Is there any hope for "lazy" and @nogc?

[Jacob Carlborg]

On 2018-07-31 09:17, Shachar Shemesh wrote:
> I'm trying to figure out what's the signature of the built-in assert. It
> does not seem that I can define a similar function myself.
>
> First attempt:
> void myAssert(bool cond, string msg) @nogc nothrow;
>
> No, because msg gets evaluated unconditionally.
>
> void myAssert(bool cond, lazy string msg) @nogc nothrow;
>
> test.d(8): Error: @nogc function test.myAssert cannot call non- at nogc
> delegate msg
>
> void myAssert(bool cond, lazy string msg @nogc nothrow ) @nogc nothrow;
>
> test.d(4): Error: found @ when expecting )
> test.d(4): Error: semicolon expected following function declaration
> test.d(4): Error: no identifier for declarator nogc
> test.d(4): Error: declaration expected, not )
> test.d(9): Error: unrecognized declaration
>
> Templates to the rescue!!!
> void myAssert(STR)(bool cond, lazy STR msg );
>
> test.d(14): Error: @nogc function D main cannot call non- at nogc function
> test.myAssert!string.myAssert


Based on what Steven said and what I found in the spec, this seems to 
work with the current language [1]:

extern (C) int printf(in char*, ...) @nogc nothrow;

void myAssert(bool cond, string delegate() @nogc nothrow[1] msg ...) 
@nogc nothrow
{
     msg[0]();
}

string param() @nogc nothrow
{
     printf("bar\n");
     return "foo";
}

void main() @nogc nothrow
{
     myAssert(false, param());
}

This prints "bar". But if the delegate is not called, then nothing is 
printed.

The signature of "myAssert" is bit ugly and not straightforward to 
understand, but it works. I think it should be possible to specify 
attributes for lazy parameters or they should be automatically inferred. 
Is there an issue for this reported?

[1] https://run.dlang.io/is/g4Rm1w

-- 
/Jacob Carlborg