`add C to the context` code action doesn't appear for instances

[isovector]

Consider the following:

class Yo f where
    yo :: f x -> Int

go :: M1 i c f x -> Int
go (M1 fx) = yo fx

This gives me an underline on the call to yo, complaining that there is no instance Yo f. It comes with a code action to insert it into the type.

Now consider the analogous:

class Yo f where
    yo :: f x -> Int

instance Yo (M1 _1 _2 f) where
  yo (M1 fx) = yo fx

Same diagnostic, but this time there is no code action to fix the issue!

[isovector]

It looks like this is supposed to work:

haskell-language-server/ghcide/src/Development/IDE/Plugin/CodeAction.hs

Lines 1045 to 1077 in aa4bd29

	suggestInstanceConstraint :: DynFlags -> ParsedSource -> Diagnostic -> T.Text -> [(T.Text, Rewrite)]
	suggestInstanceConstraint df (L _ HsModule {hsmodDecls}) Diagnostic {..} missingConstraint
	| Just instHead <- instanceHead
	= [(actionTitle missingConstraint , appendConstraint (T.unpack missingConstraint) instHead)]
	| otherwise = []
	where
	instanceHead
	-- Suggests a constraint for an instance declaration with no existing constraints.
	-- • No instance for (Eq a) arising from a use of ‘==’
	-- Possible fix: add (Eq a) to the context of the instance declaration
	-- • In the expression: x == y
	-- In an equation for ‘==’: (Wrap x) == (Wrap y) = x == y
	-- In the instance declaration for ‘Eq (Wrap a)’
	| Just [instanceDeclaration] <- matchRegexUnifySpaces _message "In the instance declaration for ‘([^`]*)’"
	, Just instHead <- findInstanceHead df (T.unpack instanceDeclaration) hsmodDecls
	= Just instHead
	-- Suggests a constraint for an instance declaration with one or more existing constraints.
	-- • Could not deduce (Eq b) arising from a use of ‘==’
	-- from the context: Eq a
	-- bound by the instance declaration at /path/to/Main.hs:7:10-32
	-- Possible fix: add (Eq b) to the context of the instance declaration
	-- • In the second argument of ‘(&&)’, namely ‘x' == y'’
	-- In the expression: x == y && x' == y'
	-- In an equation for ‘==’:
	-- (Pair x x') == (Pair y y') = x == y && x' == y'
	| Just [instanceLineStr, constraintFirstCharStr]
	<- matchRegexUnifySpaces _message "bound by the instance declaration at .+:([0-9]+):([0-9]+)"
	, Just (L _ (InstD _ (ClsInstD _ ClsInstDecl {cid_poly_ty = HsIB{hsib_body}})))
	<- findDeclContainingLoc (Position (readPositionNumber instanceLineStr) (readPositionNumber constraintFirstCharStr)) hsmodDecls
	= Just hsib_body
	| otherwise
	= Nothing

but the error message I get on 8.8 is:

• No instance for (Yo f) arising from a use of ‘yo’
  Possible fix:
    add (Yo f) to the context of
      the type signature for:
        yo :: forall x. Bool -> (:+:) f g x -> Int
      or the instance declaration
• In the expression: yo b fx
  In an equation for ‘fx_c’: fx_c = yo b fx
  In the expression: let fx_c = yo b fx in fx_c

[berberman]

Sorry, it should be a bug. I just took a look, and found that because of the "the type signature for" in diagnostics message, we entered suggestFunctionConstraint wrongly:

haskell-language-server/ghcide/src/Development/IDE/Plugin/CodeAction.hs

Lines 1027 to 1033 in aa4bd29

	suggestConstraint :: DynFlags -> ParsedSource -> Diagnostic -> [(T.Text, Rewrite)]
	suggestConstraint df parsedModule diag@Diagnostic {..}
	| Just missingConstraint <- findMissingConstraint _message
	= let codeAction = if _message =~ ("the type signature for:" :: String)
	then suggestFunctionConstraint df parsedModule
	else suggestInstanceConstraint df parsedModule
	in codeAction diag missingConstraint

This could be fixed easily by changing the condition, and I've tested

if not $ "instance declaration" `T.isInfixOf` _message

worked.

Meanwhile, I found another case it didn't work:

instance  Eq (Three a b c) where
   (==) (Three x y z) (Three x' y' z') = x == x' && y == y' && z == z'

with the error message:

• No instance for (Eq a) arising from a use of ‘==’
  Possible fix: add (Eq a) to the context of the instance declaration
• In the first argument of ‘(&&)’, namely ‘x == x'’
  In the expression: x == x' && y == y' && z == z'
  In an equation for ‘==’:
      (Three x y z) == (Three x' y' z') = x == x' && y == y' && z == z'

This was because the error message didn't contain In the instance declaration for ... like other cases, so we can't find the instance head to rewrite.

[isovector]

How annoying. We could cross-reference the diagnostic location against the ParsedModule, and get the instance head from there --- though I'm not feeling plucky enough to wire it up.