why does a constructor with default arguments fail to match, when default construction is disabled?

[Nick Treleaven]

On Saturday, 22 August 2026 at 04:19:54 UTC, DanielG wrote:
> This is actually a reduced example - I first noticed the 
> problem when my class contained a struct with `@disable this`.
...
> I would expect the human-provided constructor with default 
> arguments to be matched even in the absence of a 
> default/zero-arg constructor. From the output we can see that 
> the provided constructor is the only one that ever matches/runs 
> ... but if we `@disable this`, suddenly it refuses to match. 
> This is surprising.

There's no need to `@disable this();` for a class with a defined 
constructor, as one would not be generated:
https://dlang.org/spec/class.html#implicit-base-construction

There seems to be a bug with default constructor arguments here, 
however you can workaround it by using a delegating constructor:

```d
import std.stdio;

struct S
{
	int i;
	@disable this();
}

class MyClass {
	S s;

	//this(string y = "hi") { // bug
	// workaround:
	this() { this("hi"); }
	this(string y) {
		writeln("hello from MyClass constructor");
		S tmp = {1};
		s = tmp;
	}
}

void main()
{
	// always works
	auto x = new MyClass("hi");

	// works now
	auto y = new MyClass;
}
```