Use a class to check validity of withDictwip/withdict

This moves handling of the magic 'withDict' function from the desugarer
to the typechecker. Details in Note [withDict].

I've extracted a part of T16646Fail to a separate file T16646Fail2,
because the new error in 'reify' hides the errors from 'f' and 'g'.

WithDict now works with casts, this fixes #21328.

Part of #19915

2 files changed, 18 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_compile/T21328.hs b/testsuite/tests/typecheck/should_compile/T21328.hs
new file mode 100644
index 0000000000..9b72df193f
--- /dev/null
+++ b/testsuite/tests/typecheck/should_compile/T21328.hs
@@ -0,0 +1,17 @@
+{-# LANGUAGE TypeFamilies #-}
+module T21328 where
+
+import GHC.Exts
+import Type.Reflection
+
+type family F x
+type instance F x = x
+
+type family G x
+type instance G x = x
+
+class C a where
+  m :: G a
+
+cast :: forall a. F a -> (C a => Int) -> Int
+cast = withDict  @(C a) @(F a)
diff --git a/testsuite/tests/typecheck/should_compile/all.T b/testsuite/tests/typecheck/should_compile/all.T
index f08828eeea..a258f6de03 100644
--- a/testsuite/tests/typecheck/should_compile/all.T
+++ b/testsuite/tests/typecheck/should_compile/all.T
@@ -829,6 +829,7 @@ test('T21023', normal, compile, ['-ddump-types'])
 test('T21205', normal, compile, ['-O0'])
 test('T21323', normal, compile, [''])
 test('T21315', normal, compile, ['-Wredundant-constraints'])
+test('T21328', normal, compile, [''])
 test('T21516', normal, compile, [''])
 test('T21519', normal, compile, [''])
 test('T21519a', normal, compile, [''])