Clarify hs-boot file default method restrictions

  The user guide wrongly stated that default methods should not
  be included in hs-boot files. In fact, if the class is not left
  abstract (no methods, no superclass constraints, ...) then the
  defaults must be provided and match with those given in the .hs
  file.

  We add some tests for this, as there were no tests in the testsuite
  that gave rise to the "missing default methods" error.

  Fixes #20588

1 files changed, 15 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_fail/T20588.hs b/testsuite/tests/typecheck/should_fail/T20588.hs
new file mode 100644
index 0000000000..475c68ef2d
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T20588.hs
@@ -0,0 +1,15 @@
+
+{-# LANGUAGE TypeFamilies #-}
+
+module T20588 where
+
+import Data.Kind
+
+class C (a :: Type) where
+  {-# MINIMAL meth #-}
+  meth :: a -> a
+  meth = id
+
+class D (a :: Type) where
+  type family T a :: Type
+  type instance T a = Int