Add test for #25428

testsuite/tests/linear/should_compile/T25428.hs

+{-# language LinearTypes #-}
+
+module T25428 where
+
+f :: () %1 -> Int
+f x = let !() = x in 0

testsuite/tests/linear/should_compile/all.T

@@ -48,3 +48,4 @@ test('LinearLetPoly', normal, compile, [''])
 test('LinearListComprehension', normal, compile, ['-dlinear-core-lint'])
 test('OmitFieldPat', normal, compile, ['-dcore-lint'])
 test('T25515', normal, compile, ['-dcore-lint'])
+test('T25428', expect_broken(25428), compile, [''])