eval-expression-debug-on-error

eval-expression-debug-on-error is a variable defined in `simple.el'.
Its value is

t

Documentation:
If non-nil set `debug-on-error' to t in `eval-expression'.
If nil, don't change the value of `debug-on-error'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 21.1 of Emacs.