std.conv: Fix ambiguity when target has ctor and source has opCast

[CyberShadow]

For dlang/druntime#711

[monarchdodra]

Place an assert in here. We're supposed to call opCast, right?

[monarchdodra]

I have just 1 question: There's an ambiguity between opCast and T().

You chose to default to opCast (I think). Apparently, dmd favors calling construction in this case. Shouldn't we default to calling construction too?

[CyberShadow]

Apparently, dmd favors calling construction in this case.

For assignment? I think this is something from C++.

Shouldn't we default to calling construction too?

I chose to prefer opCast because its meaning seems closer to the idea of converting one type to another, thus better suitable for conv. But since they'll probably be synonymous in most cases I don't have a strong opinion. Do you think I should change it?

[monarchdodra]

For assignment? I think this is something from C++.

I'm thinking:

    S s;
    T t = s; //Calls constructor

For assignement, you need an explict cast or contruction anyways (unlike C++).

T t;
t = s; //Fails
T = cast(T)s; //OK
T = T(s); //OK

Do you think I should change it?

Given that conv does "whatever it takes to build a T from an S", I think it would make more to do the same as the compiler. IMO.

[CyberShadow]

Right, I meant initialization, not assignment.

Updated.

[monarchdodra]

LGTM. Anyone else?

[ghost]

Auto-merge toggled on

[ghost]

LG.