Bug in ifthen (GAMS)

Consider the models:

   1  variable x;
  2  equation e1,e2;
  3
  4  scalar s /1/;
  5
  6  e1.. x$(s>0) =e= 0;
  7  e2.. ifthen(s>0,x,0) =e= 0;
  8
  9  model m1 /e1/;
 10  solve m1 using cns;
 11
 12  model m2 /e2/;
 13  solve m2 using cns;

The models are the same. The ifthen is exogenous but GAMS does not recognize this. As a result the fist model solves fine and the second model leads to numeric problems:

** Error in Square System: Pivot too small.

Turns out the gradients are not correct for ifthen:

---- e2  =E= 
e2..  (0)*x =E= 0 ; (LHS = 0)

Advice: don't use ifthen in equations as the derivatives of this function are wrong. If possible use $ conditions (exogenous if) or a smooth approximation (endogenous if). Smooth approximations for ifthen are discussed in this blog.