[Bug c++/90126] gcc can not correctly deal with its own preprocessed output

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90126

Richard Biener  changed:

   What|Removed |Added

   Keywords||diagnostic
 Status|UNCONFIRMED |RESOLVED
 Resolution|--- |INVALID

--- Comment #3 from Richard Biener  ---
Confirmed.  I think this is odd behavior of the warning (not sure what it is
about).  Note you get no warning when you pass -fpreprocessed.  The key
to the diagnostic is that the anonymous namespace appears in a file
(through a #line directive) that is not the same as the file compiled.
Thus, the following testcase warns:

# 1 "t.C"
namespace {
struct Receiver { int object; };
}
struct Node
{
Receiver receiverQueue;
Node() { }
};
int main(int argc, char* argv[]) { return 0; }

> g++ t2.C

note to put the testcase into a file named t2.C, it doesn't warn when
the filename is t.C.  This is because the warning intends to warn about
anonymous namespaces in headers which, when included from multiple sources
may cause issues.

So I think this behaves as intended and you need to compile preprocessed
source with -fpreprocessed (or use t2.ii filenames) or retain the original
filename.  Or disable this particular warning.

[Bug c++/90126] gcc can not correctly deal with its own preprocessed output

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90126

--- Comment #2 from Jonathan Wakely  ---
tl;dr the preprocessor should only be used once. You're running it twice on the
same input.

[Bug c++/90126] gcc can not correctly deal with its own preprocessed output

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90126

--- Comment #1 from Jonathan Wakely  ---
I don't think this is a bug.

If you tell gcc that the preprocessed output is preprocessed output, then the
behaviour is consistent. So either:

g++ -E namespace_anonymous_1_min_ok.cpp > temp_namespace_anonymous.ii
g++ temp_namespace_anonymous.ii

Or:

g++ -E namespace_anonymous_1_min_ok.cpp > temp_namespace_anonymous.cpp
g++ -x c++-cpp-output temp_namespace_anonymous.cpp


The warning is suppressed when the type is defined in the "main input context"
i.e. not in a header file. When you treat the preprocessed output as
unpreprocessed source you end up with two sets of line markers, which makes GCC
think the type is defined in a header.

If you want to use preprocessed output this way you should probably generate it
without line markers:

g++ -E -P namespace_anonymous_1_min_ok.cpp > temp_namespace_anonymous.cpp
g++ temp_namespace_anonymous.cpp