Sections with undefined operators have non-standard behavior

docs/users_guide/bugs.rst

@@ -348,6 +348,29 @@ The Foreign Function Interface
         single: hs_init
         single: hs_exit
 
+.. _infelicities-operator-sections:
+
+Operator sections
+^^^^^^^^^^^^^^^^^
+
+The Haskell Report demands that, for infix operators ``%``, the following
+identities hold:
+
+::
+    (% expr) = \x -> x % expr
+    (expr %) = \x -> expr % x
+
+However, the second law is violated in the presence of undefined operators,
+
+::
+    (%) = error "urk"
+    (() %)         `seq` () -- urk
+    (\x -> () % x) `seq` () -- OK, result ()
+
+The operator section is treated like function application of an undefined
+function, while the lambda form is in WHNF that contains an application of an
+undefined function.
+
 .. _haskell-98-2010-undefined:
 
 GHC's interpretation of undefined behaviour in Haskell 98 and Haskell 2010