Use mutable update to defer out-of-scope errorswip/T17812

Previously, we let-bound an identifier to use to carry
the erroring evidence for an out-of-scope variable. But
this failed for levity-polymorphic out-of-scope variables,
leading to a panic (#17812). The new plan is to use
a mutable update to just write the erroring expression directly
where it needs to go.

Close #17812.

Test case: typecheck/should_compile/T17812

1 files changed, 7 insertions, 1 deletions

diff --git a/compiler/GHC/Data/IOEnv.hs b/compiler/GHC/Data/IOEnv.hs
index 439f101ecc..850d111818 100644
--- a/compiler/GHC/Data/IOEnv.hs
+++ b/compiler/GHC/Data/IOEnv.hs
@@ -29,7 +29,7 @@ module GHC.Data.IOEnv (
         tryM, tryAllM, tryMostM, fixM,
 
         -- I/O operations
-        IORef, newMutVar, readMutVar, writeMutVar, updMutVar,
+        IORef, newMutVar, readMutVar, writeMutVar, updMutVar, updMutVarM,
         atomicUpdMutVar, atomicUpdMutVar'
   ) where
 
@@ -193,6 +193,12 @@ readMutVar var = liftIO (readIORef var)
 updMutVar :: IORef a -> (a -> a) -> IOEnv env ()
 updMutVar var upd = liftIO (modifyIORef var upd)
 
+updMutVarM :: IORef a -> (a -> IOEnv env a) -> IOEnv env ()
+updMutVarM ref upd
+  = do { contents     <- liftIO $ readIORef ref
+       ; new_contents <- upd contents
+       ; liftIO $ writeIORef ref new_contents }
+
 -- | Atomically update the reference.  Does not force the evaluation of the
 -- new variable contents.  For strict update, use 'atomicUpdMutVar''.
 atomicUpdMutVar :: IORef a -> (a -> (a, b)) -> IOEnv env b