[C++20] [Modules] Clang produce incorrect linkage if we export things in language linkage

[JohelEGP]

A linkage-specification is a name-declaration
and has a terminal extern unevaluated-string name-declaration.
An export-declaration isn't a name-declaration,
but Clang accepts one after a linkage-specification.

GCC diagnoses it: https://compiler-explorer.com/z/nozWPbxbG.
But Clang accepts it with an unexpected result: https://compiler-explorer.com/z/eefn3sWMs.

Note that if the export comes before the extern, both accept.
GCC: https://compiler-explorer.com/z/Y3zn6EEKK.
Clang: https://compiler-explorer.com/z/zb5qhxYEa.

module;
#include <iostream>
export module module_utility;
extern "C" __attribute__ ((visibility ("default")))
export void utility() {
  std::cout << "Hello from utility!" << std::endl;
}

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: Johel Ernesto Guerrero Peña (JohelEGP)

A _linkage-specification_ is a _name-declaration_ and has a terminal `extern` _unevaluated-string_ _name-declaration_. An _export-declaration_ isn't a _name-declaration_, but Clang accepts one after a _linkage-specification_.

GCC diagnoses it: <https://compiler-explorer.com/z/nozWPbxbG>.
But Clang accepts it with an unexpected result: <https://compiler-explorer.com/z/eefn3sWMs>.

Note that if the export comes before the extern, both accept.
GCC: <https://compiler-explorer.com/z/Y3zn6EEKK>.
Clang: <https://compiler-explorer.com/z/zb5qhxYEa>.

module;
#include <iostream>
export module module_utility;
extern "C" __attribute__ ((visibility ("default")))
export void utility() {
  std::cout << "Hello from utility!" << std::endl;
}

[shafik]

CC @ChuanqiXu9

[ChuanqiXu9]

There are questions:
(1) Is it valid to export things from language linkage? From https://godbolt.org/z/hnx3KToKc it looks like both GCC and clang accept it. (I don't know how to invoke msvc for modules in godbolt.) And I remember I checked the wording. There is no saying that we can't export things from language linkage.
(2) If the answer of (1) is yes, then the question becomes why the result is not good.

[JohelEGP]

We can't use the IDE tree feature of Compiler Explorer with MSVC: compiler-explorer/compiler-explorer#2763 (reply in thread).

(2) If the answer of (1) is yes, then the question becomes why the result is not good.

The result is good.
But if you put the export after the language linkage,
that's not valid grammar and Clang should reject just like GCC,
rather than accept and give an unexpected answer.

[ChuanqiXu9]

But if you put the export after the language linkage, that's not valid grammar and Clang should reject just like GCC, rather than accept and give an unexpected answer.

Yeah, this is my first question. It looks like GCC will accept it in my example. And I need to figure why it is allowed or not from the standard wording.

[JohelEGP]

Well, in the case of a block, at least it's valid grammar.

[JohelEGP]

I remember that name-declaration was introduced to avoid export on declarations without a name.
A linkage-specification is a name-declaration, so we can export it.
If a single export extern "C" ... is valid,
it makes sense that extern "C" { export ...; } also would.

[ChuanqiXu9]

So extern "C" { export ...; } is valid but extern "C" export ... is not? This is pretty odd to me. I need to investigate more on that...

[JohelEGP]

Turns out I was wrong.
An export-declaration is a special-declaration which is a declaration.