Give warning if losing value to defvaralias (Bug#5950)

* src/eval.c (Fdefvaralias): Call `display-warning' if the alias
target has a non-eq value to the variable being aliased.
* test/src/eval-tests.el (defvaralias-overwrite-warning): New test.

1 files changed, 10 insertions, 0 deletions

diff --git a/src/eval.c b/src/eval.c
index 86011a234c0..5c7cb3196a6 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -623,6 +623,16 @@ The return value is BASE-VARIABLE.  */)
   if (NILP (Fboundp (base_variable)))
     set_internal (base_variable, find_symbol_value (new_alias),
                   Qnil, SET_INTERNAL_BIND);
+  else if (!NILP (Fboundp (new_alias))
+           && !EQ (find_symbol_value (new_alias),
+                   find_symbol_value (base_variable)))
+    call2 (intern ("display-warning"),
+           list3 (intern ("defvaralias"), intern ("losing-value"), new_alias),
+           CALLN (Fformat_message,
+                  build_string
+                  ("Overwriting value of `%s' by aliasing to `%s'"),
+                  new_alias, base_variable));
+
   {
     union specbinding *p;