Fix leading for zero polynomial

Author: blegat

src/polynomial.jl

@@ -251,7 +251,11 @@ Returns the coefficient of the leading term, i.e. `first(terms(p))`.
 Calling `leadingterm` on ``4x^2y + xy + 2x`` should return ``4x^2y``.
 """
 function leadingterm(p::AbstractPolynomialLike)
-    first(terms(p))
+    if iszero(p)
+        zeroterm(p)
+    else
+        first(terms(p))
+    end
 end
 leadingterm(t::AbstractTermLike) = term(t)
 

test/monovec.jl

@@ -3,9 +3,13 @@
     @test x > y
     @test x^2 > y^2
     X = [x^2, x*y, y^2]
+    #@test isempty(@inferred monomials((x, y), 1:0))
     for (i, m) in enumerate(monomials((x, y), 2))
         @test m == X[i]
     end
+    for (i, m) in enumerate(monomials((x, y), 2:2))
+        @test m == X[i]
+    end
     X = [x^2, y^2]
     for (i, m) in enumerate(monomials((x, y), 2, m -> m != x*y))
         @test m == X[i]

test/polynomial.jl

@@ -7,6 +7,9 @@
 
     @test polynomial(1 + x) == 1 + x
     @test leadingterm(1 + x) == x
+    @test leadingterm(x - x) == 0
+    @test leadingmonomial(x - x) == 1
+    @test leadingcoefficient(x - x) == 0
     @test one(1 + x) == one(1.0 + x) == 1
     @test zero(1 + x) == zero(1.0 + x) == 0
     @test 1 != 1 + x