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

It seems this is a limitation in the syntax. It works with an explicit 
delegate, but then that is required at the call site as well. A lazy 
parameter is basically just a delegate with a nicer syntax.

void myAssert(bool cond, string delegate() @nogc nothrow msg) @nogc nothrow
{
     auto a = msg();
}

Support for adding UDAs to function parameters was recently added. 
Perhaps we need to support other attributes as well.

Please report an issue to http://issues.dlang.org.

-- 
/Jacob Carlborg