Issue 2803 - template + default argument = doesn't work

[9rnsr]

https://issues.dlang.org/show_bug.cgi?id=2803

[ghost]

I had a gut feeling you were going to work on this! Very very nice. :)

[WalterBright]

This makes me uneasy. First, there's the (deleted) comment that default parameters do not participate in type deduction. I don't remember the reason for that decision. The next issue it the comment about a workaround for existing code - doesn't this make the language more complex than necessary?

[WalterBright]

Please show the cases being worked around here, rather than a reference to something in template9.d which is a 4000+ line file and would surely get out of sync with this comment regardless.

[9rnsr]

I had just specified the issue 2803 test case. It's only 500 lines so finding it would be easy.
But OK, I added acceptable cases in the comment.

[9rnsr]

First, there's the (deleted) comment that default parameters do not participate in type deduction. I don't remember the reason for that decision.

I know the comment, but I couldn't understand the reason. As far as I see, issue 2803 is possible.

The next issue it the comment about a workaround for existing code - doesn't this make the language more complex than necessary?

This PR will add just only one more IFTI rule - if a default argument is used for the type deduction, it can refer already deduced types. For example:

auto foo(A, B)(A a, B b = A.stringof) {}
foo(1);

At the function parameter B b = A.stringof, B depends on the deduced type A (it's already deduced to int).
The example case was actually existing in earlier std.conv code.

[MartinNowak]

The next issue it the comment about a workaround for existing code

Seems to me, that existing code here means the newly added test cases.

doesn't this make the language more complex than necessary?

I'd say it makes the language simpler because default arguments should behave like normal arguments.

void foo(T)(T t = 0) {}

void main() {
    foo();
    foo(0);
}

And the possibility to refer to already deduced types matches default template arguments.

void foo(T, U = T.U)(T t, U u) {}

struct Foo {
    alias U = size_t;
}

void main() {
    foo(Foo(), 12);
}

[yebblies]

This is failing with ddmd because of a goto-skips-init error.

[9rnsr]

@yebblies Thanks, updated.

[WalterBright]

Auto-merge toggled on

[MartinNowak]

This seems to have introduced a small regression Issue 14468 – [Reg 2.068.0-devel] overload mismatch for template instance with typesafe variadic parameter.

[WalterBright]

Another regression: https://issues.dlang.org/show_bug.cgi?id=15668