Forbid build metadata ordering (See #18)

SemVer 2.0.0 states that "Build metadata SHOULD be ignored when
determining version precedence".

This means that, when comparing ``0.1.0+1`` to ``0.1.0+bcd``::

    >>> Version('0.1.0+1') == Version('0.1.0+bcd')
    False
    >>> Version('0.1.0+1') != Version('0.1.0+bcd')
    True
    >>> Version('0.1.0+1') < Version('0.1.0+bcd')
    False
    >>> Version('0.1.0+1') > Version('0.1.0+bcd')
    False
    >>> Version('0.1.0+1') <= Version('0.1.0+bcd')
    False
    >>> Version('0.1.0+1') >= Version('0.1.0+bcd')
    False
    >>> compare(Version('0.1.0+1'), Version('0.1.0+bcd'))
    NotImplemented

This change also has the following effects:
- When including build metadata in a ``Spec``, the only valid options
  are ``Spec('==0.1.0+sth')`` and ``Spec('!=0.1.0+sth')``
- The meaning of ``Spec('==0.1.0+')`` is now "Only version 0.1.0 without
  build metadata"
- ``Spec('==0.1.0')`` now matches ``Version('0.1.0+anything')``

1 files changed, 6 insertions, 7 deletions

diff --git a/README.rst b/README.rst
index 5923a30..484692e 100644
--- a/README.rst
+++ b/README.rst
@@ -247,19 +247,18 @@ definition or (for the empty pre-release number) if a single dash is appended
     False
 
 
-Including build identifiers in specifications
-"""""""""""""""""""""""""""""""""""""""""""""
+Including build metadata in specifications
+""""""""""""""""""""""""""""""""""""""""""
 
-The same rule applies for the build identifier: comparisons will include it only
-if it was included in the :class:`Spec` definition, or - for the unnumbered build
-version - if a single + is appended to the definition(``1.0.0+``, ``1.0.0-alpha+``):
+Build metadata has no ordering; thus, the only meaningful comparison including
+build metadata is equality.
 
 
 .. code-block:: pycon
 
-    >>> Version('1.0.0+build2') in Spec('<=1.0.0')   # Build identifier ignored
+    >>> Version('1.0.0+build2') in Spec('<=1.0.0')   # Build metadata ignored
     True
-    >>> Version('1.0.0+build2') in Spec('<=1.0.0+')  # Include build in checks
+    >>> Version('1.0.0+build2') in Spec('==1.0.0+build2')  # Include build in checks
     False