Use addUsedDataCons more judiciously in TcDeriv (#17324)

If you derive an instance like this:

```hs
deriving <...> instance Foo C
```

And the data constructors for `C` aren't in scope, then
`doDerivInstErrorChecks1` throws an error. Moreover, it will
_only_ throw an error if `<...>` is either `stock` or `newtype`.
This is because the code that the `anyclass` or `via` strategies
would generate would not require the use of the data constructors
for `C`.

However, `doDerivInstErrorChecks1` has another purpose. If you
write this:

```hs
import M (C(MkC1, ..., MkCn))

deriving <...> instance Foo C
```

Then `doDerivInstErrorChecks1` will call `addUsedDataCons` on
`MkC1` through `MkCn` to ensure that `-Wunused-imports` does not
complain about them. However, `doDerivInstErrorChecks1` was doing
this for _every_ deriving strategy, which mean that if `<...>` were
`anyclass` or `via`, then the warning about `MkC1` through `MkCn`
being unused would be suppressed!

The fix is simple enough: only call `addUsedDataCons` when the
strategy is `stock` or `newtype`, just like the other code paths
in `doDerivInstErrorChecks1`.

Fixes #17324.

3 files changed, 22 insertions, 0 deletions

diff --git a/testsuite/tests/deriving/should_compile/T17324.hs b/testsuite/tests/deriving/should_compile/T17324.hs
new file mode 100644
index 0000000000..7373af8936
--- /dev/null
+++ b/testsuite/tests/deriving/should_compile/T17324.hs
@@ -0,0 +1,17 @@
+{-# LANGUAGE DeriveAnyClass #-}
+{-# LANGUAGE DerivingStrategies #-}
+{-# LANGUAGE DerivingVia #-}
+{-# LANGUAGE StandaloneDeriving #-}
+{-# OPTIONS_GHC -Wunused-imports #-}
+module T17324 where
+
+import Data.Monoid (Sum(Sum), Product(Product), Dual(Dual))
+
+class C1 a
+deriving anyclass instance C1 (Sum a)
+
+class C2 a
+deriving anyclass instance C2 (Product a)
+
+class C3 a
+deriving via Dual a instance C3 (Dual a)
diff --git a/testsuite/tests/deriving/should_compile/T17324.stderr b/testsuite/tests/deriving/should_compile/T17324.stderr
new file mode 100644
index 0000000000..54e6534462
--- /dev/null
+++ b/testsuite/tests/deriving/should_compile/T17324.stderr
@@ -0,0 +1,4 @@
+
+T17324.hs:8:1: warning: [-Wunused-imports (in -Wextra)]
+    The import of ‘Dual, Product, Sum’
+    from module ‘Data.Monoid’ is redundant
diff --git a/testsuite/tests/deriving/should_compile/all.T b/testsuite/tests/deriving/should_compile/all.T
index a12cf95c28..04fd02518f 100644
--- a/testsuite/tests/deriving/should_compile/all.T
+++ b/testsuite/tests/deriving/should_compile/all.T
@@ -118,3 +118,4 @@ test('T15637', normal, compile, [''])
 test('T15831', normal, compile, [''])
 test('T16179', normal, compile, [''])
 test('T16518', normal, compile, [''])
+test('T17324', normal, compile, [''])