Convert diagnostics in GHC.Tc.Validity to proper TcRnMessage.

- Add 19 new messages. Update test outputs accordingly.

- Pretty print suggest-extensions hints: remove space before
interspersed commas.

- Refactor Rank's MonoType constructors. Each MonoType constructor
should represent a specific case. With the Doc suggestion belonging
to the TcRnMessage diagnostics instead.

- Move Rank from Validity to its own `GHC.Tc.Types.Rank` module.

- Remove the outdated `check_irred_pred` check.

- Remove the outdated duplication check in `check_valid_theta`, which
was subsumed by `redundant-constraints`.

- Add missing test cases for quantified-constraints/T16474 & th/T12387a.

3 files changed, 22 insertions, 0 deletions

diff --git a/testsuite/tests/th/T12387a.hs b/testsuite/tests/th/T12387a.hs
new file mode 100644
index 0000000000..c8ca5c942a
--- /dev/null
+++ b/testsuite/tests/th/T12387a.hs
@@ -0,0 +1,16 @@
+{-# LANGUAGE TemplateHaskell #-}
+{-# LANGUAGE TypeFamilies #-}
+
+module T12387a where
+
+import GHC.Generics
+import Language.Haskell.TH.Lib
+
+data Foo = Foo
+
+$(do d <- instanceD (cxt []) (conT ''Eq `appT` conT ''Foo)
+            [tySynInstD $ tySynEqn Nothing (conT ''Rep `appT` conT ''Foo) (conT ''Maybe)]
+     return [d])
+
+main :: IO ()
+main = print $ Foo == Foo
diff --git a/testsuite/tests/th/T12387a.stderr b/testsuite/tests/th/T12387a.stderr
new file mode 100644
index 0000000000..00645866d4
--- /dev/null
+++ b/testsuite/tests/th/T12387a.stderr
@@ -0,0 +1,5 @@
+
+T12387a.hs:11:2:
+    Class ‘Eq’ does not have an associated type ‘Rep’
+    In the type instance declaration for ‘Rep’
+        In the instance declaration for ‘Eq Foo’
\ No newline at end of file
diff --git a/testsuite/tests/th/all.T b/testsuite/tests/th/all.T
index f280ab7f57..306af0a911 100644
--- a/testsuite/tests/th/all.T
+++ b/testsuite/tests/th/all.T
@@ -372,6 +372,7 @@ test('T12073', normal, compile_and_run, [''])
 test('T12130', [], multimod_compile,
      ['T12130', '-v0 ' + config.ghc_th_way_flags])
 test('T12387', normal, compile_fail, ['-v0'])
+test('T12387a', normal, compile_fail, ['-v0'])
 test('T12403', omit_ways(['ghci']),
               compile_and_run, ['-v0 -ddump-splices -dsuppress-uniques'])
 test('T12407', omit_ways(['ghci']), compile, ['-v0'])