Don't lint erroneous programs.

newFamInst lints its types. This is good. But it's not so good
when there have been errors and thus recovery tycons are about.
So we now don't.

Fixes #15796.

Test case: typecheck/should_fail/T15796

compiler/typecheck/FamInst.hs

@@ -150,7 +150,7 @@ See #9562.
 -- It is defined here to avoid a dependency from FamInstEnv on the monad
 -- code.
 
-newFamInst :: FamFlavor -> CoAxiom Unbranched -> TcRnIf gbl lcl FamInst
+newFamInst :: FamFlavor -> CoAxiom Unbranched -> TcM FamInst
 -- Freshen the type variables of the FamInst branches
 newFamInst flavor axiom@(CoAxiom { co_ax_tc = fam_tc })
   = ASSERT2( tyCoVarsOfTypes lhs `subVarSet` tcv_set, text "lhs" <+> pp_ax )
@@ -162,7 +162,10 @@ newFamInst flavor axiom@(CoAxiom { co_ax_tc = fam_tc })
        ; let lhs'     = substTys subst lhs
              rhs'     = substTy  subst rhs
              tcvs'    = tvs' ++ cvs'
-       ; when (gopt Opt_DoCoreLinting dflags) $
+       ; ifErrsM (return ()) $ -- Don't lint when there are errors, because
+                               -- errors might mean TcTyCons.
+                               -- See Note [Recover from validity error] in TcTyClsDecls
+         when (gopt Opt_DoCoreLinting dflags) $
            -- Check that the types involved in this instance are well formed.
            -- Do /not/ expand type synonyms, for the reasons discussed in
            -- Note [Linting type synonym applications].

testsuite/tests/typecheck/should_fail/T15796.hs

+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE TypeFamilies #-}
+module Bug where
+
+newtype N a where
+  MkN :: Show a => a -> N a
+type family T a
+type instance T (N a) = N a

testsuite/tests/typecheck/should_fail/T15796.stderr

+
+T15796.hs:6:3: error:
+    • A newtype constructor cannot have a context in its type
+      MkN :: forall a. Show a => a -> N a
+    • In the definition of data constructor ‘MkN’
+      In the newtype declaration for ‘N’

testsuite/tests/typecheck/should_fail/all.T

@@ -486,3 +486,4 @@ test('T15552a', normal, compile_fail, [''])
 test('T15629', normal, compile_fail, [''])
 test('T15767', normal, compile_fail, [''])
 test('T15648', [extra_files(['T15648a.hs'])], multimod_compile_fail, ['T15648', '-v0 -fprint-equality-relations'])
+test('T15796', normal, compile_fail, [''])