Hello @mayjs,

interesting and thanks for tracing it so far. Can you give more details about the build environment. Was it a out-of-clang build? What did CMake initially print out?

Andreas

This is a problem with include path ordering. The order is:

1. -isystem paths
2. Implicit system include paths
3. -I paths

This combined with the fact that /usr/include absolutely has to be the last of the system include paths can cause problems. What happens here is that specifying only -isystem/usr/include makes /usr/include come before all other implicit system include paths.

A few more bits of information:

• Include paths can be examined with clang++ -E -v.

• The reason why the order matters is in the line right after what @mayjs posted:

  #include_next <stdlib.h>

With include paths reordered, #include_next from cstdlib is unable to find anything in /usr/include.

I do tamper a bit with the includes in CMakeLists.txt.

I'm not sure what a better solution might be.

That line turns user defined include paths (-I) into system include paths (-isystem) which should be fine unless someone explicitly declared the implicit include paths as -I. And even then you'd be fine if all implicit paths got "transformed" (for lack of a better word). The problem would appear if only some of implicit include paths got turned explicit with -isystem.

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