Default value for external generic types

[momvart]

The C++ translator assumes there's a get_XXXX_default() for each external type.

dafny/Source/DafnyCore/Backends/Cplusplus/CppCodeGenerator.cs

Line 1046 in e7e3ed4

return String.Format("{1}::get_{0}_default()", IdProtect(udt.Name), udt.ResolvedClass.EnclosingModuleDefinition.GetCompileName(Options));

While this works for non-generic types, for generic ones, even if such implementation exists, it still requires the generic (template) parameters to be passed to it.

For example:

type {:extern "struct"} XXXX<T>

class A {
   var x: XXXX<B>
}

The generated code will be:

class A {
XXXX<B> x = get_XXXX_default();
}

If we provide a default function like below:

template <typename T>
XXXX<T> get_XXXX_default() {
    // ...
}

the C++ compiler does not infer B at the call site, and complains.

The solution would be to provide B to the function call, or use the same get_default so the user specializes it for the external type.