Add regression test for #15321

testsuite/tests/th/T15321.hs

+{-# LANGUAGE TemplateHaskell #-}
+
+module T15321 where
+
+foo :: String
+foo = test
+
+bar :: String
+bar = $(_ "baz")

testsuite/tests/th/T15321.stderr

+
+T15321.hs:9:9: error:
+    • Found hole: _ :: [Char] -> Language.Haskell.TH.Lib.Internal.ExpQ
+    • In the expression: _
+      In the expression: _ "baz"
+      In the untyped splice: $(_ "baz")
+    • Valid hole fits include
+        fail :: forall (m :: * -> *) a. Monad m => String -> m a
+          with fail @Language.Haskell.TH.Syntax.Q
+                    @Language.Haskell.TH.Syntax.Exp
+          (imported from ‘Prelude’ at T15321.hs:3:8-13
+           (and originally defined in ‘GHC.Base’))

testsuite/tests/th/all.T

@@ -415,3 +415,4 @@ test('T14885a', normal, compile, [''])
 test('T14885b', normal, compile, [''])
 test('T14885c', normal, compile, [''])
 test('T15243', normal, compile, ['-dsuppress-uniques'])
+test('T15321', normal, compile_fail, [''])