[NF] Various improvements.

- Implement expansion for diagonal().
- Mark slice subscripts in equations as structural.

Belonging to [master]:
  - #2707

Author: perost

Compiler/NFFrontEnd/NFExpandExp.mo

@@ -271,10 +271,11 @@ public
   algorithm
     (outExp, expanded) := match Absyn.pathFirstIdent(fn_path)
       case "cat" then expandBuiltinCat(args, call);
-      case "promote" then expandBuiltinPromote(args);
       case "der" then expandBuiltinGeneric(call);
+      case "diagonal" then expandBuiltinDiagonal(listHead(args));
       case "pre" then expandBuiltinGeneric(call);
       case "previous" then expandBuiltinGeneric(call);
+      case "promote" then expandBuiltinPromote(args);
       case "transpose" then expandBuiltinTranspose(listHead(args));
     end match;
   end expandBuiltinCall;
@@ -313,6 +314,18 @@ public
     exp := Expression.promote(eexp, Expression.typeOf(eexp), n);
   end expandBuiltinPromote;
 
+  function expandBuiltinDiagonal
+    input Expression arg;
+    output Expression outExp;
+    output Boolean expanded;
+  algorithm
+    (outExp, expanded) := expand(arg);
+
+    if expanded then
+      outExp := Ceval.evalBuiltinDiagonal(outExp);
+    end if;
+  end expandBuiltinDiagonal;
+
   function expandBuiltinTranspose
     input Expression arg;
     output Expression outExp;

Compiler/NFFrontEnd/NFTyping.mo

@@ -1378,13 +1378,17 @@ algorithm
         if Type.isArray(ty) then
           outSubscript := Subscript.SLICE(e);
           ty := Type.unliftArray(ty);
+
+          if ExpOrigin.flagSet(origin, ExpOrigin.EQUATION) then
+            Inst.markStructuralParamsExp(e);
+          end if;
         else
           outSubscript := Subscript.INDEX(e);
         end if;
       then
         (ty, variability);
 
-    //// Other subscripts have already been typed, but still need to be type checked.
+    // Other subscripts have already been typed, but still need to be type checked.
     case Subscript.INDEX(index = e) then (Expression.typeOf(e), Expression.variability(e));
     case Subscript.SLICE(slice = e) then (Type.unliftArray(Expression.typeOf(e)), Expression.variability(e));
     case Subscript.WHOLE() then (Type.UNKNOWN(), Dimension.variability(dimension));