clang/test/Preprocessor/cxx_true.cpp

/* RUN: clang -E t.cpp -x=c++ | grep block_1 &&
   RUN: clang -E t.cpp -x=c++ | not grep block_2 &&
   RUN: clang -E t.cpp -x=c | not grep block
*/

#if true
block_1
#endif

#if false
block_2
#endif