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

3 files changed, 15 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_fail/T15796.hs b/testsuite/tests/typecheck/should_fail/T15796.hs
new file mode 100644
index 0000000000..450064d4cc
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T15796.hs
@@ -0,0 +1,8 @@
+{-# 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
diff --git a/testsuite/tests/typecheck/should_fail/T15796.stderr b/testsuite/tests/typecheck/should_fail/T15796.stderr
new file mode 100644
index 0000000000..3aa7ae8d65
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T15796.stderr
@@ -0,0 +1,6 @@
+
+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’
diff --git a/testsuite/tests/typecheck/should_fail/all.T b/testsuite/tests/typecheck/should_fail/all.T
index f80f5cda01..c3a9f510d0 100644
--- a/testsuite/tests/typecheck/should_fail/all.T
+++ b/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, [''])