size>
struct Buffer /*{ char buf[size]; }*/;

template <class T>
struct Wrapper { void operator+() {} };

Wrapper<Buffer<-5>> a;

void f() { +a; }

The reason, the uncommented version does not compile: +a triggers ADL, and to collect all candidates for operator+, all associated classes must be checked for friend functions. Buffer<-5> is an associated class, so it must be instantiated. Instantiation fails, hence the compilation error. See this question.

I wonder if Buffer<-5> must be instantiated, why don't we have a compilation error, if Buffer is not defined?

As incomplete, so no instantiated, its potential `friend` functions cannot participate in overload resolution. — Jarod42, Jun 12 '20 at 19:40
@Jarod42: So it has to instantiate `Buffer` only if it has a defninition? I see your point: [this works](https://godbolt.org/z/vdqp_U). Compiler does not feel the need to look into class `Incomplete` for `friend` functions. — Dr. Gut, Jun 12 '20 at 19:48
@AsteroidsWithWings: In that context, I don't see difference for ADL. (and `Wrapper` doesn't need definition). — Jarod42, Jun 12 '20 at 19:54
@AsteroidsWithWings: in both cases, we should instantiate `Incomplete` (if definition is available (name suggest it is not ;) )) to look for `friend` functions. — Jarod42, Jun 12 '20 at 19:59
@AsteroidsWithWings: see [Demo](https://godbolt.org/z/y5oGpW), we look at `friend` functions of that class. — Jarod42, Jun 12 '20 at 20:19
For both `Wrapper` and `Wrapper`, we look (also) into class `C` for `friend` functions. — Jarod42, Jun 12 '20 at 22:01

score 1 · Accepted Answer · edited Jun 19 '20 at 00:26

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You can (implicitly) instantiate a class template from only a declaration; you get an incomplete class type, just like from struct A; ([temp.inst]/2). It’s not an error, of course, to do ADL with an incomplete associated class; the class in question is simply not searched for friend declarations.

edited Jun 19 '20 at 00:26

Dr. Gut

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answered Jun 13 '20 at 01:55

Davis Herring

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Undefined class template is not instantiated to check for friend functions

1 Answers1