variable template question

[ag0aep6g]

On Sunday, 14 January 2018 at 16:23:18 UTC, kdevel wrote:
> Why does this compile while both of the commented lines give a 
> compile error.

The code boils down to this:

----
struct decimal32
{
     this(int x) {}
}

immutable decimal32 c = 3; /* works */

void main ()
{
    immutable decimal32 i = 1; /* error */
}
----

I think this is CTFE being unexpectedly smart.

If you add the `pure` attribute to the constructor, then the `i` 
line works as well. That's because a strongly pure constructor is 
guaranteed to return a unique object, and a unique object can be 
converted implicitly to other mutability levels.

The `pure` attribute is needed for `i`, because here the compiler 
only looks at the function attributes to determine purity. No 
`pure` attribute -> function is regarded as impure.

But for `c`, the constructor goes through CTFE, and CTFE doesn't 
care all that much about the `pure` attribute. Instead, CTFE just 
tries to evaluate the function and aborts when it encounters an 
action that would be impure.