Set logger flags in --backpack mode

  Backpack used to initialise the logger before obtaining the
  DynFlags. This meant that logging options (such as dump flags)
  were not set.
  Initialising the logger after the session flags have been set
  fixes the issue.

  fixes #20396

4 files changed, 33 insertions, 2 deletions

diff --git a/compiler/GHC/Driver/Backpack.hs b/compiler/GHC/Driver/Backpack.hs
index a4dbe7052b..5d0a6a828c 100644
--- a/compiler/GHC/Driver/Backpack.hs
+++ b/compiler/GHC/Driver/Backpack.hs
@@ -91,8 +91,6 @@ import qualified Data.Set as Set
 -- | Entry point to compile a Backpack file.
 doBackpack :: [FilePath] -> Ghc ()
 doBackpack [src_filename] = do
-    logger <- getLogger
-
     -- Apply options from file to dflags
     dflags0 <- getDynFlags
     let dflags1 = dflags0
@@ -100,6 +98,9 @@ doBackpack [src_filename] = do
     src_opts <- liftIO $ getOptionsFromFile parser_opts1 src_filename
     (dflags, unhandled_flags, warns) <- liftIO $ parseDynamicFilePragma dflags1 src_opts
     modifySession (hscSetFlags dflags)
+    logger <- getLogger -- Get the logger after having set the session flags,
+                        -- so that logger options are correctly set.
+                        -- Not doing so caused #20396.
     -- Cribbed from: preprocessFile / GHC.Driver.Pipeline
     liftIO $ checkProcessArgsResult unhandled_flags
     liftIO $ handleFlagWarnings logger (initDiagOpts dflags) warns
diff --git a/testsuite/tests/backpack/should_compile/T20396.bkp b/testsuite/tests/backpack/should_compile/T20396.bkp
new file mode 100644
index 0000000000..eff8b66316
--- /dev/null
+++ b/testsuite/tests/backpack/should_compile/T20396.bkp
@@ -0,0 +1,7 @@
+
+{-# OPTIONS_GHC -ddump-simpl -dsuppress-all -dsuppress-uniques #-}
+
+unit unit where
+  module A where
+    f :: Int -> Int
+    f x = x
diff --git a/testsuite/tests/backpack/should_compile/T20396.stderr b/testsuite/tests/backpack/should_compile/T20396.stderr
new file mode 100644
index 0000000000..702ab87ffe
--- /dev/null
+++ b/testsuite/tests/backpack/should_compile/T20396.stderr
@@ -0,0 +1,22 @@
+[1 of 1] Processing unit
+  Instantiating unit
+  [1 of 1] Compiling A                ( unit\A.hs, T20396.out\unit\A.o )
+
+==================== Tidy Core ====================
+Result size of Tidy Core
+  = {terms: 17, types: 8, coercions: 0, joins: 0/0}
+
+f = \ x -> x
+
+$trModule1 = "unit"#
+
+$trModule2 = TrNameS $trModule1
+
+$trModule3 = "A"#
+
+$trModule4 = TrNameS $trModule3
+
+$trModule = Module $trModule2 $trModule4
+
+
+
diff --git a/testsuite/tests/backpack/should_compile/all.T b/testsuite/tests/backpack/should_compile/all.T
index ac8c2a7ed8..07c1ed27ec 100644
--- a/testsuite/tests/backpack/should_compile/all.T
+++ b/testsuite/tests/backpack/should_compile/all.T
@@ -58,3 +58,4 @@ test('T13149', expect_broken(13149), backpack_compile, [''])
 test('T13214', normal, backpack_compile, [''])
 test('T13250', normal, backpack_compile, [''])
 test('T13323', normal, backpack_compile, [''])
+test('T20396', normal, backpack_compile, [''])