GHC can suggest "Probable fixes" that are impossible

Let's say I type in ["foo", 4] to GHCI:

Prelude> ["foo", 4]

<interactive>:1:8:

No instance for (Num [Char])

arising from the literal 4' at <interactive>:1:8 Probable fix: add an instance declaration for (Num [Char]) In the list element: 4 In the definition of it': it = ["foo", 4]

It suggests a "Probable fix": add an instance declaration for (Num [Char]). OK, great. However, as soon as I try to evaluate "instance Num [Char] where" I get

Illegal instance declaration for Num [Char]' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for Num [Char]'

Now, I'm not enough of an expert on the type class system to know if this means that it's absolutely impossible to declare instances on [Char], but it certainly seems like that. Thus, the "Probable fix" is in error, since [Char] can't get a Num instance. (Though I guess (Num a) => [a] can, so maybe this whole issue is a little moot.)

Trac metadata

Trac field	Value
Version	6.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown