type inference changing dimensionality

Because of return type inference, my mat gets bumped to a cube. How do I “downcast” that cube again in order to pass it to a function accepting an input of type mat?
For example:

foo = randn(2, 2)
print size(foo)  --> [ 2, 2]
print type(foo)  --> mat

% this is some generic function,
% working for 1D, 2D or 3D data
function [B:cube] = generic_function(A:cube)
    B = A
end

bar = generic_function(foo)
print size(bar)  --> [ 2, 2]
print type(bar)  --> cube!

This breaks calling the following function:

% this is a more specific function,
% only working for 1D or 2D data
function [B:mat] = specific_function(A:mat)
    B = A
end

_ = specific_function(bar)  --> error

The context is some image processing function of which I know some are compatible with arbitrary dimensions, whilst another function is not.
I know I could get rid of the :cube type specifier on generic_function its input and output parameters, but that does not solve the underlying problem.
Having read the quick reference, I thought a assert(type(bar, "mat")) should fix this, but it doesn’t.