Hi Roberto,
the program below, when compiled with GCC 4.0.2 and given the input "-3e-abcd", prints
input = `-3e-abcd' good f = -3 left = `abcd'
I changed this behavior during the 3.4.x development cycle (3.3.x did behave like Comeau), see testcase num_get/get/char/10.cc. Basically, my rationale was that of matching as closely as possible the usual usage of scanf in C: if '-3e-' is handed to scanf, it definitely assigns -3.0 and the user happily proceeds.
Then, we come to the detailed interpretation of C++ 22.2.2.1.2/11, where it says: "would have caused scanf to report an input failure". In my reading of the C standard (frankly, I have available only C99, which sometimes is subtly different from C90, the real reference), an input failure is a very specific kind of failure (see 7.19.6.2/4), which may lead to scanf returning EOF (7.19.6.2/16).
Anyway, more generally, if I read again the entire 7.19.6.2 (in particular /9 is important when it defines an input item as *a prefix* of a matching input sequence), I don't see how possibly a scanf of a single float can *fail* (either input failure or matching failure) when '-3e-' is passed. Indeed, in practice, it assigns the value and returns 1, nothing externally visible signals that something went wrong.
Paolo.