P3868R0
Allow #line before module declarations

1. The Problem

After implementing part of [P1857R3], Clang soon discovered that it makes the following code ill-formed:

#line 1 "A.cppm"
export module a;

This is a problem because tools such as build2, ccache, and distcc, among many others, rely on feeding the preprocessed output back into the compiler. This output generally contains #line (or GNU line marker) directives at the start of the file.

2. The Fix

The standard should allow #line directives to appear on any line.

3. Wording

Modify [cpp.pre]:

module-file:
group_opt pp-global-module-fragment_opt pp-module group_opt pp-private-module-fragment_opt

4 At the start of phase 4 of translation, the group of a pp-global-module-fragment shall contain neither a text-line nor a pp-import.

5 At the start of phase 4 of translation, the leading group of a module-file, if present, shall consist only of #line directives.