Always false float comparisons

[Joseph Rushton Wakeling via Digitalmars-d]

On Monday, 16 May 2016 at 09:54:51 UTC, Iain Buclaw wrote:
> On 16 May 2016 at 10:52, Ola Fosheim Grøstad via Digitalmars-d 
> <digitalmars-d at puremagic.com> wrote:
>> On Monday, 16 May 2016 at 08:47:03 UTC, Iain Buclaw wrote:
>>>
>>> But you *didn't* request coercion to 32 bit floats.  
>>> Otherwise you would have used 1.30f.
>>
>>
>>         const float f = 1.3f;
>>         float c = f;
>>         assert(c*1.0 == f*1.0); // Fails! SHUTDOWN!
>>
>>
>
> Your still using doubles.  Are you intentionally missing the 
> point?

I think you're missing what Ola means when he's talking about 
32-bit floats.  Of course when you multiply a float by a double 
(here, 1.0) you promote it to a double; but you'd expect the 
result to reflect the data available in the 32-bit float.

Whereas in Ola's example, the fact that `const float f` is known 
at compile-time means that the apparent 32-bit precision is in 
fact entirely ignored when doing the * 1.0 calculation.

In other words, Ola's expecting

     float * double == float * double

but is getting something more like

     float * double == double * double

or maybe even

     float * double == real * real

... because of the way FP constants are treated at compile time.