Troubles with template constraints on string and static if

[sungal via Digitalmars-d-learn]

I have this piece of code and I can't understand why the `static 
if` conditionals are always false.

```
import std.digest.sha;
import std.file;
import std.stdio;
void main()
{
    auto hash1 = produceHash!string("prova.d");
    auto hash2 = produceHash!File(File("./prova.d"));
}

string produceHash(Type)(Type data)
    if(is(Type == string) || is(Type == File))
{
    string h;
    static if(is(Type == File)) {
        enforce(data.exists, "File does not exist.");
        // Hashing both name and file, to avoid having the same 
Hash on empty files
        return 
toHexString(digest!SHA256(data.byChunk(4096*1024))).dup;
    } else static if(is(Type == string)){
        return toHexString(digest!SHA256(data)).dup;
    }
    static assert(false, "Data must be string or file. ");
}
```

```
prova.d(22): Error: static assert  "Data must be string or file. "
prova.d(7):        instantiated from here: produceHash!string
```