Since IsString is a broken interface, I suggest to raise a GHC and CLC proposal (since the class will need to be added to base).

I think the ByteString discussion is enough evidence that there is a problem.

The new OsString/OsPath interface in filepath is another evidence: it will never support IsString and OverloadedStrings.

So we need a new class and a new GHC extension.

I can't say how this will go on GHC steering committee side, but I can assure you my CLC vote will be +1 and I will vehemently promote it.

I don't have any knowledge about an actual implementation, but my guess is it would follow a similar pattern as OverloadedString and just inject class function at string literals?

One concern was that it will require TH and will make it hard to write trustworthy code. I believe @angerman had an idea of an IO-free TH. Maybe that will complement such an extension, but I think it could be proposed regardless.

Wdyt?

When an invalid literal is used to create an IO x in ghci, no error occurs.

Prelude> :set -XTemplateHaskell
Prelude> :m Functional.Iota.Free ValidLiterals 
Prelude Functional.Iota.Free ValidLiterals> ($$(valid '(') :: IFree)

<interactive>:3:5: error:
    • Invalid input used for type-safe validated literal!
An error occured during compile-time validation!

    • In the Template Haskell splice $$(valid '(')
      In the expression: ($$(valid '(') :: IFree)
      In an equation for ‘it’: it = ($$(valid '(') :: IFree)

<interactive>:3:5: error:
    • Exception when trying to run compile-time code:
        ValidationFailure "An error occured during compile-time validation!"
      Code: valid '('
    • In the Template Haskell splice $$(valid '(')
      In the expression: ($$(valid '(') :: IFree)
      In an equation for ‘it’: it = ($$(valid '(') :: IFree)
Prelude Functional.Iota.Free ValidLiterals> print ($$(valid '(') :: IFree)
Prelude Functional.Iota.Free ValidLiterals> print ($$(valid 'a') :: IFree)
IFree {fromIFree = 'a'}
Prelude Functional.Iota.Free ValidLiterals>

Prolly just in the type class, e.g.

quasiQuoter :: Proxy a -> Proxy b -> QuasiQuoter