[Bug preprocessor/79948] New: _Pragma(“GCC error”) is processed differently inside and outside of a #if directive

[duparq at free dot fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79948

            Bug ID: 79948
           Summary: _Pragma(“GCC error”) is processed differently inside
                    and outside of a #if directive
           Product: gcc
           Version: 5.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: duparq at free dot fr
  Target Milestone: ---

Created attachment 40919
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40919&action=edit
Source file that produces the unexpected behavior

I process this with gcc -E -std=c1x -Wall -Wextra -Wpedantic main.c >output.

I get this on the command line (almost normal):

'''
main.c:16:11: error: This is an error
 "ISVOID(E):" ISVOID(E)
           ^
'''

I get that in the output:

'''
# 1 "main.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "main.c"
# 14 "main.c"
"ISVOID(something):" 0
"ISVOID():" 1
"ISVOID(E):"
# 16 "main.c"

# 16 "main.c"
 1





"ISVOID(something)==1: false"



"ISVOID()==1: true"







"ISVOID(E)==1: false"
'''

Then, ISVOID(E) expands to 1 outside the #if directive and it expands to 0
inside, meaning that _Pragma("GCC error") is processed differently inside and
outside a #if directive.

Moreover, there is no emission of a "This is an error" related to the #if
ISVOID(E) == 1.

Is that behavior normal?