https://issues.dlang.org/show_bug.cgi?id=23994

          Issue ID: 23994
           Summary: Compiler fails to derive lambda to be const
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody@puremagic.com
          Reporter: andrej.mitrovich@gmail.com

DMD 2.102.0

-----
module test;

void call(void delegate(int i) const cb) {
    cb(42);
}

struct C {
    void callback(int i) const { }
}

void main() {
    C c;
    call(some_int => c.callback(some_int));
}
-----

$ dmd test.d
> test.d(13): Error: function `test.call(void delegate(int i) const cb)` is not callable using argument types `(void)
> test.d(13):        cannot pass argument `__lambda2` of type `void` to parameter `void delegate(int i) const cb`

Commenting out `const` from both the parameter and the ballback function fixes the issue.

Another workaround is to explicitly specify the callback type at the call site:

-----
// Works with `const` with the example above
call((int some_int) const => c.callback(some_int));
-----

But the compiler should be able to derive this on its own.

This workaround also works, where the type is omitted:

-----
call((some_int) const => c.callback(some_int));
-----

--