Don't infer CallStacks

We originally wanted CallStacks to be opt-in, but dealing with let
binders complicated things, forcing us to infer CallStacks. It turns
out that the inference is actually unnecessary though, we can let the
wanted CallStacks bubble up to the outer context by refusing to
quantify over them. Eventually they'll be solved from a given CallStack
or defaulted to the empty CallStack if they reach the top.

So this patch prevents GHC from quantifying over CallStacks, getting us
back to the original plan. There's a small ugliness to do with
PartialTypeSignatures, if the partial theta contains a CallStack
constraint, we *do* want to quantify over the CallStack; the user asked
us to!

Note that this means that

  foo :: _ => CallStack
  foo = getCallStack callStack

will be an *empty* CallStack, since we won't infer a CallStack for the
hole in the theta. I think this is the right move though, since we want
CallStacks to be opt-in. One can always write

  foo :: (HasCallStack, _) => CallStack
  foo = getCallStack callStack

to get the CallStack and still have GHC infer the rest of the theta.

Test Plan: ./validate

Reviewers: goldfire, simonpj, austin, hvr, bgamari

Reviewed By: simonpj, bgamari

Subscribers: bitemyapp, thomie

Projects: #ghc

Differential Revision: https://phabricator.haskell.org/D1912

GHC Trac Issues: #11573

2 files changed, 5 insertions, 10 deletions

diff --git a/testsuite/tests/partial-sigs/should_compile/ExtraConstraints3.stderr b/testsuite/tests/partial-sigs/should_compile/ExtraConstraints3.stderr
index e0daa4e87d..c34b1396c7 100644
--- a/testsuite/tests/partial-sigs/should_compile/ExtraConstraints3.stderr
+++ b/testsuite/tests/partial-sigs/should_compile/ExtraConstraints3.stderr
@@ -62,8 +62,7 @@ TYPE SIGNATURES
   enumFromThen :: forall a. Enum a => a -> a -> [a]
   enumFromThenTo :: forall a. Enum a => a -> a -> a -> [a]
   enumFromTo :: forall a. Enum a => a -> a -> [a]
-  error ::
-    forall a. (?callStack::GHC.Stack.Types.CallStack) => [Char] -> a
+  error :: forall a. [Char] -> a
   even :: forall a. Integral a => a -> Bool
   exp :: forall a. Floating a => a -> a
   exponent :: forall a. RealFloat a => a -> Int
@@ -213,7 +212,7 @@ TYPE SIGNATURES
   toRational :: forall a. Real a => a -> Rational
   truncate :: forall a b. (RealFrac a, Integral b) => a -> b
   uncurry :: forall a b c. (a -> b -> c) -> (a, b) -> c
-  undefined :: forall t. (?callStack::GHC.Stack.Types.CallStack) => t
+  undefined :: forall t. t
   unlines :: [String] -> String
   until :: forall a. (a -> Bool) -> (a -> a) -> a -> a
   unwords :: [String] -> String
@@ -232,4 +231,4 @@ TYPE CONSTRUCTORS
 COERCION AXIOMS
 Dependent modules: []
 Dependent packages: [base-4.9.0.0, ghc-prim-0.5.0.0,
-                     integer-gmp-1.0.0.0]
+                     integer-gmp-1.0.0.1]
diff --git a/testsuite/tests/partial-sigs/should_fail/T10999.stderr b/testsuite/tests/partial-sigs/should_fail/T10999.stderr
index ef4d9f83c1..c74719addf 100644
--- a/testsuite/tests/partial-sigs/should_fail/T10999.stderr
+++ b/testsuite/tests/partial-sigs/should_fail/T10999.stderr
@@ -1,7 +1,6 @@
 
 T10999.hs:5:6: error:
-    Found constraint wildcard ‘_’
-      standing for ‘(?callStack::GHC.Stack.Types.CallStack, Ord a)’
+    Found constraint wildcard ‘_’ standing for ‘Ord a’
     To use the inferred type, enable PartialTypeSignatures
     In the type signature:
       f :: _ => () -> _
@@ -9,10 +8,7 @@ T10999.hs:5:6: error:
 T10999.hs:5:17: error:
     • Found type wildcard ‘_’ standing for ‘Set.Set a’
       Where: ‘a’ is a rigid type variable bound by
-               the inferred type of
-               f :: (?callStack::GHC.Stack.Types.CallStack, Ord a) =>
-                    () -> Set.Set a
-               at T10999.hs:6:1
+               the inferred type of f :: Ord a => () -> Set.Set a at T10999.hs:6:1
       To use the inferred type, enable PartialTypeSignatures
     • In the type signature:
         f :: _ => () -> _