P3868R1
Allow #line before module declarations

1. Changelog

• R1 - Change wording to not use a group, and be compatible with Alisdair’s editorial change to the line directive wording.

2. 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.

3. The Fix

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

4. Wording

Modify [cpp.pre]:

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

control-line:
# include pp-tokens new-line
pp-import
line-directive
# embed pp-tokens new-line
# define identifier replacement-list new-line
# define identifier lparen identifier-list_opt ) replacement-list new-line
# define identifier lparen ... ) replacement-list new-line
# define identifier lparen identifier-list , ... ) replacement-list new-line
# undef identifier new-line
# line pp-tokens new-line
# error pp-tokens_opt new-line
# warning pp-tokens_opt new-line
# pragma pp-tokens_opt new-line
# new-line
line-directives:
line-directive line-directives_opt

Add before the first paragraph in [cpp.line]:

line-directive:
# line pp-tokens new-line