• kdevel (30/30) Jan 14 2018 vartmpl.d
• ag0aep6g (25/27) Jan 14 2018 The code boils down to this:

kdevel <kdevel vogtner.de> writes:

vartmpl.d
```
import std.stdio : writeln;
import decimal : decimal32;

template F(T) {
    immutable T c = 3;
}

void foo (T) ()
{
    immutable T t = 1;
}

void main ()
{
//   immutable decimal32 i = 1; // Error: none of the overloads 
of '__ctor' are
callable using a immutable object
//   foo!decimal32; //  Error: none of the overloads of '__ctor' 
are callable us
ing a immutable object, candidates are:
    alias c = F!decimal32.c;
    c.writeln;
    writeln (typeof (c).stringof);
}
```

$ dmd -g vartmpl.d decimal.git/libdecimal.a
$ ./vartmpl

3
immutable(Decimal!32)

Why does this compile while both of the commented lines give a 
compile error. decimal ist http://rumbu13.github.io/decimal/

ag0aep6g <anonymous example.com> writes:

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.