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-fwarn-incomplete-patterns doesn't seem to generate any warnings when -fno-code is specified.

To reproduce, save this module as foo.hs.

module Foo where
foo True = 4

and compile with -fwarn-incomplete-patterns, with and without -fno-code.

% ghc -fwarn-incomplete-patterns -fforce-recomp foo.hs
[1 of 1] Compiling Foo              ( foo.hs, foo.o )

foo.hs:2:1: Warning:
    Pattern match(es) are non-exhaustive
    In an equation for ‘foo’: Patterns not matched: False
% ghc -fwarn-incomplete-patterns -fforce-recomp -fno-code foo.hs
[1 of 1] Compiling Foo              ( foo.hs, nothing )
%