Add static_asserts for higher-order functions in kernel/for_each.h and kernel/pp.cpp

src/kernel/for_each.h

@@ -13,6 +13,8 @@ template<typename F>
 class for_each_fn {
     expr_cell_offset_set m_visited;
     F                    m_f;
+    static_assert(std::is_same<typename std::result_of<F(expr const &, unsigned)>::type, void>::value,
+                  "for_each_fn: return type of m_f is not void");
 
     void apply(expr const & e, unsigned offset) {
         if (is_shared(e)) {

src/kernel/pp.cpp

@@ -117,6 +117,8 @@ struct pp_fn {
 
     template<typename It>
     format pp_bnames(It const & begin, It const & end, bool use_line) {
+        static_assert(std::is_same<typename std::iterator_traits<It>::value_type, expr>::value,
+                      "pp_bnames takes an argument which is not an iterator containing expr.");
         auto it = begin;
         format r = pp_bname(*it);
         ++it;