Module, preamble, global module fragment and legacy header unit

[Loïc Joly]

I've been reading the merged proposal, and I have a question.


Assume I have some code not yet fully migrated to modules, but that
makes use of modules. It can look that way:

// header1.h
#pragma once
#include "other.h"
import std.code;
void g(std::string s);


Now, if I want to use this code from a module, I have two options:

*Option 1**: Legacy header unit*

If I do:
export module mainModule;
import "header1.h"

Then a module is generated out of header1.h, 10.7.3§4 tells "Within a
legacy header unit, certain
semantic restrictions are relaxed:
— A module-import-declaration may appear, and is considered to be exported."

Does it means that this declaration may appear /anywhere/, including in
places where it would not be normally allowed?

*Option 2**: Global module*

module;
#include "header1.h"
export module mainModule;
// ...

If I do so, the preprocessed code will contain an import directive
outside of mainModule preamble, and the code is incorrect (10.7.3§1 In a
module unit, a module-import-declaration shall appear only within the
preamble.).

Do I miss something?
I not, I find this difference of behaviour between the global module
fragment and a legacy header unit disturbing.

---
Loïc

[Richard Smith]

On Mon, Sep 10, 2018 at 3:10 PM Loïc Joly <loic.act...@numericable.fr> wrote:

I've been reading the merged proposal, and I have a question.


Assume I have some code not yet fully migrated to modules, but that
makes use of modules. It can look that way:

// header1.h
#pragma once
#include "other.h"
import std.code;
void g(std::string s);


Now, if I want to use this code from a module, I have two options:

*Option 1**: Legacy header unit*

If I do:
export module mainModule;
import "header1.h"

Then a module is generated out of header1.h, 10.7.3§4 tells "Within a
legacy header unit, certain
semantic restrictions are relaxed:
— A module-import-declaration may appear, and is considered to be exported."

Does it means that this declaration may appear /anywhere/, including in
places where it would not be normally allowed?

Yes. The preamble restriction only applies to modular code, not to legacy source files.

*Option 2**: Global module*

module;
#include "header1.h"
export module mainModule;
// ...

If I do so, the preprocessed code will contain an import directive
outside of mainModule preamble, and the code is incorrect (10.7.3§1 In a
module unit, a module-import-declaration shall appear only within the
preamble.).

Do I miss something?
I not, I find this difference of behaviour between the global module
fragment and a legacy header unit disturbing.

That seems like a reasonable concern to me. I think the Modules TS permitted this, so removing that ability would be an unintended functionality regression in the merged proposal.

I'd be very interested to hear Gaby's thoughts on this.