[C++20] [Modules] <ranges> cannot be included in more than one module

[camio]

// A.cppm
module;
#include <ranges>
export module A;

// B.cppm
module;
#include <ranges>
export module B;
import A;

clang++ -fprebuilt-module-path=. -std=c++20  --precompile A.cppm -o A.pcm
clang++ -fprebuilt-module-path=. -std=c++20  --precompile B.cppm -o B.pcm

Compilation of the second file results in several errors like this

In module 'A' imported from B.cppm:6:
/usr/sbin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/ranges:1258:2: error: 'std::ranges::views::_All::operator()' from module 'A.<global>' is not present in definition of 'std::ranges::views::_All' provided earlier
        operator() [[nodiscard]] (_Range&& __r) const
        ^
/usr/sbin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/ranges:1258:2: note: declaration of 'operator()' does not match
        operator() [[nodiscard]] (_Range&& __r) const
        ^

This is using Clang trunk and libstdc++ that is distributed with GCC 12.2.1.

[llvmbot]

@llvm/issue-subscribers-clang-modules

[ChuanqiXu9]

Thanks for reporting this! Would you like to reduce the defect report? It will be pretty helpful.

We can reduce the defect report by:

1. preprocess <ranges> to a local header my_ranges.
2. include my_ranges in the reproducer.
3. Remove unnecessary things in my_ranges until we can't make it any more.

[camio]

This reduced version reproduces the issue.

// A.cppm
module;
#include "foo.h"
export module A;

// B.cppm
module;
#include "foo.h"
export module B;
import A;

// foo.h
#ifndef _FOO
#define _FOO

template <typename T> struct Foo {
  Foo(T) {}
};

template <typename T> Foo(T &) -> Foo<T>;

struct Bar {
  template <typename T>
    requires requires { Foo{T()}; }
  void baz() const {}
};

#endif

The issue seems to be related to the deduction guide. If the requires clause is changed to Foo<T>{T()};, the issue appears to go away.

[davidstone]

This feels a lot like #60486. Note in particular "The practical effect of this bug is that #include <ranges> fails when used in two modules with libstdc++."

[camio]

Thank you!

[mizvekov]

I have come across this change while working on another problem.

The fix which was committed is not correct : The profile function is supposed to support structural comparison, not referential comparison. It's even inconsistent that it would arbitrarily pick referential comparisons for 'TemplateDecl' names, but pick structural for everything else.

This can't be the real solution: When we build canonical types, we start with canonical components, and profiling these still does the right thing, because for canonical stuff, structural and referential comparisons are equivalent.

This fix causes issues where TemplateNames would become canonicalized on the first spelling appearing in source, and their spellings in diagnostics becoming inconsistent.

For example, some libcxx tests emitting diagnostics with different spellings depending on whether they were built with modules or not.