fix pre-release checks in constraints

Pre-releases should not be automatically accepted as candidate versions
for constraints without a pre-release component. This follows the logic
documented for RubyGems which appears to be the inspiration for the
constraint operators, and matches the behavior of other version
packages.

Adding a pre-release component to a pessimistic constraint will limit
accepted versions to pre-releases only. Adding a pre-release to other
constraint types will accept any comparable version, including
pre-releases.

constraint.go

@@ -88,6 +88,12 @@ func (cs Constraints) String() string {
 
 // Check tests if a constraint is validated by the given version.
 func (c *Constraint) Check(v *Version) bool {
+	// A constraint without a pre-release can only match a version without a
+	// pre-release.
+	if c.check.Prerelease() == "" && v.Prerelease() != "" {
+		return false
+	}
+
 	return c.f(v, c.check)
 }
 
@@ -142,6 +148,12 @@ func constraintLessThanEqual(v, c *Version) bool {
 }
 
 func constraintPessimistic(v, c *Version) bool {
+	// Using a pessimistic constraint with a pre-release, restricts versions to
+	// pre-releases.
+	if c.Prerelease() != "" && v.Prerelease() == "" {
+		return false
+	}
+
 	// If the version being checked is naturally less than the constraint, then there
 	// is no way for the version to be valid against the constraint
 	if v.LessThan(c) {

constraint_test.go

@@ -62,6 +62,14 @@ func TestConstraintCheck(t *testing.T) {
 		{"~> 1.0.9.5", "1.0.9.6", true},
 		{"~> 1.0.9.5", "1.0.9.5.0", true},
 		{"~> 1.0.9.5", "1.0.9.5.1", true},
+		{">=2.1.0-a", "2.1.1-beta", true},
+		{"~> 2.0", "2.1.0-beta", false},
+		{">= 1.0.0", "2.1.0-beta", false},
+		{"> 2.0", "2.1.0-beta", false},
+		{">= 2.1.0-a", "2.1.1", true},
+		{"~> 2.1.0-a", "2.2.0", false},
+		{">= 2.1.0-a", "2.1.0", true},
+		{"<= 2.1.0-a", "2.0.0", true},
 	}
 
 	for _, tc := range cases {