Here is an assert that is static if possible:

  #ifndef __clang__
  #define assume(expr) \
      __extension__ ({ \
          static_assert(__builtin_choose_expr( \
              __builtin_constant_p(expr), expr, true), #expr); \
          assert(__builtin_choose_expr( \
              __builtin_constant_p(expr), true, expr)); \
      })
  #else
  #define assume(expr) \
      __extension__ ({ \
          static_assert(__builtin_constant_p(expr) ? (expr) : true, #expr); \
          assert(__builtin_choose_expr( \
              __builtin_constant_p(expr), true, expr)); \
      })
  #endif

Note that the clang semantics of language extensions sometimes differ from GCC.

I also like to use __auto_type very much:

  #define var __auto_type
  #define let __auto_type const

As in:

  #define m_max(a, b) \
      __extension__ ({ \
          let _a = (a); \
          let _b = (b); \
          _a > _b ? _a : _b; \
      })