Fix #10045

Summary:
SPJ's solution is to only bring the `TcId` (which includes the type) of a
binder into scope when it had a non-partial type signature.

Take care of this by only storing the `TcId` in `TcSigInfo` of non-partial
type signatures, hence the change to `sig_poly_id :: Maybe TcId`. Only in case
of a `Just` will we bring the `TcId` in scope. We still need to know the name
of the binder, even when it has a partial type signature, so add a `sig_name
:: Name` field. The field `sig_partial :: Bool` is no longer necessary, so
reimplement `isPartialSig` in terms of `sig_poly_id`.

Note that the new test case fails, but not because of a panic, but because the
`Num a` constraint is missing. Adding an extra-constraints wildcard to
`copy`'s signature would fix it.

Test Plan: validate

Reviewers: simonpj, austin

Reviewed By: simonpj

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D646

GHC Trac Issues: #10045

3 files changed, 54 insertions, 0 deletions

diff --git a/testsuite/tests/partial-sigs/should_fail/Trac10045.hs b/testsuite/tests/partial-sigs/should_fail/Trac10045.hs
new file mode 100644
index 0000000000..e7c07470aa
--- /dev/null
+++ b/testsuite/tests/partial-sigs/should_fail/Trac10045.hs
@@ -0,0 +1,8 @@
+module Trac10045 where
+
+newtype Meta = Meta ()
+
+foo (Meta ws1) =
+    let copy :: _
+        copy w from = copy w 1
+    in copy ws1 1
diff --git a/testsuite/tests/partial-sigs/should_fail/Trac10045.stderr b/testsuite/tests/partial-sigs/should_fail/Trac10045.stderr
new file mode 100644
index 0000000000..8c8e42f704
--- /dev/null
+++ b/testsuite/tests/partial-sigs/should_fail/Trac10045.stderr
@@ -0,0 +1,45 @@
+
+Trac10045.hs:6:17:
+    Found hole ‘_’ with type: t1 -> a -> t2
+    Where: ‘t1’ is a rigid type variable bound by
+                the inferred type of copy :: Num a => t1 -> a -> t2
+                at Trac10045.hs:7:9
+           ‘t2’ is a rigid type variable bound by
+                the inferred type of copy :: Num a => t1 -> a -> t2
+                at Trac10045.hs:7:9
+           ‘a’ is a rigid type variable bound by
+               the inferred type of copy :: Num a => t1 -> a -> t2
+               at Trac10045.hs:7:9
+    To use the inferred type, enable PartialTypeSignatures
+    Relevant bindings include
+      ws1 :: () (bound at Trac10045.hs:5:11)
+      foo :: Meta -> t (bound at Trac10045.hs:5:1)
+    In the type signature for ‘copy’: _
+    In the expression:
+      let
+        copy :: _
+        copy w from = copy w 1
+      in copy ws1 1
+    In an equation for ‘foo’:
+        foo (Meta ws1)
+          = let
+              copy :: _
+              copy w from = copy w 1
+            in copy ws1 1
+
+Trac10045.hs:7:9:
+    No instance for (Num a)
+    When checking that ‘copy’ has the specified type
+      copy :: forall t t1 a. t -> a -> t1
+    Probable cause: the inferred type is ambiguous
+    In the expression:
+      let
+        copy :: _
+        copy w from = copy w 1
+      in copy ws1 1
+    In an equation for ‘foo’:
+        foo (Meta ws1)
+          = let
+              copy :: _
+              copy w from = copy w 1
+            in copy ws1 1
diff --git a/testsuite/tests/partial-sigs/should_fail/all.T b/testsuite/tests/partial-sigs/should_fail/all.T
index 7e56d15c71..c49a36fe34 100644
--- a/testsuite/tests/partial-sigs/should_fail/all.T
+++ b/testsuite/tests/partial-sigs/should_fail/all.T
@@ -18,6 +18,7 @@ test('ScopedNamedWildcardsBad', normal, compile_fail, [''])
 test('TidyClash', normal, compile_fail, [''])
 # Bug
 test('TidyClash2', expect_fail, compile_fail, [''])
+test('Trac10045', normal, compile_fail, [''])
 test('UnnamedConstraintWildcard1', normal, compile_fail, [''])
 test('UnnamedConstraintWildcard2', normal, compile_fail, [''])
 test('WildcardInADT1', normal, compile_fail, [''])