Sliced subscripts in Modelica.ComplexMath.sum fails at code generation

[Per Östlund]

Modelica.ComplexMath.sum is defined as:

function sum 
  input Complex[:] v ;
  output Complex result;
algorithm
  result := Complex(.sum(v[:].re), .sum(v[:].im));
  annotation(Inline = true); 
end sum;

The new frontend flattens this to:

function Modelica.ComplexMath.sum
  input Complex[:] v;
  output Complex result;
algorithm
  result := Complex.'constructor'.fromReal(sum(v[:].re), sum(v[:].im));
end Modelica.ComplexMath.sum;

This causes the following error to occur during code generation:

[CodegenCFunctions.tpl:7515:14-7515:14:writable] Error: Template error: non INDEX(_)
  (i.e., slice) subscripts probably should not reach here.
  Check indexedAssign template..

The question is whether this is something that should be fixed in the frontend or the backend. The subscripts in e.g. v[:].re are superfluous since v.re would mean the same thing, but the same issue occurs with more complicated slice subscripts that can't just be removed. Removing the subscripts also makes the code generation succeed in this case, but the generated code is wrong.

Unfortunately I can't just check that the old frontend does in this case, since it flatten the function to:

function Modelica.ComplexMath.sum
  input Complex[:] v;
  output Complex result;
algorithm
  result := Complex(0.0, 0.0);
end Modelica.ComplexMath.sum;

which is obviously completely wrong.

This issue affects several of the Modelica.Electrical.Batteries models, for example ​ShowImpedance (output might be outdated, I just fixed the NF so the model passes the frontend).