boxing, struct opAssign and constructors

[gena]

Let's say I have some boxing implementation using structs.

With D1 one could have

struct Value {
   Value opAssign(double arg){...}
}

Value v1, v2;
v1 = v2;


With D2 (at least 014 and 023) I get compilation error:
function Value.opAssign (double arg) does not match parameter types (Value)

This sounds like as soon as you have at least one opAssign you MUST 
write the 'copy-constructor-like' version of opAssign
BTW, what's the right jargon for such 'reflective' opAssign?

Why not giving an OPTION to let compiler create the fastest 
copy-ctor-like opAssign automatically, as it does if I haven't defined 
any opAssigns?


-------------------------------
having the above struct definition,

Value v = 10

gives compilation error:
Error: cannot cast int to Value[]

why no line number in error?
Where does [] assumption come from?

whereas

Value v = 10.0
gives
test.d(25): Error: cannot cast double to Value

and finally

Value v; v = 10;
Value v; v = 10.0;

do compile and work the same.

NOTE: there is NO opAssign for int, only for double and Value.

-------------------------------
I couldn't find other way of calling struct constructors except using 
new operator, shouldn't there be a way to do the same on stack? Say

struct Value {
   this(double arg) {...}
}

Value v(10)
AND/OR maybe the above
Value v = 10
to be interpreted as calling the constructor?

right now only following initializer seems to work:
Value v; v = 10;
But It's ugly and I tend to question the efficiency of the produced code