1 files changed, 35 insertions, 6 deletions
diff --git a/compiler/main/GHC.hs b/compiler/main/GHC.hs
index d04f092c17..d6aa2273dc 100644
--- a/compiler/main/GHC.hs
+++ b/compiler/main/GHC.hs
@@ -570,17 +570,19 @@ checkBrokenTablesNextToCode' dflags
 --
 setSessionDynFlags :: GhcMonad m => DynFlags -> m [PackageKey]
 setSessionDynFlags dflags = do
-  (dflags', preload) <- liftIO $ initPackages dflags
-  modifySession $ \h -> h{ hsc_dflags = dflags'
-                         , hsc_IC = (hsc_IC h){ ic_dflags = dflags' } }
+  dflags' <- checkNewDynFlags dflags
+  (dflags'', preload) <- liftIO $ initPackages dflags'
+  modifySession $ \h -> h{ hsc_dflags = dflags''
+                         , hsc_IC = (hsc_IC h){ ic_dflags = dflags'' } }
   invalidateModSummaryCache
   return preload
 
 -- | Sets the program 'DynFlags'.
 setProgramDynFlags :: GhcMonad m => DynFlags -> m [PackageKey]
 setProgramDynFlags dflags = do
-  (dflags', preload) <- liftIO $ initPackages dflags
-  modifySession $ \h -> h{ hsc_dflags = dflags' }
+  dflags' <- checkNewDynFlags dflags
+  (dflags'', preload) <- liftIO $ initPackages dflags'
+  modifySession $ \h -> h{ hsc_dflags = dflags'' }
   invalidateModSummaryCache
   return preload
 
@@ -619,7 +621,8 @@ getProgramDynFlags = getSessionDynFlags
 -- 'pkgState' into the interactive @DynFlags@.
 setInteractiveDynFlags :: GhcMonad m => DynFlags -> m ()
 setInteractiveDynFlags dflags = do
-  modifySession $ \h -> h{ hsc_IC = (hsc_IC h) { ic_dflags = dflags }}
+  dflags' <- checkNewDynFlags dflags
+  modifySession $ \h -> h{ hsc_IC = (hsc_IC h) { ic_dflags = dflags' }}
 
 -- | Get the 'DynFlags' used to evaluate interactive expressions.
 getInteractiveDynFlags :: GhcMonad m => m DynFlags
@@ -631,6 +634,32 @@ parseDynamicFlags :: MonadIO m =>
                   -> m (DynFlags, [Located String], [Located String])
 parseDynamicFlags = parseDynamicFlagsCmdLine
 
+{- Note [GHCi and -O]
+~~~~~~~~~~~~~~~~~~~~~
+When using optimization, the compiler can introduce several things
+(such as unboxed tuples) into the intermediate code, which GHCi later
+chokes on since the bytecode interpreter can't handle this (and while
+this is arguably a bug these aren't handled, there are no plans to fix
+it.)
+
+While the driver pipeline always checks for this particular erroneous
+combination when parsing flags, we also need to check when we update
+the flags; this is because API clients may parse flags but update the
+DynFlags afterwords, before finally running code inside a session (see
+T10052 and #10052).
+-}
+
+-- | Checks the set of new DynFlags for possibly erroneous option
+-- combinations when invoking 'setSessionDynFlags' and friends, and if
+-- found, returns a fixed copy (if possible).
+checkNewDynFlags :: MonadIO m => DynFlags -> m DynFlags
+checkNewDynFlags dflags
+  -- See Note [GHCi and -O]
+  | Left e <- checkOptLevel (optLevel dflags) dflags
+    = do liftIO $ warningMsg dflags (text e)
+         return (dflags { optLevel = 0 })
+  | otherwise
+    = return dflags
 
 -- %************************************************************************
 -- %*                                                                      *