Sequester deriving-related validity check into cond_stdOK

Currently, any standalone-derived instance must satisfy the
property that the tycon of the data type having an instance being
derived for it must be either a normal ADT tycon or a data family
tycon. But there are several other primitive tycons—such as `(->)`,
`Int#`, and others—which cannot have standalone-derived instances
(via the `anyclass` strategy) as a result of this check! See
https://ghc.haskell.org/trac/ghc/ticket/13154#comment:8 for an
example of where this overly conservative restriction bites.

Really, this validity check only makes sense in the context of
`stock` deriving, where we need the property that the tycon is that
of a normal ADT or a data family in order to inspect its data
constructors. Other deriving strategies don't require this validity
check, so the most sensible way to fix this error is to move the
logic of this check into `cond_stdOK`, which is specific to
`stock` deriving.

This makes progress towards fixing (but does not entirely fix)

Test Plan: make test TEST=T13154a

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #13154

Differential Revision: https://phabricator.haskell.org/D4337

2 files changed, 15 insertions, 0 deletions

diff --git a/testsuite/tests/deriving/should_compile/T13154a.hs b/testsuite/tests/deriving/should_compile/T13154a.hs
new file mode 100644
index 0000000000..4d60785eb2
--- /dev/null
+++ b/testsuite/tests/deriving/should_compile/T13154a.hs
@@ -0,0 +1,14 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE DeriveAnyClass #-}
+{-# LANGUAGE KindSignatures #-}
+{-# LANGUAGE MagicHash #-}
+{-# LANGUAGE StandaloneDeriving #-}
+module T13154a where
+
+import GHC.Exts
+
+class C1 a
+deriving instance C1 (a -> b)
+
+class C2 (a :: TYPE 'IntRep)
+deriving instance C2 Int#
diff --git a/testsuite/tests/deriving/should_compile/all.T b/testsuite/tests/deriving/should_compile/all.T
index 3360c81850..a06cd27c84 100644
--- a/testsuite/tests/deriving/should_compile/all.T
+++ b/testsuite/tests/deriving/should_compile/all.T
@@ -85,6 +85,7 @@ test('T12594', normal, compile, [''])
 test('T12616', normal, compile, [''])
 test('T12688', normal, compile, [''])
 test('T12814', normal, compile, ['-Wredundant-constraints'])
+test('T13154a', normal, compile, [''])
 test('T13272', normal, compile, [''])
 test('T13272a', normal, compile, [''])
 test('T13297', normal, compile, [''])