Catch type-checker exceptions when splicing

In GHC.Tc.Gen.Splice.tcTopSpliceExpr we were forgetting to
catch exceptions. As a result we missed the kind error
in the unsolved constraints.

This patch has an easy fix, which cures #20179

(cherry picked from commit eff51e2aca978ebd0ca08345fb1790d70f6bb7c8)

4 files changed, 28 insertions, 4 deletions

diff --git a/compiler/GHC/Tc/Gen/Splice.hs b/compiler/GHC/Tc/Gen/Splice.hs
index 998f03a5d5..f2fd31f02a 100644
--- a/compiler/GHC/Tc/Gen/Splice.hs
+++ b/compiler/GHC/Tc/Gen/Splice.hs
@@ -735,11 +735,17 @@ tcTopSpliceExpr isTypedSplice tc_action
                    -- is expected (#7276)
     setStage (Splice isTypedSplice) $
     do {    -- Typecheck the expression
-         (expr', wanted) <- captureConstraints tc_action
-       ; const_binds     <- simplifyTop wanted
+         (mb_expr', wanted) <- tryCaptureConstraints tc_action
+             -- If tc_action fails (perhaps because of insoluble constraints)
+             -- we want to capture and report those constraints, else we may
+             -- just get a silent failure (#20179). Hence the 'try' part.
 
-          -- Zonk it and tie the knot of dictionary bindings
-       ; return $ mkHsDictLet (EvBinds const_binds) expr' }
+       ; const_binds <- simplifyTop wanted
+
+       ; case mb_expr' of
+            Nothing    -> failM   -- In this case simplifyTop should have
+                                  -- reported some errors
+            Just expr' -> return $ mkHsDictLet (EvBinds const_binds) expr' }
 
 {-
 ************************************************************************
diff --git a/testsuite/tests/th/T20179.hs b/testsuite/tests/th/T20179.hs
new file mode 100644
index 0000000000..a01f573979
--- /dev/null
+++ b/testsuite/tests/th/T20179.hs
@@ -0,0 +1,7 @@
+{-# LANGUAGE TemplateHaskell #-}
+module Bug where
+
+$(let slurmp :: [Maybe]
+      slurmp = []
+
+   in pure [])
diff --git a/testsuite/tests/th/T20179.stderr b/testsuite/tests/th/T20179.stderr
new file mode 100644
index 0000000000..28eb54d698
--- /dev/null
+++ b/testsuite/tests/th/T20179.stderr
@@ -0,0 +1,10 @@
+
+T20179.hs:4:18: error:
+    • Expecting one more argument to ‘Maybe’
+      Expected a type, but ‘Maybe’ has kind ‘* -> *’
+    • In the type signature: slurmp :: [Maybe]
+      In the expression:
+        let
+          slurmp :: [Maybe]
+          slurmp = []
+        in pure []
diff --git a/testsuite/tests/th/all.T b/testsuite/tests/th/all.T
index 6d4a5036d7..c89b748f82 100644
--- a/testsuite/tests/th/all.T
+++ b/testsuite/tests/th/all.T
@@ -513,3 +513,4 @@ test('T18102b', extra_files(['T18102b_aux.hs']), compile_and_run, [''])
 test('T18121', normal, compile, [''])
 test('T18123', normal, compile, [''])
 test('T18388', normal, compile, [''])
+test('T20179', normal, compile_fail, [''])