If I run "typeset -F 1 pi=3.14159", I receive a warning of an invalid base,
and the parameter is not initialised. However, the parameter appears to be
created, and thereafter has the expected rounding attribute:

$ typeset -F 1 pi=3.14159
typeset: invalid base (must be 2 to 36 inclusive): 1
$ echo $pi
0.0
$ pi=2.71828
$ echo $pi
2.7

With "-F 0", the warning is also shown, and the parameter is also
configured as a floating point, but when I expand the parameter its
precision is set to the default 10 decimal places:

$ unset pi
$ typeset -F 0 pi=3.14159
typeset: invalid base (must be 2 to 36 inclusive): 0
$ echo $pi
0.0000000000
$ pi=2
$ echo $pi
2.0000000000

The behaviour I expect is that 'pi' should be rounded to one decimal place
with "-F 1", and to the nearest integer with "-F 0".  As a fabricated
example:

$ typeset -F 1 pi=3.14159
$ echo $pi
3.1
$ typeset -F 0 e=2.71828
$ echo $e
3

For values of "-F" greater than one, zsh behaves as I expect: no warnings
are issued, and the expanded value is rounded to the specified number of
places:

$ typeset -F 3 pi=3.14159
$ echo $pi
3.142

I have replicated this in zsh 5.0.5, and I do not see any related bug when
searching http://sourceforge.net/p/zsh/bugs/ for "typeset" or "float", so I
guess this issue is still extant in the dev version.

I didn't have enough C skill to understand the relevant parameter code and
structs from bin_typeset() down to typeset_setbase() within an hour, but
please let me know if I can provide more information at a simpler level.