What is the reasoning behind the lack of conversions when passing parameters

[Carl Sturtivant]

```
import std.variant, std.math, std.stdio;

void f(Variant z) {
	writeln(z);
}

void main() {
	auto x = sqrt(2.0);
	Variant v = x;
	f(v);
	//f(x);
}
```

The commented out call `f(x)` is illegal in D.
`f(VariantN!32LU v) is not callable using argument types (double)`

Yet in principle it is an attempt to create and initialize a 
variable `z` local to `f` of type Variant.

Exactly like the legal `Variant v = x` declaration.

Why exactly is vanilla parameter passing not given the same 
semantics as initialization?