Linux: list all matches in 'which' command

'man which' says:

   --all, -a
       Print all matching executables in PATH, not just the first.

But when I do that, I get:

> which --all g++
--all: Command not found.
/usr/bin/g++
> which -a g++
-a: Command not found.
/usr/bin/g++

Tried placing --all or -a after "g++" too, no luck.

- - , .
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