Merge branch 'master' of github.com:leanprover/lean4 into joachim/kernel-comments2

I just spent too much time being confused about the kernel type checker
until I noticed that `lazy_delta_reduction` modifies its arguments.

src/kernel/type_checker.cpp

@@ -981,6 +981,7 @@ lbool type_checker::is_def_eq_offset(expr const & t, expr const & s) {
     return l_undef;
 }
 
+/** \remark t_n, s_n are updated. */
 lbool type_checker::lazy_delta_reduction(expr & t_n, expr & s_n) {
     while (true) {
         lbool r = is_def_eq_offset(t_n, s_n);
@@ -1098,6 +1099,7 @@ bool type_checker::is_def_eq_core(expr const & t, expr const & s) {
     r = is_def_eq_proof_irrel(t_n, s_n);
     if (r != l_undef) return r == l_true;
 
+    /* NB: `lazy_delta_reduction` updates `t_n` and `s_n` even when returning `l_undef`. */
     r = lazy_delta_reduction(t_n, s_n);
     if (r != l_undef) return r == l_true;