Allow generation of TTH syntax with TH

In other words allow generation of typed splices and brackets with
Untyped Template Haskell.

That is useful in cases where a library is build with TTH in mind,
but we still want to generate some auxiliary declarations,
where TTH cannot help us, but untyped TH can.
Such example is e.g. `staged-sop` which works with TTH,
but we would like to derive `Generic` declarations with TH.

An alternative approach is to use `unsafeCodeCoerce`, but then the
derived `Generic` instances would be type-checked only at use sites,
i.e. much later. Also `-ddump-splices` output is quite ugly:
user-written instances would use TTH brackets, not `unsafeCodeCoerce`.

This commit doesn't allow generating of untyped template splices
and brackets with untyped TH, as I don't know why one would want to do
that (instead of merging the splices, e.g.)

1 files changed, 4 insertions, 0 deletions

diff --git a/compiler/GHC/ThToHs.hs b/compiler/GHC/ThToHs.hs
index 724f15f602..39da7e0c51 100644
--- a/compiler/GHC/ThToHs.hs
+++ b/compiler/GHC/ThToHs.hs
@@ -1157,6 +1157,10 @@ cvtl e = wrapLA (cvt e)
                                          (L noSrcSpanA (DotFieldOcc noAnn (L noSrcSpanA (FieldLabelString (fsLit f))))) }
     cvt (ProjectionE xs) = return $ HsProjection noAnn $ fmap
                                          (L noSrcSpanA . DotFieldOcc noAnn . L noSrcSpanA . FieldLabelString  . fsLit) xs
+    cvt (TypedSpliceE e) = do { e' <- parenthesizeHsExpr appPrec <$> cvtl e
+                              ; return $ HsTypedSplice (noAnn, noAnn) e' }
+    cvt (TypedBracketE e) = do { e' <- cvtl e
+                               ; return $ HsTypedBracket noAnn e' }
 
 {- | #16895 Ensure an infix expression's operator is a variable/constructor.
 Consider this example: