Fix inference of partial signatures

When we had
  f :: ( _ ) => blah
we were failing to call growThetaTyVars, as we do in the
no-type-signature case, and that meant that we weren't generalising
over the right type variables.  I'm quite surprised this didn't cause
problems earlier.

Anyway Trac #12844 showed it up and this patch fixes it

3 files changed, 46 insertions, 0 deletions

diff --git a/testsuite/tests/partial-sigs/should_compile/T12844.hs b/testsuite/tests/partial-sigs/should_compile/T12844.hs
new file mode 100644
index 0000000000..d47b82cc64
--- /dev/null
+++ b/testsuite/tests/partial-sigs/should_compile/T12844.hs
@@ -0,0 +1,20 @@
+{-# LANGUAGE DataKinds             #-}
+{-# LANGUAGE PartialTypeSignatures #-}
+{-# LANGUAGE PolyKinds             #-}
+{-# LANGUAGE TypeFamilies          #-}
+{-# LANGUAGE TypeOperators         #-}
+
+module T12844 where
+
+barWraper :: ('(r,r') ~ Head rngs, Foo rngs) => FooData rngs
+barWraper = bar
+
+bar :: (_) => FooData rngs
+bar = foo
+
+data FooData rngs
+
+class Foo xs where foo :: (Head xs ~ '(r,r')) => FooData xs
+
+type family Head (xs :: [k]) where Head (x ': xs) = x
+
diff --git a/testsuite/tests/partial-sigs/should_compile/T12844.stderr b/testsuite/tests/partial-sigs/should_compile/T12844.stderr
new file mode 100644
index 0000000000..8ad3777f2a
--- /dev/null
+++ b/testsuite/tests/partial-sigs/should_compile/T12844.stderr
@@ -0,0 +1,25 @@
+
+T12844.hs:12:9: warning: [-Wpartial-type-signatures (in -Wdefault)]
+    • Found type wildcard ‘_’
+        standing for ‘(Head rngs ~ '(r, r'), Foo rngs)’
+      Where: ‘r’ is a rigid type variable bound by
+               the inferred type of
+               bar :: (Head rngs ~ '(r, r'), Foo rngs) => FooData rngs
+               at T12844.hs:13:1-9
+             ‘r'’ is a rigid type variable bound by
+               the inferred type of
+               bar :: (Head rngs ~ '(r, r'), Foo rngs) => FooData rngs
+               at T12844.hs:13:1-9
+             ‘rngs’ is a rigid type variable bound by
+               the inferred type of
+               bar :: (Head rngs ~ '(r, r'), Foo rngs) => FooData rngs
+               at T12844.hs:13:1-9
+             ‘k’ is a rigid type variable bound by
+               the inferred type of
+               bar :: (Head rngs ~ '(r, r'), Foo rngs) => FooData rngs
+               at T12844.hs:13:1-9
+             ‘k1’ is a rigid type variable bound by
+               the inferred type of
+               bar :: (Head rngs ~ '(r, r'), Foo rngs) => FooData rngs
+               at T12844.hs:13:1-9
+    • In the type signature: bar :: _ => FooData rngs
diff --git a/testsuite/tests/partial-sigs/should_compile/all.T b/testsuite/tests/partial-sigs/should_compile/all.T
index b3208518f4..10cdfaaf35 100644
--- a/testsuite/tests/partial-sigs/should_compile/all.T
+++ b/testsuite/tests/partial-sigs/should_compile/all.T
@@ -68,3 +68,4 @@ test('T11670', normal, compile, [''])
 test('T12156', normal, compile_fail, ['-fdefer-typed-holes'])
 test('T12531', normal, compile, ['-fdefer-typed-holes'])
 test('T12845', normal, compile, [''])
+test('T12844', normal, compile, [''])