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, 6 insertions, 4 deletions

diff --git a/docs/users_guide/separate_compilation.rst b/docs/users_guide/separate_compilation.rst
index c6e9b7d0b6..b580c9ab18 100644
--- a/docs/users_guide/separate_compilation.rst
+++ b/docs/users_guide/separate_compilation.rst
@@ -864,10 +864,12 @@ A hs-boot file is written in a subset of Haskell:
    You cannot use ``deriving`` on a data type declaration; write an
    ``instance`` declaration instead.
 
--  Class declarations is exactly as in Haskell, except that you may not
-   put default method declarations. You can also omit all the
-   superclasses and class methods entirely; but you must either omit
-   them all or put them all in.
+-  Class declarations can either be given in full, exactly as in Haskell,
+   or they can be given abstractly by omitting everything other than the
+   instance head: no superclasses, no class methods, no associated types.
+   If the class declaration is given in full, the default delarations
+   must also match; this applies to both default methods and default
+   declarations for associated types.
 
 -  You can include instance declarations just as in Haskell; but omit
    the "where" part.