id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
6124	Spurious non-exhaustive warning with GADT and newtypes	joeyadams		"This may be related to #3927 or similar, but here's another case where the compiler produces a ""Pattern match(es) are non-exhaustive"" warning for patterns on a GADT that are impossible to implement:

{{{
newtype A = MkA Int
newtype B = MkB Char

data T a where
    A :: T A
    B :: T B

f :: T A -> A
f A = undefined
}}}

This produces the following warning:

{{{
    Warning: Pattern match(es) are non-exhaustive
             In an equation for `f': Patterns not matched: B
}}}

It is impossible to write a pattern for {{{B}}} because {{{B :: T B}}} does not match {{{T A}}}.

If I replace {{{newtype}}} with {{{data}}} for both {{{A}}} and {{{B}}}, the warning goes away.  If I replace only one instance of either {{{newtype}}}, it will still produce the warning."	bug	new	normal	7.8.1	Compiler (Type checker)	7.4.1				Unknown/Multiple	Unknown/Multiple	Incorrect warning at compile-time	Unknown