scala/scala — pull request #2439 by retronym (2013-04-24 06:11:21+00:00)
Title: SI-7369 Avoid spurious unreachable warnings in patterns

Unreachability analysis draws on the enumerated domain of types
(e.g sealed subclasses + null, or true/false), and also looks at all
stable identifier patterns tested for equality against the same 'slot'
in a pattern.

It was drawing the wrong conclusions about stable identifier patterns.
Unlike the domain constants, two such values may hold the same value,
so we can't assume that matching X precludes matching Y in the same
slot in a subsequent case.

For example:

val X: Boolean = true; val Y: Boolean = true
def m1(t1: Tuple1[Boolean]) = t1 match {
case Tuple1(true) =>
case Tuple1(false) =>
case Tuple1(false) => // correctly unreachable
}
def m2(t1: Tuple1[Boolean]) = t1 match {
case Tuple1(X) =>
case Tuple1(Y) => // spurious unreachable warning
}

//
// Before
//
reachability, vars:
V2: Boolean ::= true | false// Set(false, Y, X, true) // = x1._1
V1: (Boolean,) ::= null | ... // = x1
equality axioms:
V2=true#4 \/ V2=false#5 /\
-V2=false#5 \/ -V2=Y#3 /\
-V2=false#5 \/ -V2=X#2 /\
-V2=false#5 \/ -V2=true#4 /\
-V2=Y#3 \/ -V2=X#2 /\
-V2=Y#3 \/ -V2=true#4 /\
-V2=X#2 \/ -V2=true#4

//
// After
//
reachability, vars:
V2: Boolean ::= true | false// Set(false, Y, X, true) // = x1._1
V1: (Boolean,) ::= null | ... // = x1
equality axioms:
V2=true#4 \/ V2=false#5 /\
-V2=false#5 \/ -V2=true#4

Review by @adriaanm, who diagnosed the problem the other day by harnessing the emanations of the Scala Staircase. (Or was it the coffee?)