Typeinfo inconsistencies

[Maxim Fomin]

On Tuesday, 13 August 2013 at 17:42:31 UTC, H. S. Teoh wrote:
> So, I got some free time today and decided to look into issue 
> 6210, and
> found out that the problem was caused by this:
>
> 	char[]        a = "h".dup;
> 	const(char)[] b = "h";
> 	string        c = "h";
>
> 	writeln(typeid(a).getHash(&a)); // prints 104
> 	writeln(typeid(b).getHash(&b)); // prints 703014434222502
> 	writeln(typeid(c).getHash(&c)); // prints 104
>
> This is completely b0rken, because it causes the following code 
> to fail:
>
> 	int[char[]] aa;
> 	aa["h"] = 1;
>
> 	assert(aa.dup == aa);	// fails
>
> The reason for this inconsistency is that char[] and 
> immutable(char)[]
> (aka string) have their getHash functions overridden in 
> druntime's
> src/rt/typeinfo/ti_Ag.d, but there is no such override for
> const(char)[].
>
> I tried adding another subclass for const(char)[]'s typeid that 
> inherits
> the correct version of getHash, but it didn't work because 
> presumably
> this stuff is hardcoded into the compiler somewhere.
>
> So my question is, where in the compiler decides to use the 
> specific
> typeinfos for char[] and immutable(char)[], but not 
> const(char)[]?
>
>
> T

It seems that important path is taken here: 
https://github.com/D-Programming-Language/dmd/blob/master/src/typinf.c#L138

And root of the issue maybe here: 
https://github.com/D-Programming-Language/dmd/blob/master/src/typinf.c#L793 
(essentially if(char && immutable))

Based on ad-hoc dmd debug:

If type is "AssociativeArray!(const(char)[], int)" 
Type::builtinTypeInfo returns 0.

If type is "char[]" !t->vtinfo is false, so entire branch is 
skipped.

If type is "const(char)[]" then !t->builtinTypeInfo() is true and 
dmd executes COMDAT generation branch.

If type is "const(char)" then !t->builtinTypeInfo() is true and 
dmd executes COMDAT generation branch (also if (t->isConst()) is 
also true).

If type is "char" then "!t->builtinTypeInfo()" is false.

It seems that it is consistent with your observations.