Use precise language

The error message and documentation specifically refers to the variants
of Enumerable#sum/product that do not support union types.

spec/std/enumerable_spec.cr

@@ -1373,9 +1373,9 @@ describe "Enumerable" do
         require "prelude"
         [1, 10000000000_u64].sum
         CRYSTAL
-        "Enumerable#sum and #product do not support Union types. Instead, " +
-        "use Enumerable#sum/product(initial) with an initial value of " +
-        "the expected type of the call."
+        "Enumerable#sum() and #product() do not support Union types. " +
+        "Instead, use Enumerable#sum and #product(initial), respectively, " +
+        "with an initial value of the intended type of the call."
     end
 
     it "uses additive_identity from type" do
@@ -1427,9 +1427,9 @@ describe "Enumerable" do
         require "prelude"
         [1, 10000000000_u64].product
         CRYSTAL
-        "Enumerable#sum and #product do not support Union types. Instead, " +
-        "use Enumerable#sum/product(initial) with an initial value of " +
-        "the expected type of the call."
+        "Enumerable#sum() and #product() do not support Union types. " +
+        "Instead, use Enumerable#sum and #product(initial), respectively, " +
+        "with an initial value of the intended type of the call."
     end
   end
 

src/enumerable.cr

@@ -2299,9 +2299,10 @@ module Enumerable(T)
     def self.first
       {% if X.union? %}
         {{
-          raise("Enumerable#sum and #product do not support Union types. " +
-                "Instead, use Enumerable#sum/product(initial) with an " +
-                "initial value of the expected type of the call.")
+          raise("Enumerable#sum() and #product() do not support Union types. " +
+                "Instead, use Enumerable#sum and #product(initial), " +
+                "respectively, with an initial value of the intended type " +
+                "of the call.")
         }}
       {% else %}
         X