Compile-time immutable AA?

[Syniurge]

This is my first code in D, I feel like I'm already abusing mixin 
macros but I'm trying to avoid as much duplicate 
code/redundancies as possible. I would love a "static switch" as 
proposed in http://d.puremagic.com/issues/show_bug.cgi?id=6921 to 
replace the first AA, but since there isn't atm, a compile-time 
AA with a mixin is more elegant than seven "static if/else if" in 
single file.

enum TypeIndex : uint {
     Nil = 0,  // Undefined or dynamic value, depending on the 
context
     Number,
     String,
     Table,
     Classinst,
     Function,
     Bool   // Only used internally to avoid unnecessary 
bool->float conversions
}

// The rest below is for compile time only

private immutable string[TypeIndex] TyIdxtoDType = [
     TypeIndex.Nil : "DynamicValue",
     TypeIndex.Number : "Number",
     TypeIndex.String : "string",
     TypeIndex.Table : "TableHeader*",
     TypeIndex.Classinst : "ClassHeader*",
     TypeIndex.Function : "FunctionValue*",
     TypeIndex.Bool : "bool"
];

private template DType(TypeIndex tyIdx) {
     mixin("alias " ~ TyIdxtoDType(tyIdx) ~ " DType;");
}

But LDC gives me this error:
TalesRuntime.d(51): Error: expression 
'[cast(TypeIndex)0u:"DynamicValue", cast(TypeIndex)1u:"Number", 
cast(TypeIndex)2u:"string", cast(TypeIndex)3u:"TableHeader*", 
cast(TypeIndex)4u:"ClassHeader*", 
cast(TypeIndex)5u:"FunctionValue*", cast(TypeIndex)6u:"bool"]' is 
not a constant

And if I remove the immutable qualifier it says that the AA isn't 
available at compile time.
Probably related: 
http://d.puremagic.com/issues/show_bug.cgi?id=6238 , but yebblies 
says that associative arrays work fine in CTFE, so I don't 
understand what I'm doing wrong. Is there a better way to create 
an AA mapping an enum to D type names?