Yes I am sure. If you change the source to
program isinfx;
uses
math;
var
x: extended;
begin
x := 1e2345;
writeln('Value of x:', x);
writeln('No crash up to now');
writeln(IsInfinite(x));
end.
You get
Value of x: 9.99999999999999999980E+2344
No crash up to now
An unhandled exception occurred at $004015ED:
EOverflow: Floating point overflow
$004015ED
and no crash if you omit the IsInfinite. The reason is clear: IsInfinite expects a double and x (=1e2345) cannot converted to double.
Anyway I've tested your code and it worked (FPC 3.0.0 + Xubuntu 16.04.4).
What does this mean? Do you get an EOverflow as decribed? Otherwise do you compile a 64-bit-Exe? Then there are no 10-Bytes-Extended and the `x := 1e2345;` may crash, I get x=inf for 64-bit.