how to instantiate explicitly template parameters in struct A(T1){this(T2)(){...}}

[Timothee Cour]

While writing DIP40, I came upon the following question:
how to instantiate explicitly template parameters for both the
class/struct AND the constructor?
for example: struct A(T1){this(T2)(){...}}  ? (T2 could be used
somehow inside the ctor body for example)
// auto a=A!double!int(); // CT error
The example is a bit contrived but there could be less contrived ones
where both A and ctor need explicit instantiations.