[NF] Improve subscripting in EvalFunction.

- Implement handling of scalar subscripts on the lhs of assignments.

Belonging to [master]:
  - OpenModelica/OMCompiler#2496
  - OpenModelica/OpenModelica-testsuite#971

Compiler/NFFrontEnd/NFEvalFunction.mo

@@ -452,8 +452,7 @@ algorithm
 
     case (Expression.SUBSCRIPTED_EXP(exp = Expression.MUTABLE(exp = var_ptr)), _)
       algorithm
-        Error.assertion(false, getInstanceName() +
-          ": missing handling of subscripted assignment", sourceInfo());
+        assignSubscriptedVariable(var_ptr, variable.subscripts, value);
       then
         ();
 
@@ -467,6 +466,55 @@ algorithm
   end match;
 end assignVariable;
 
+function assignSubscriptedVariable
+  input Mutable<Expression> variable;
+  input list<Expression> subscripts;
+  input Expression value;
+protected
+  list<Expression> subs;
+algorithm
+  subs := list(Ceval.evalExp(s) for s in subscripts);
+  Mutable.update(variable, assignArrayElement(Mutable.access(variable), subs, value));
+end assignSubscriptedVariable;
+
+function assignArrayElement
+  input Expression arrayExp;
+  input list<Expression> subscripts;
+  input Expression value;
+  output Expression result;
+protected
+  Expression sub;
+  list<Expression> rest_subs;
+  Integer idx;
+algorithm
+  result := match (arrayExp, subscripts)
+    case (Expression.ARRAY(), {sub}) guard Expression.isScalarLiteral(sub)
+      algorithm
+        idx := Expression.toInteger(sub);
+        arrayExp.elements := List.set(arrayExp.elements, idx, value);
+      then
+        arrayExp;
+
+    case (Expression.ARRAY(), sub :: rest_subs) guard Expression.isScalarLiteral(sub)
+      algorithm
+        idx := Expression.toInteger(sub);
+        arrayExp.elements := List.set(arrayExp.elements, idx,
+          assignArrayElement(listGet(arrayExp.elements, idx), rest_subs, value));
+      then
+        arrayExp;
+
+    else
+      algorithm
+        Error.assertion(false, getInstanceName() + ": unimplemented case for " +
+          Expression.toString(arrayExp) +
+          List.toString(subscripts, Expression.toString, "", "[", ", ", "]", false) + " = " +
+          Expression.toString(value), sourceInfo());
+      then
+        fail();
+
+  end match;
+end assignArrayElement;
+
 function assignExp
   input Expression lhs;
   input Expression rhs;