Tuple Constructors in Interface Instance Declarations Cause Illegal Instance Type Errors

Consider the following module:

module TupleConstructorInstance where

class C a where
  methodC :: a Bool Bool

instance C (,) where
  methodC = (True, False)

testExp :: (Bool, Bool)
testExp = methodC

Compiling this module works without problems, but using it interactively in PAKCS causes an Illegal instance type error:

TupleConstructorInstance> testExp

Tests/.curry/pakcs-3.2.0/TupleConstructorInstance.icurry:8:12-8:14 Error:
    Illegal instance type (,)
    The instance type must be of the form (T u_1 ... u_n),
    where T is not a type synonym and u_1, ..., u_n are
    mutually distinct, non-anonymous type variables.
   |
 8 | instance C (,) {
   |            ^^^
ERROR occurred during parsing!

The cause of this error is the checkTypeConstructor function in the interface syntax check (only the first part of it is displayed here):

checkTypeConstructor :: SpanInfo -> QualIdent -> ISC TypeExpr
checkTypeConstructor spi tc = do
  tyEnv <- getTypeEnv
  case qualLookupTypeKind tc tyEnv of
    [] | not (isQualified tc) -> return $ VariableType spi $ unqualify tc
       | otherwise            -> do
          report $ errUndefinedType tc
          return $ ConstructorType spi tc

Unlike its counterpart checkType in the type syntax check, checkTypeConstructor does not have a special case for tuple constructors. You can see the isQTupleId tc guard of the checkType function here:

checkType :: TypeExpr -> TSCM TypeExpr
checkType c@(ConstructorType spi tc) = do
  m <- getModuleIdent
  tEnv <- getTypeEnv
  case qualLookupTypeKind tc tEnv of
    []
      | isQTupleId tc -> return c
      | not (isQualified tc) -> return $ VariableType spi $ unqualify tc
      | otherwise -> report (errUndefinedType tc) >> return c

This missing guard in checkTypeConstructor means that the instance type (,) is identified as a type variable in the interface syntax check. Type variables are not allowed as instance types however, therefore an Illegal instance type error is reported.