Clang incorrectly complains about default template argument not being reachable

[ilya-biryukov]

Run a compiler on this code:

// --- foo.cppm
export module foo;

export struct partial {};
export template <class T, class Cat = partial> concept is_comparable = true;

// --- bar.cppm
export module bar;

import foo;
template <class T>
void func() requires is_comparable<T> {}

The exact commands are:

$ clang++ -std=c++20 --precompile foo.cppm
$ clang++ -std=c++20 --precompile bar.cppm -fmodule-file=foo.pcm

Expected results: code compiles with no errors.
Actual: Clang produces the following error when compiling the module bar:

bar.cppm:6:22: error: default argument of 'is_comparable' must be imported from module 'foo' before it is required
void func() requires is_comparable<T> {}
                     ^
foo.cppm:5:39: note: default argument declared here is not reachable
export template <class T, class Cat = partial> concept is_comparable = true;
                                      ^
1 error generated.

[llvmbot]

@llvm/issue-subscribers-clang-modules

Run a compiler on this code: ```cpp // --- foo.cppm export module foo;

export struct partial {};
export template <class T, class Cat = partial> concept is_comparable = true;

// --- bar.cppm
export module bar;

import foo;
template
void func() requires is_comparable {}

The exact commands are:
```sh
$ clang++ -std=c++20 --precompile foo.cppm
$ clang++ -std=c++20 --precompile bar.cppm -fmodule-file=foo.pcm

Expected results: code compiles with no errors.
Actual: Clang produces the following error when compiling the module bar:

bar.cppm:6:22: error: default argument of 'is_comparable' must be imported from module 'foo' before it is required
void func() requires is_comparable<T> {}
                     ^
foo.cppm:5:39: note: default argument declared here is not reachable
export template <class T, class Cat = partial> concept is_comparable = true;
                                      ^
1 error generated.

[llvmbot]

@llvm/issue-subscribers-c-20

Run a compiler on this code: ```cpp // --- foo.cppm export module foo;

export struct partial {};
export template <class T, class Cat = partial> concept is_comparable = true;

// --- bar.cppm
export module bar;

import foo;
template
void func() requires is_comparable {}

The exact commands are:
```sh
$ clang++ -std=c++20 --precompile foo.cppm
$ clang++ -std=c++20 --precompile bar.cppm -fmodule-file=foo.pcm

Expected results: code compiles with no errors.
Actual: Clang produces the following error when compiling the module bar:

bar.cppm:6:22: error: default argument of 'is_comparable' must be imported from module 'foo' before it is required
void func() requires is_comparable<T> {}
                     ^
foo.cppm:5:39: note: default argument declared here is not reachable
export template <class T, class Cat = partial> concept is_comparable = true;
                                      ^
1 error generated.

[ilya-biryukov]

I believe this particular bug is only applicable to older Clang versions (definitely present in clang 14).
I could not actually reproduce at head.

But we are seeing a similar issue with a generated module map we're using for libc++, I will keep this issue open for a few days while I'm trying to get a reduced reproducer.

[cor3ntin]

@ilya-biryukov can we close?