Change the meaning of -fextended-default-rules
-fextended-default-rules applies to source code modules. GHCi has the extended defaulting behaviour by default for expressions typed at the prompt, and there's no way to change that.
We propose the following change: extended defaulting would only ever apply to expressions typed at the GHCi prompt, and it would be disabled with
-fno-extended-default-rules (on by default).
See also #1200 (closed), where
-fextended-default-rules was introduced.