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

3 files changed, 30 insertions, 0 deletions

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, [''])