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author | Jason Zaugg <jzaugg@gmail.com> | 2013-04-23 23:55:05 +0200 |
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committer | Jason Zaugg <jzaugg@gmail.com> | 2013-04-24 10:09:25 +0200 |
commit | 62713964b96f64f9c0fd0070c89aa6571679856d (patch) | |
tree | 36141e60bec629274a7b716386672f8e3d897509 /src | |
parent | d484c17dbc47559dad8a7ae544d14a285cdfb8b2 (diff) | |
download | scala-62713964b96f64f9c0fd0070c89aa6571679856d.tar.gz scala-62713964b96f64f9c0fd0070c89aa6571679856d.tar.bz2 scala-62713964b96f64f9c0fd0070c89aa6571679856d.zip |
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
Diffstat (limited to 'src')
-rw-r--r-- | src/compiler/scala/tools/nsc/transform/patmat/Logic.scala | 39 |
1 files changed, 28 insertions, 11 deletions
diff --git a/src/compiler/scala/tools/nsc/transform/patmat/Logic.scala b/src/compiler/scala/tools/nsc/transform/patmat/Logic.scala index 0fab48028e..dbe08315f4 100644 --- a/src/compiler/scala/tools/nsc/transform/patmat/Logic.scala +++ b/src/compiler/scala/tools/nsc/transform/patmat/Logic.scala @@ -385,18 +385,35 @@ trait ScalaLogic extends Interface with Logic with TreeAndTypeAnalysis { // else debug.patmat("NOT implies: "+(lower, upper)) - /** does V = C preclude V having value `other`? - (1) V = null is an exclusive assignment, - (2) V = A and V = B, for A and B value constants, are mutually exclusive unless A == B - we err on the safe side, for example: - - assume `val X = 1; val Y = 1`, then - (2: Int) match { case X => case Y => <falsely considered reachable> } - - V = 1 does not preclude V = Int, or V = Any, it could be said to preclude V = String, but we don't model that - - (3) for types we could try to do something fancy, but be conservative and just say no + /** Does V=A preclude V=B? + * + * (0) A or B must be in the domain to draw any conclusions. + * + * For example, knowing the the scrutinee is *not* true does not + * statically exclude it from being `X`, because that is an opaque + * Boolean. + * + * val X = true + * (true: Boolean) match { case true => case X <reachable> } + * + * (1) V = null excludes assignment to any other constant (modulo point #0). This includes + * both values and type tests (which are both modelled here as `Const`) + * (2) V = A and V = B, for A and B domain constants, are mutually exclusive unless A == B + * + * (3) We only reason about test tests as being excluded by null assignments, otherwise we + * only consider value assignments. + * TODO: refine this, a == 0 excludes a: String, or `a: Int` excludes `a: String` + * (since no value can be of both types. See also SI-7211) + * + * NOTE: V = 1 does not preclude V = Int, or V = Any, it could be said to preclude + * V = String, but we don't model that. */ - def excludes(a: Const, b: Const): Boolean = - a != b && ((a == NullConst || b == NullConst) || (a.isValue && b.isValue)) + def excludes(a: Const, b: Const): Boolean = { + val bothInDomain = domain exists (d => d(a) && d(b)) + val eitherIsNull = a == NullConst || b == NullConst + val bothAreValues = a.isValue && b.isValue + bothInDomain && (eitherIsNull || bothAreValues) && (a != b) + } // if(r) debug.patmat("excludes : "+(a, a.tp, b, b.tp)) // else debug.patmat("NOT excludes: "+(a, b)) |