Not-in-scope variables are always errors

This fixes Trac #12406.  A not-in-scope error shoudl be an error
even if you have -fdefer-typed-holes.

compiler/typecheck/TcErrors.hs

@@ -621,9 +621,15 @@ maybeReportHoleError ctxt ct err
        HoleWarn  -> reportWarning (Reason Opt_WarnPartialTypeSignatures) err
        HoleDefer -> return ()
 
-  -- Otherwise this is a typed hole in an expression
+  | isOutOfScopeCt ct
+  = -- Always report an error for out-of-scope variables
+    -- See Trac #12170, #12406
+    reportError err
+
+  -- Otherwise this is a typed hole in an expression,
+  -- but not for an out-of-scope variable
   | otherwise
-  = -- If deferring, report a warning only if -Wtyped-holds is on
+  = -- If deferring, report a warning only if -Wtyped-holes is on
     case cec_expr_holes ctxt of
        HoleError -> reportError err
        HoleWarn  -> reportWarning (Reason Opt_WarnTypedHoles) err

testsuite/tests/partial-sigs/should_compile/T12156.stderr

 
-T12156.hs:3:14: warning: [-Wtyped-holes (in -Wdefault)]
-    Variable not in scope: v
+T12156.hs:3:14: error: Variable not in scope: v

testsuite/tests/partial-sigs/should_compile/all.T

@@ -64,4 +64,4 @@ test('SuperCls', normal, compile, [''])
 test('T12033', normal, compile, [''])
 test('T11339a', normal, compile, [''])
 test('T11670', normal, compile, [''])
-test('T12156', normal, compile, ['-fdefer-typed-holes'])
+test('T12156', normal, compile_fail, ['-fdefer-typed-holes'])

testsuite/tests/typecheck/should_fail/T12406.hs

+{-# LANGUAGE TypeFamilies #-}
+{-# OPTIONS_GHC -fdefer-typed-holes #-}
+
+module T12406 where
+
+-- import Control.Monad  -- comment this out to cause error
+import Data.IORef
+
+class MonadRef m where
+    type Ref m :: * -> *
+    newRef :: a -> m (Ref m a)
+    readRef :: Ref m a -> m a
+
+instance MonadRef IO where
+    type Ref IO = IORef
+    newRef = newIORef
+    readRef = readIORef
+
+foo :: IO ()
+foo = newRef (pure ()) >>= join . readRef

testsuite/tests/typecheck/should_fail/T12406.stderr

+
+T12406.hs:20:7: error:
+    • Couldn't match type ‘Ref m0’ with ‘IORef’
+      Expected type: IO (Ref m0 (f0 ()))
+        Actual type: IO (Ref IO (f0 ()))
+      The type variable ‘m0’ is ambiguous
+    • In the first argument of ‘(>>=)’, namely ‘newRef (pure ())’
+      In the expression: newRef (pure ()) >>= join . readRef
+      In an equation for ‘foo’: foo = newRef (pure ()) >>= join . readRef
+
+T12406.hs:20:28: error:
+    Variable not in scope: join :: m0 (f0 ()) -> IO ()

testsuite/tests/typecheck/should_fail/all.T

@@ -423,4 +423,5 @@ test('T12063', [ expect_broken(12063), extra_clean(['T12063.hi-boot', 'T12063.o-
 test('T11974b', normal, compile_fail, [''])
 test('T12151', normal, compile_fail, [''])
 test('T7437', normal, compile_fail, [''])
-test('T12177', normal, compile_fail, [''])
\ No newline at end of file
+test('T12177', normal, compile_fail, [''])
+test('T12406', normal, compile_fail, [''])