Need clear error msg. (template instantiating)

[Andrei Alexandrescu]

CLXX wrote:
> I need clear error-messages of compiling.
> See this code.
> 
> // example.d
> import std.conv: to;
> void main( ){
>     S s;
>     to!( int )( s ); // 'to' does not support struct-argument.
> }
> struct S{
> }
> 
> 'to' cannot convert 'S' to 'int', so that this is error.
> Then I got mysterious messages.
> 
> "src\phobos\std\conv.d(344): Error: cannot implicitly convert expression (value) of type S to int"
> "src\phobos\std\conv.d(206): template instance std.conv.toImpl!(S,int) error instantiating"
> "src\phobos\std\conv.d(4): template instance std.conv.to!(int).to!(S) error instantiating"
> 
> We know 'std.conv.d' has no error, and assume 'example.d' has some.
> The problem is 'std.conv.d' seems to have errors.
> 
> The message should be as follows.
> 
> "example.d(5): template instance to!(S)(int,S) does not match template declaration."
> 
> Thank you for your reading.

Thanks, that's a good point. We need to make a few changes to the way 
the compiler and static assert work to get to that point. Phobos in my 
tree (i.e., as of two releases from now) already points at your file and 
line, however with not such a clear message:

./example.d(5): template std.conv.to(T,S) if (!implicitlyConverts!(S,T) 
&& isSomeString!(T) && isSomeString!(S)) does not match any function 
template declaration
./example.d(5): template std.conv.to(T,S) if (!implicitlyConverts!(S,T) 
&& isSomeString!(T) && isSomeString!(S)) cannot deduce template function 
from argument types!(int)(S)

Showing the if clause is completely superfluous (I just filed a bug 
report about that, see 
http://d.puremagic.com/issues/show_bug.cgi?id=2696) so at least you'll 
see something like:

./example.d(5): template std.conv.to(T,S) does not match any function 
template declaration
./example.d(5): template std.conv.to(T,S) cannot deduce template 
function from argument types!(int)(S)

which is rather passable.


Andrei