[OB] Fix simplification of if-equation (#13935) (#13941)

Fixes #13927

Author: phannebohm

OMCompiler/Compiler/BackEnd/BackendDAEOptimize.mo

@@ -76,6 +76,7 @@ import ExpressionSimplify;
 import Error;
 import Flags;
 import GCExt;
+import HashSet;
 import HashTableExpToIndex;
 import HpcOmTaskGraph;
 import List;
@@ -2489,7 +2490,7 @@ algorithm
         iHt;
     case (c::explst,eqns::rest,_)
       equation
-        ht = simplifySolvedIfEqns1(c,eqns,iHt);
+        ht = simplifySolvedIfEqns1(c,eqns,iHt, HashSet.emptyHashSet());
       then
         simplifySolvedIfEqns(explst,rest,ht);
   end match;
@@ -2501,35 +2502,47 @@ protected function simplifySolvedIfEqns1
   input DAE.Exp condition;
   input list<BackendDAE.Equation> brancheqns;
   input HashTable2.HashTable iHt;
+  input HashSet.HashSet iHs;
   output HashTable2.HashTable oHt;
 algorithm
+  // ticket #13927
+  // Only proceed if `cr` was not already assigned within current branch.
+  // This is checked by collecting all `cr` in `hs`.
+  // Prevents errors like:
+  // if b then a = r1; a = r2; else a = r3; c = r4; end if;
+  // to
+  // a = if b then r2 else if b then r1 else r3; c = r4;
+
   oHt := match(condition,brancheqns,iHt)
     local
       DAE.ComponentRef cr;
       DAE.Exp e,exp;
       DAE.ElementSource source;
       HashTable2.HashTable ht;
+      HashSet.HashSet hs;
       list<BackendDAE.Equation> rest;
     case (_,{},_)
       then
         iHt;
     case (_,BackendDAE.EQUATION(exp=DAE.CREF(componentRef=cr), scalar=e)::rest,_)
       equation
         false = Expression.expHasCref(e, cr);
+        hs = BaseHashSet.addUnique(cr, iHs);
         exp = BaseHashTable.get(cr, iHt);
         exp = DAE.IFEXP(condition, e, exp);
         ht = BaseHashTable.add((cr,exp), iHt);
       then
-        simplifySolvedIfEqns1(condition,rest,ht);
+        simplifySolvedIfEqns1(condition,rest,ht, hs);
     case (_,BackendDAE.EQUATION(exp=DAE.UNARY(operator=DAE.UMINUS(), exp=DAE.CREF(componentRef=cr)), scalar=e)::rest,_)
       equation
         false = Expression.expHasCref(e, cr);
+        hs = BaseHashSet.addUnique(cr, iHs);
         exp = BaseHashTable.get(cr, iHt);
         e = Expression.negate(e);
         exp = DAE.IFEXP(condition, e, exp);
         ht = BaseHashTable.add((cr,exp), iHt);
       then
-        simplifySolvedIfEqns1(condition,rest,ht);
+        simplifySolvedIfEqns1(condition,rest,ht, hs);
   end match;
 end simplifySolvedIfEqns1;