Struct template cannot deduce function from argument types

[H. S. Teoh]

On Wed, Jun 27, 2018 at 04:19:56PM +0000, Luka Aleksic via Digitalmars-d-learn wrote:
[...]
> struct pair(T, U) {
> 	T first;
> 	U second;
> 
> 	this(T arg_first, U arg_second) {
> 		first = arg_first;
> 		second = arg_second;
> 	}
> };
> 
> void main() {
> 
> 	pair!(char, uint) p1 = pair('a', 1);

The usual way I'd write this is:

	auto p1 = pair!(char, uint)('a', 1);

This saves having to retype a complicated type, and also gives the
compiler the template arguments in the place where it needs them.


[...]
> I am getting the following error:
> 
> scratch.d(14): Error: struct scratch.pair cannot deduce function from
> argument types !()(char, int), candidates are:
> scratch.d(2):        scratch.pair(T, U)
> Failed: ["/usr/bin/dmd", "-v", "-o-", "scratch.d", "-I."]

The reason is that the compiler runs semantic on the right-hand side of
the assignment first, before it looks at the type of p1.  The expression
`pair('a', 1)` is ambiguous, since the compiler doesn't (yet) know which
instantiation of `pair` you intended.

Writing it the way I recommend above avoids this problem.


T

-- 
Answer: Because it breaks the logical sequence of discussion.
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