Capture and simplify constraints arising from running typed splices

This fixes a regression caused by #15471 where splicing in a trivial
program such as `[|| return () ||]` would fail as the dictionary for
`return` would never get bound in the module containing the splice.

Arguably this is symptomatic of a major problem affecting TTH where we
serialise renamed asts and then retype check them. The reference to the
dictionary should be fully determined at the quote site so that splicing
doesn't have to solve any implicits at all. It's a coincidence this
works due to coherence but see #15863 and #15865 for examples where
things do go very wrong.

Fixes #16195

4 files changed, 37 insertions, 3 deletions

diff --git a/compiler/typecheck/TcSplice.hs b/compiler/typecheck/TcSplice.hs
index 548dc7201e..c6e5740b35 100644
--- a/compiler/typecheck/TcSplice.hs
+++ b/compiler/typecheck/TcSplice.hs
@@ -524,9 +524,14 @@ runTopSplice (DelayedSplice lcl_env orig_expr res_ty q_expr)
         -- Rename and typecheck the spliced-in expression,
         -- making sure it has type res_ty
         -- These steps should never fail; this is a *typed* splice
-       ; addErrCtxt (spliceResultDoc zonked_q_expr) $ do
-         { (exp3, _fvs) <- rnLExpr expr2
-         ; unLoc <$> tcMonoExpr exp3 (mkCheckExpType zonked_ty)} }
+       ; (res, wcs) <-
+            captureConstraints $
+              addErrCtxt (spliceResultDoc zonked_q_expr) $ do
+                { (exp3, _fvs) <- rnLExpr expr2
+                ; tcMonoExpr exp3 (mkCheckExpType zonked_ty)}
+       ; ev <- simplifyTop wcs
+       ; return $ unLoc (mkHsDictLet (EvBinds ev) res)
+       }
 
 
 {-
diff --git a/testsuite/tests/th/T16195.hs b/testsuite/tests/th/T16195.hs
new file mode 100644
index 0000000000..70e936548d
--- /dev/null
+++ b/testsuite/tests/th/T16195.hs
@@ -0,0 +1,15 @@
+{-# LANGUAGE TemplateHaskell #-}
+module T16195 where
+
+import T16195A
+
+main2 :: IO ()
+main2 = return ()
+
+main :: IO ()
+main = $$foo
+
+main3 :: IO ()
+main3 = putStrLn ($$showC $$unitC)
+
+
diff --git a/testsuite/tests/th/T16195A.hs b/testsuite/tests/th/T16195A.hs
new file mode 100644
index 0000000000..b79aff77af
--- /dev/null
+++ b/testsuite/tests/th/T16195A.hs
@@ -0,0 +1,13 @@
+{-# LANGUAGE TemplateHaskell #-}
+module T16195A where
+
+import Language.Haskell.TH
+
+foo :: Q (TExp (IO ()))
+foo = [|| return () ||]
+
+showC :: Q (TExp (() -> String))
+showC = [|| show ||]
+
+unitC :: Q (TExp ())
+unitC = [|| () ||]
diff --git a/testsuite/tests/th/all.T b/testsuite/tests/th/all.T
index 0d34c69c5c..a92cef42f1 100644
--- a/testsuite/tests/th/all.T
+++ b/testsuite/tests/th/all.T
@@ -463,3 +463,4 @@ test('T16133', normal, compile_fail, [''])
 test('T15471', normal, multimod_compile, ['T15471.hs', '-v0'])
 test('T16180', normal, compile_and_run, ['-package ghc'])
 test('T16183', normal, compile, ['-v0 -ddump-splices -dsuppress-uniques'])
+test('T16195', normal, multimod_compile, ['T16195.hs', '-v0'])