[Bug c++/61412] Warnings incorrectly suppressed when compiling previously preprocessed file

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61412

--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
cat > sys/h.h <<EOT
#define X(I) char c = { I }
EOT
cat > C.C << EOT
#include <h.h>
int main () {
  int i = 256;
  X(i);
}
EOT
g++ -isystem sys C.C -Wall
g++ -isystem sys C.C -Wall -Wsystem-headers
In file included from C.C:1:0:
C.C: In function ‘int main()’:
C.C:4:3: warning: narrowing conversion of ‘i’ from ‘int’ to ‘char’ inside { }
is ill-formed in C++11 [-Wnarrowing]
   X(i);
   ^
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From: "matt at godbolt dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/61412] Warnings incorrectly suppressed when compiling previously preprocessed file
Date: Wed, 04 Jun 2014 17:04:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?ida412

--- Comment #12 from Matt Godbolt <matt at godbolt dot org> ---
I appreciate your patience on this! I'm also now confused.  My 250k+
preprocessed file does not warn on a reordered constructor initializer, despite
there being no system code or macros involved in it.  Compiling the original
file (not the preprocessed version) directly warns as expected.

While attempting the reduce to an absolute minimal case I found the (or maybe
just one) issue that caused the suppression of the warning was the "#" line.
This is many hundreds of lines above the constructor and its initializer and is
unconnected to a system include.

The "#" line itself refers only to a non-system file, which confuses me.

I will try and get a non-preprocessed reproducible case to try and clear up any
confusion.