IFTI fails with lazy variadic function in some cases

[dlangBugzillaToGithub]

Mario Kroeplin reported this on 2016-08-01T09:12:44Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=16345

CC List

• ag0aep6g

Description

The following code does not compile:

void all(string file = __FILE__, size_t line = __LINE__)(void delegate()[] dgs ...)
{
    foreach (dg; dgs)
    {
        dg();
    }
}

void main()
{
    all(
        assert(true),
    );
}

Error: template all cannot deduce function from argument types !()(void), candidates are:
       all(string file = __FILE__, ulong line = __LINE__)(void delegate()[] dgs...)

Workaround: call `all!()(...)` instead of `all(...)`.

[dlangBugzillaToGithub]

ag0aep6g commented on 2016-08-01T10:55:01Z

For reference, the spec describes lazy variadic functions here:
http://dlang.org/spec/function.html#lazy_variadic_functions

The test case doesn't need the template parameters to fail. This fails, too:

----
void all()(void delegate()[] dgs ...) {}
void f() {}
void main() { all(f()); } /* Error: template test.all cannot deduce function from argument types !()(void) */
----

It works when the return type is int or another primitive type, including pointers and arrays:

----
void all()(int delegate()[] dgs ...) {}
int f() { return 0; }
void main() { all(f()); } /* no error */
----

But it fails with Object and user-defined aggregates. Most peculiarly, it also fails with an alias to a primitive type:

----
alias T = int;
void all()(T delegate()[] dgs ...) {}
T f() { return T.init; }
void main() { all(f()); } /* Error: template test.all cannot deduce function from argument types !()(int) */
----