std library hooks

[Vladimir Panteleev]

On Saturday, 14 April 2012 at 02:46:03 UTC, Walter Bright wrote:
> all assert(exp) does when it trips is call a function in the 
> library. If you provide your own version of that function, the 
> one in the library won't be linked in.

This indeed works, but how exactly? In what case does the linker 
pick one function (which?) and in what case does it complain 
about duplicate symbol definitions? Do they have to be in 
different libraries?