Arguments to function-like macros are not expanded before substitution

The preprocessor does not expand arguments of function-like macros before substitution. Instead, it seems to expand the result of a function-like macro only after substitution.

#define BEFORE AFTER
#define SHOW(a, b) show_##a show_##b
#define TEST(a) SHOW(test_##a, a)
TEST(BEFORE)

• QtCreator result: show_test_BEFORE show_BEFORE
• g++ -E result: show_test_BEFORE show_AFTER

I think the rule is that macro arguments are expanded unless they're used with stringification or token pasting. See https://gcc.gnu.org/onlinedocs/cpp/Argument-Prescan.html .

This causes failures with most uses of the boost preprocessor library and is likely the root cause behind QTCREATORBUG-11243.