fix another spurious expint domainerror

Author: stevengj

src/expint.jl

@@ -188,7 +188,11 @@ function En_cf_nogamma(ν::Number, z::Number, n::Int=1000)
 
     exppart = exp(-z)
     if isinf(exppart)
-        return exp(-z + log(A) - log(B)), iters
+        if A isa Real
+            return sign(A)*sign(B)*exp(-z + log(abs(A)) - log(abs(B))), iters
+        else
+            exp(-z + log(A) - log(B)), iters
+        end
     else
         cfpart = A/B
         return cfpart * exppart, iters

test/expint.jl

@@ -130,6 +130,7 @@ using Base.MathConstants
     @test expint(-2.2, 3.2) ≈ 0.024950173497409329191241353395358 rtol=1e-14
     @test expint(+2.2, 3.2) ≈ 0.008044603700773423319087602010 rtol=1e-12
     @test expint(3.2, 1.3) ≈ 0.070147692224611216675759479422283315452559216337905 rtol=1e-14
+    @test expint(-20, -1000) === -Inf
 end
 
 expinti_real(x) = invoke(expinti, Tuple{Real}, x)