tidy up expression graphs

src/nlp.jl

@@ -1018,6 +1018,9 @@ end
 
 # currently don't merge duplicates (this isn't required by MPB standard)
 function affToExpr(aff::AffExpr, constant::Bool)
+    if length(aff.vars) == 0 && !constant
+        return 0
+    end
     ex = Expr(:call,:+)
     for k in 1:length(aff.vars)
         push!(ex.args, Expr(:call,:*,aff.coeffs[k],:(x[$(aff.vars[k].col)])))
@@ -1055,6 +1058,11 @@ function tapeToExpr(k, nd::Vector{NodeData}, adj, const_values, parameter_values
         op = nod.index
         opsymbol = operators[op]
         children_idx = nzrange(adj,k)
+        if opsymbol == :+ && length(children_idx) == 0
+            return 0
+        elseif opsymbol == :* && length(children_idx) == 0
+            return 1
+        end
         ex = Expr(:call,opsymbol)
         for cidx in children_idx
             push!(ex.args, tapeToExpr(children_arr[cidx], nd, adj, const_values, parameter_values, subexpressions))

test/nonlinear.jl

@@ -663,6 +663,17 @@ facts("[nonlinear] Expression graph for ifelse") do
     @fact MathProgBase.obj_expr(d) --> :(ifelse( x[1] <= 1, x[1]^2, x[1]))
 end
 
+facts("[nonlinear] Expression graphs for corner cases") do
+    m = Model()
+    @variable(m, x, start = 2)
+    @constraint(m, 0 <= 1)
+    @NLconstraint(m, x <= sum{0, i in []} + prod{1, i in []})
+    d = JuMP.NLPEvaluator(m)
+    MathProgBase.initialize(d, [:ExprGraph])
+    @fact MathProgBase.constr_expr(d,1) --> :(0 <= 1.0)
+    @fact MathProgBase.constr_expr(d,2) --> :(x[1] - (0 + 1) <= 0.0)
+end
+
 facts("[nonlinear] Hessians through MPB") do
     # Issue 435
     m = Model()