Remove equations like "{} = f(...)"

This should not be done, but breaks the Chemical library if we don't
and this was the previous behaviour of the compiler. Should be fixed
properly by patching index reduction, sorting/matching, or adding
assertions for lhs/rhs that the expressions evaluate to zero-dimension
(or at least that f(...) does not cause run-time errors).

Author: sjoelund

Compiler/BackEnd/RemoveSimpleEquations.mo

@@ -1150,11 +1150,33 @@ protected
   list<list<DAE.Subscript>> subslst;
   list<DAE.Exp> elst1, elst2;
   Boolean hasInlineAfterIndexReduction, expandLhs, expandRhs;
+  DAE.ElementSource source;
+  BackendDAE.EquationAttributes attr;
+  BackendDAE.Equation eq;
 algorithm
   dims := Expression.arrayDimension(ty);
   ds := Expression.dimensionsSizes(dims);
   subslst := List.map(ds, Expression.dimensionSizeSubscripts);
   subslst := Expression.rangesToSubscripts(subslst);
+  if listEmpty(subslst) then
+    (source,attr) := eqnAttributes;
+    outTpl := inTpl;
+    return;
+    /* TODO: Shouldn't we generate some sort of assertion for the lhs/rhs actually being of zero dimension or not throwing errors at run-time? */
+    for e in {lhs,rhs} loop
+      if Expression.isEvaluatedConst(e) then
+        continue;
+      end if;
+      eq := BackendDAE.WHEN_EQUATION(0, BackendDAE.WHEN_STMTS(
+        DAE.BCONST(false),
+        {BackendDAE.ASSERT(DAE.BCONST(false), DAE.SCONST("Failed assertion exp is 0"), DAE.ASSERTIONLEVEL_ERROR, source)},
+        NONE()
+      ), source, attr);
+      outTpl := simpleEquationsFinder(eq, outTpl);
+    end for;
+    return;
+    /* End TODO. Note: The above code does not execute */
+  end if;
   (,hasInlineAfterIndexReduction) := Expression.traverseExpTopDown(lhs, Expression.findCallIsInlineAfterIndexReduction, false);
   (,hasInlineAfterIndexReduction) := Expression.traverseExpTopDown(rhs, Expression.findCallIsInlineAfterIndexReduction, hasInlineAfterIndexReduction);
   (elst1,expandLhs) := List.mapFold(subslst, function Expression.applyExpSubscriptsFoldCheckSimplify(exp=lhs), false);
@@ -1163,6 +1185,9 @@ algorithm
     // If inlining after index reduction or i, we need to expand equations to pass sorting+matching
     // Note: We *should* be looking for the derivative annotation here, but it's not available directly
     //       and better would be if sorting+matching could expand/split equations when necessary
+    if false and not (expandLhs and expandRhs) then
+      print(getInstanceName() + " not expanding " + ExpressionDump.printExpStr(lhs) + " = " + ExpressionDump.printExpStr(rhs) + "\n");
+    end if;
     true := expandLhs and expandRhs "Do not expand equation if it doesn't help with anything... Like x=f(...); => x[1]=f()[1], ..., x[n]=f()[n]";
   else
   end if;