http://d.puremagic.com/issues/show_bug.cgi?id=8737
Summary: Associative Array (AA) KeyType is not Unqual-able
Product: D
Version: unspecified
Platform: All
OS/Version: All
Status: NEW
Severity: major
Priority: P2
Component: DMD
AssignedTo: nob...@puremagic.com
ReportedBy: monarchdo...@gmail.com
--- Comment #0 from monarchdo...@gmail.com 2012-09-29 15:42:26 PDT ---
There is an issue with AA's, where it is not possible to extract the exact
"mutable KeyType" of an AA. This is a built in protection for when iterating on
reference keys.
The problem is that it becomes impossible to create new keys, without knowing
the "real" type.
This was revealed in a bug in conv.to : #8705
http://d.puremagic.com/issues/show_bug.cgi?id=8705
Reduced test:
--------
import std.stdio;
import std.traits;
import std.conv;
void main()
{
alias short[short[short]] S;
alias int[int[int]] T;
S value;
T result;
alias Unqual!(KeyType!T) K2; // const(int)[int]
alias Unqual!(ValueType!T) V2; // int
foreach (k1, v1; value)
{
K2 k2 = to2!K2(k1); //request a cast to "const(int)[int]" !!!
V2 v2 = to!V2(v1);
result[k2] = v2;
}
}
T to2(T, S)(S value) //T is const(int)[int]
{
alias Unqual!(KeyType!T) K2;
alias Unqual!(ValueType!T) V2;
Unqual!T result;
foreach (k1, v1; value)
{
K2 k2 = to!K2(k1); //const(int)
V2 v2 = to!V2(v1); //int
result[k2] = v2; //Error: result[k2] isn't mutable (naturally)
}
return result;
}
--------
Here, the implementer of "foo" is unable to tranform his keys, because their
types are not mutable.
The real problem is that KeyType returned "const(int)[int]", is not really
const, but not mutable either. Because of this Unqual doesn't work.
Suggestion: The type returned by KeyType should be "full const", eg:
const(int[int]).
This would make it just as safe, but a user can still request an Unqual on the
type.
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