Add another example for GHC-39999

Author: Bodigrim

message-index/messages/GHC-39999/overloaded-number/after/Lib.hs

@@ -0,0 +1,4 @@
+module Lib where
+
+foo :: [Int]
+foo = map (+ 1) [0]

message-index/messages/GHC-39999/overloaded-number/before/Lib.hs

@@ -0,0 +1,4 @@
+module Lib where
+
+foo :: [Int]
+foo = map (+ 1) 0

message-index/messages/GHC-39999/overloaded-number/index.md

@@ -0,0 +1,31 @@
+---
+title: No instance for ‘Num [Int]’ arising from the literal ‘0’
+order: 5
+---
+
+## Error message
+
+```
+Lib.hs:4:17: error: [GHC-39999]
+    • No instance for ‘Num [Int]’ arising from the literal ‘0’
+    • In the second argument of ‘map’, namely ‘0’
+      In the expression: map (+ 1) 0
+      In an equation for ‘foo’: foo = map (+ 1) 0
+  |
+4 | foo = map (+ 1) 0
+  |                 ^
+```
+
+## Explanation
+
+Sometimes, when GHC encounters a type error, it suggests solving
+it from an unexpected end. In this case, when it encouters a literal
+instead of a list, it does not just complain about it. Instead it suspects
+that maybe an author meant to overload numeric literals so that they can mean lists too.
+
+Every time you write a numeric literal in Haskell,
+it gets desugared using `fromInteger :: Num a => Integer -> a`. So the program above
+would be absolutely valid if only `[Int]` itself (not just `Int`) was an instance of `Num`.
+While such interpretation of lists is not entirely out of the realm of possibility
+(say, imagine that lists represent coefficients of polynomials), it's much more likely
+that `map (+ 1) 0` is just a typo.