Following on from issue 22812, while new lines can't be ignored, they can however be escaped.

From C11 5.1.1.2-2:
---
Each instance of a backslash character ( \) immediately followed by a new-line
character is deleted, splicing physical source lines to form logical source lines. Only the last backslash on any physical source line shall be eligible for being part of such a splice. A source file that is not empty shall end in a new-line character, which shall not be immediately preceded by a backslash character before any such splicing takes place.
---


So whilst issue 22812 is an error, this should be accepted as valid:
---
# \
    pragma \
pack \
    ( \
2 \
    )
struct S
{
    int i;
    short j;
    double k;
};
# \
    pragma \
pack \
    ( \
)
---