Improve pretty-printer defaulting logic (#19361)

When determining whether to default a RuntimeRep or Multiplicity
variable, use isMetaTyVar to distinguish between metavariables
(which can be hidden) and skolems (which cannot).

7 files changed, 42 insertions, 16 deletions

diff --git a/testsuite/tests/linear/should_fail/T19361.hs b/testsuite/tests/linear/should_fail/T19361.hs
new file mode 100644
index 0000000000..503b299a0b
--- /dev/null
+++ b/testsuite/tests/linear/should_fail/T19361.hs
@@ -0,0 +1,9 @@
+{-# LANGUAGE LinearTypes #-}
+
+module T19361 where
+
+f :: a %m -> a
+f x = g x
+
+g :: a -> a
+g x = x
diff --git a/testsuite/tests/linear/should_fail/T19361.stderr b/testsuite/tests/linear/should_fail/T19361.stderr
new file mode 100644
index 0000000000..1d7746786a
--- /dev/null
+++ b/testsuite/tests/linear/should_fail/T19361.stderr
@@ -0,0 +1,10 @@
+
+T19361.hs:6:3: error:
+    • Couldn't match type ‘m’ with ‘'Many’
+        arising from multiplicity of ‘x’
+      ‘m’ is a rigid type variable bound by
+        the type signature for:
+          f :: forall a. a -> a
+        at T19361.hs:5:1-14
+    • In an equation for ‘f’: f x = g x
+    • Relevant bindings include f :: a %m -> a (bound at T19361.hs:6:1)
diff --git a/testsuite/tests/linear/should_fail/all.T b/testsuite/tests/linear/should_fail/all.T
index 89363cba85..4d8eec398e 100644
--- a/testsuite/tests/linear/should_fail/all.T
+++ b/testsuite/tests/linear/should_fail/all.T
@@ -39,3 +39,4 @@ test('T18888', normal, compile_fail, [''])
 test('T18888_datakinds', normal, compile_fail, [''])
 test('T19120', normal, compile_fail, [''])
 test('T20083', normal, compile_fail, ['-XLinearTypes'])
+test('T19361', normal, compile_fail, [''])
diff --git a/testsuite/tests/typecheck/should_fail/T15883b.stderr b/testsuite/tests/typecheck/should_fail/T15883b.stderr
index 21b9305315..b3efbc1b41 100644
--- a/testsuite/tests/typecheck/should_fail/T15883b.stderr
+++ b/testsuite/tests/typecheck/should_fail/T15883b.stderr
@@ -1,6 +1,7 @@
-T15883b.hs:14:1:
-     Can't make a derived instance of
+
+T15883b.hs:14:1: error:
+    • Can't make a derived instance of
         ‘Eq (Foo ('BoxedRep 'Lifted))’ with the stock strategy:
-        Don't know how to derive ‘Eq’ for type ‘forall a. a’
-     In the stand-alone deriving instance for
+        Don't know how to derive ‘Eq’ for type ‘forall (a :: TYPE rep). a’
+    • In the stand-alone deriving instance for
         ‘Eq (Foo (BoxedRep Lifted))’
diff --git a/testsuite/tests/typecheck/should_fail/T15883c.stderr b/testsuite/tests/typecheck/should_fail/T15883c.stderr
index 60678c4fcb..2aa1049fa5 100644
--- a/testsuite/tests/typecheck/should_fail/T15883c.stderr
+++ b/testsuite/tests/typecheck/should_fail/T15883c.stderr
@@ -1,6 +1,7 @@
-T15883c.hs:14:1:                 
-     Can't make a derived instance of
+
+T15883c.hs:14:1: error:
+    • Can't make a derived instance of
         ‘Ord (Foo ('BoxedRep 'Lifted))’ with the stock strategy:
-        Don't know how to derive ‘Ord’ for type ‘forall a. a’
-     In the stand-alone deriving instance for
+        Don't know how to derive ‘Ord’ for type ‘forall (a :: TYPE rep). a’
+    • In the stand-alone deriving instance for
         ‘Ord (Foo (BoxedRep Lifted))’
diff --git a/testsuite/tests/typecheck/should_fail/T15883d.stderr b/testsuite/tests/typecheck/should_fail/T15883d.stderr
index 162b31072e..96a294bc9e 100644
--- a/testsuite/tests/typecheck/should_fail/T15883d.stderr
+++ b/testsuite/tests/typecheck/should_fail/T15883d.stderr
@@ -1,6 +1,8 @@
-T15883d.hs:14:1:
-     Can't make a derived instance of
+
+T15883d.hs:14:1: error:
+    • Can't make a derived instance of
         ‘Show (Foo ('BoxedRep 'Lifted))’ with the stock strategy:
-        Don't know how to derive ‘Show’ for type ‘forall a. a’
-     In the stand-alone deriving instance for
+        Don't know how to derive ‘Show’
+          for type ‘forall (a :: TYPE rep). a’
+    • In the stand-alone deriving instance for
         ‘Show (Foo (BoxedRep Lifted))’
diff --git a/testsuite/tests/typecheck/should_fail/T15883e.stderr b/testsuite/tests/typecheck/should_fail/T15883e.stderr
index a20b3f5d43..c7006fb790 100644
--- a/testsuite/tests/typecheck/should_fail/T15883e.stderr
+++ b/testsuite/tests/typecheck/should_fail/T15883e.stderr
@@ -1,6 +1,8 @@
-T15883e.hs:16:1:
-     Can't make a derived instance of
+
+T15883e.hs:16:1: error:
+    • Can't make a derived instance of
         ‘Data (Foo ('BoxedRep 'Lifted))’ with the stock strategy:
-        Don't know how to derive ‘Data’ for type ‘forall a. a’
-     In the stand-alone deriving instance for
+        Don't know how to derive ‘Data’
+          for type ‘forall (a :: TYPE rep). a’
+    • In the stand-alone deriving instance for
         ‘Data (Foo (BoxedRep Lifted))’