#pragma once causes compile failures when used with includes in global module fragment

[davidstone]

Attempting to compile the following valid set of source files

// a.hpp
#pragma once
using a = int;

// b.hpp
#pragma once
#include "a.hpp"
a b;

// c.cpp
module;
#include "b.hpp"
export module c;

Causes Clang to error with

In file included from c.cpp:3:
b.hpp:4:1: error: unknown type name 'a'
a b;
^

Changing from #pragma once to #ifdnef FOO...-style include-guards causes the code to compile as expected. It appears as though #pragma once is applied to the set of all headers that have that pragma collectively?

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[davidstone]

Duplicate of #58532