Although the coding in the dmd looks almost identical for dynamic arrays and associative arrays, following example works only for dynamic arrays.

https://github.com/dlang/dmd/blob/master/src/dmd/initsem.d#L634

struct Bar
{
    string s;
}

struct Foo
{
    Bar[string] asso;
    Bar[] arr;
}

void main()
{
    Foo foo = {
        arr: [{s: "123"}],
        asso: ["0": {s: "123"}] // not an associative array initializer
    };
}

Although there is an DIP, this might be more a bug fix, as it works well for dynamic arrays. Providing the same support for associative arrays seems natural