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.
