Program with trivial polymorphism leads to out of scope dictionary

Almost certainly due to c2455e64

A trivial program now causes a core lint error due to an out-of-scope dictionary.

module A where

foo :: Code (IO ())
foo = [|| return () ||]

module B where

main :: IO ()
main = $$foo

*** Core Lint errors : in result of Desugar (before optimization) ***
<no location info>: warning:
    In the expression: return @ IO $dMonad_a4od @ () ()
    Out of scope: $dMonad_a4od :: Monad m_a4oc[tau:0]
                  [LclId]
*** Offending Program ***
Rec {
$trModule :: Module
[LclIdX]
$trModule = Module (TrNameS "main"#) (TrNameS "B"#)

main :: IO ()
[LclIdX]
main = return @ IO $dMonad_a4od @ () ()
end Rec }

*** End of Offense ***

Trac metadata

Trac field	Value
Version	8.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed milestone to %8.8.1

changed weight to 7

added Tbug TemplateHaskell Trac import + 1 deleted label

I tried adding a call to captureTopConstraints around the zonkTcGblEnv where the splices are run but it doesn't seem to capture any evidence.

MR: !131 (merged)

I understand we'd like to merge this, since the commit causing the regression is in ghc-8.8.

added 1 deleted label

Do we have a regression test?

Trac metadata

Trac field	Value
Test case	- → th/T16195

added backport label

added typed TemplateHaskell label

closed

Regression test is th/T16195 and it has been merged to 8.8

mentioned in commit ac37ab79

mentioned in issue #15471 (closed)

mentioned in issue #15863

mentioned in issue #15865

added Phigh label