Thanks for your pull request! This is your first PR since the project has permanently switched to a standard commercial OSS license, and I've emailed you the new Contributor License Agreement (CLA), which will cover this and all your future contributions. Once it's completed I can look at your PR. For more details see #1322 .

Thanks!

I am not able to make progress evaluating this because it seems to need testing...

1. Test self-build by:
   a) Building cppfront.cpp as usual which builds from the Cpp1 sources
   b) Using that cppfront.exe to build reflect.h2 and cpp2regex.h2 (this should result in no surprising diffs) -- see the main branch's build_h2.bat
   c) Building cppfront.cpp again (this should work)
2. Test generating regression tests by:
   a) Building the Cpp2 tests into Cpp1 tests -- go to regression-tests/test-results and then run run-tests.bat
   b) Build the resulting Cpp1 tests using at least one compiler -- go to regression-tests/test-results/<your preferred configuation> and then run that directory's run-tests-... (a .bat or a .sh)

I'm currently getting failures at 1(c). I've just committed the results of 1(b) for reference.

Seems the main branch have modified on my previous code base commit, I will fix it later.

I find the root cause
Here is the example

namespace A {

template<bool> struct t { };
constexpr bool f(const t<true>&) { return true; }
constexpr t<true> o{};

} // namespace ns

A : namespace = {
        t1: <_: t<o.f()>> type == bool;
}

B : namespace = {
        using A::t1;
}

int main()
{
        return 0;
}

From main branch, it would become

#line 9 "test.cpp2"
namespace A {

}

namespace B {
        using A::t1;
}


//=== Cpp2 type definitions and function declarations ===========================

#line 1 "test.cpp2"
namespace A {

template<bool> struct t { };
constexpr bool f(const t<true>&) { return true; }
constexpr t<true> o{};

} // namespace ns

t1 is aliases from stage1, but my code will become

#line 9 "test.cpp2"
namespace A {
 template<t<CPP2_UFCS_NONLOCAL(f)(o)> UnnamedTypeParam1_1> using t1 = bool;
}

namespace B {
        using A::t1;
}


//=== Cpp2 type definitions and function declarations ===========================

#line 1 "test.cpp2"
namespace A {

template<bool> struct t { };
constexpr bool f(const t<true>&) { return true; }
constexpr t<true> o{};

} // namespace ns

Looks it needs to move cpp1 namespace definition before stage0.