c++20 modules (experimenting with Xcode 11)

问题

I'm having trouble getting c++20 modules to work with Xcode (v11 beta5).

I'm trying this:

// file a.cppm (also tried naming it a.cpp or a.cc)
export module a;
int x;

// file b.cpp
import a;  // ERROR: Module 'a' not found.
int y;
y = x;

• I get the ERROR : "Build Time" -> "Parse Issue": Module 'a' not found.
• there are no other warning/errors (and, therefore, it would seem that modules are supported by compiler/linker).
• I note that the build created no precompiled module a.pcm.

My Xcode project build settings are: using clang gnu++2a language dialect + libc++. compiling with -fmodules-ts and -std=c++2a. also enabled "Enable Modules (C and Objective-C) [no option available for C++].

so, my questions:

1. does Xcode (10 or 11) clang support c++20 modules? (I wish to avoid using headers)
2. if so, what additional configurations do i have to do?

Do i perhaps have to specify explicitly/manually (as compiler arguments) any intermediate output (eg. precompiled .pmc files/paths)? that would however be contrary to the purpose of an IDE, but perhaps Xcode only provides partial support?

来源：https://stackoverflow.com/questions/57556396/c20-modules-experimenting-with-xcode-11