Catch type-checker exceptions when splicingwip/T20179

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

1 files changed, 10 insertions, 4 deletions

diff --git a/compiler/GHC/Tc/Gen/Splice.hs b/compiler/GHC/Tc/Gen/Splice.hs
index fcd1474afa..13cd3e71c9 100644
--- a/compiler/GHC/Tc/Gen/Splice.hs
+++ b/compiler/GHC/Tc/Gen/Splice.hs
@@ -754,11 +754,17 @@ 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' }
 
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