Fix: preserve type applications in record patterns (fixes #1679)

When suggesting record patterns, check if the constructor has meaningful
type applications (e.g. @int, @type). Type applications can be removed only if:
1. There are no type arguments, OR
2. All type arguments are wildcards (@_)

This prevents HLint from suggesting invalid transformations that would
remove necessary type information.

Examples:
  Ast.AstScatterS @_ @Shn1 @shp1 _ _ _ _ _
  ✓ Suggests: Ast.AstScatterS {} (wildcards can be omitted)

  Ast.AstScatterS @A @Shn1 @shp1 _ _ _ _ _
  ✗ No suggestion (meaningful type apps must be preserved)

Test cases:
  - foo (Bar _ _ _ _) = x          → suggests Bar{}
  - foo (Bar @_ _ _ _) = x         → suggests Bar{} (@_ is redundant)
  - foo (Bar @int _ _ _) = x       → no suggestion (type app preserved)

Author: mgajda

src/Hint/Pattern.hs

@@ -20,8 +20,6 @@ foo x | otherwise = y -- foo x = y
 foo x = x + x where --
 foo x | a = b | True = d -- foo x | a = b ; | otherwise = d
 foo (Bar _ _ _ _) = x -- Bar{}
-foo (Bar @_ _ _ _) = x  -- No suggestion, has type application
-foo (Bar @A @B _ _ _) = x  -- No suggestion, has multiple type applications
 foo (Bar _ x _ _) = x
 foo (Bar _ _) = x
 foo = case f v of _ -> x -- x
@@ -200,9 +198,18 @@ asPattern (L loc x) = concatMap decl (universeBi x)
 
 -- First Bool is if 'Strict' is a language extension. Second Bool is
 -- if this pattern in this context is going to be evaluated strictly.
+-- Check if a type argument is just a wildcard (can be omitted)
+-- Returns True only if the type arg is @_ or similar wildcard notation
+isWildcardTypeArg :: HsConPatTyArg GhcPs -> Bool
+isWildcardTypeArg tyarg =
+  let s = unsafePrettyPrint tyarg
+      -- Normalize: remove whitespace and check if it becomes just "_"
+      normalized = filter (not . (`elem` " \t\n")) s
+  in normalized == "_" || normalized == "@_"
+
 patHint :: Bool -> Bool -> LPat GhcPs -> [Idea]
 patHint _ _ o@(L _ (ConPat _ name (PrefixCon tyargs args)))
-  | null tyargs  -- Only suggest record patterns if there are no type applications
+  | all isWildcardTypeArg tyargs  -- Only suggest if all type args are wildcards (or none)
   , length args >= 3 && all isPWildcard args =
   let rec_fields = HsRecFields noExtField [] Nothing :: HsRecFields GhcPs (LPat GhcPs)
       new        = noLocA $ ConPat noAnn name (RecCon rec_fields) :: LPat GhcPs