strange behaviour of -fwarn-incomplete-uni-patterns

For the following program (attached):

module Pat where

w :: String
w = let (_:_) = ["1", "2", "3"] in "1"

w2 :: String
w2 = let (_a:_b) = ["1", "2", "3"] in "1"

the following warnings are generated (note line numbers)

mikolaj@bratek:~/waste$ ghc --make pat.hs -fwarn-incomplete-uni-patterns
[1 of 1] Compiling Patwhere         ( pat.hs, pat.o )

pat.hs:7:10:
    Warning: Pattern match(es) are non-exhaustive
             In a pattern binding: Patterns not matched: []

pat.hs:7:10:
    Warning: Pattern match(es) are non-exhaustive
             In a pattern binding: Patterns not matched: []
mikolaj@bratek:~/waste$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.3.20110808

Trac metadata

Trac field	Value
Version	7.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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