Remove * from valid version characters

cep-xx.md

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+<table>
+<tr><td> Title </td><td> Remove <code>&ast</code>; from valid version characters </td>
+<tr><td> Status </td><td> Draft </td></tr>
+<tr><td> Author(s) </td><td> Antoine Prouvost &lt;[email protected]&gt; </td></tr>
+<tr><td> Created </td><td> July 18, 2023 </td></tr>
+<tr><td> Updated </td><td> July 18, 2023 </td></tr>
+<tr><td> Discussion </td><td> NA </td></tr>
+<tr><td> Implementation </td><td> NA </td></tr>
+</table>
+
+
+## Abstract
+We recommend to remove ``*`` from valid ``Version`` characters, because of the confusing intersection with
+``VersionSpec`` glob expressions.
+We suggest different possibilities with how the character could be reinterpreted in ``VersionSpec``,
+including an analysis on backward compatibility.
+
+
+## Motivation
+### Suprising examples
+In conda, ``*`` is a valid version character that is smaller than any string.
+For instance ``1.1* < 1.1alpha < 1.1``.
+Strings following a separator (typically a ``.``) are implicitly assumed to start with a ``0``, so
+we further have ``1.1.* == 1.1.0* < 1.1.0``.
+
+When we further consider ``VersionSpec`` (sets of versions), both ``.*`` and ``*`` suffixes can
+alternatively be interpreted as a glob expression.
+
+We have the following in ``conda``.
+
+| VersionSpec | Version operand | ``1.7.0`` matches |
+|-------------|-----------------|-------------------|
+| ``1.7.*``   | ``1.7``         | True              |
+| ``1.7*``    | ``1.7``         | True              |
+| ``=1.7.*``  | ``1.7``         | True              |
+| ``=1.7*``   | ``1.7*``        | False             |
+| ``==1.7.*`` | ``1.7``         | True              |
+| ``==1.7*``  | ``1.7*``        | False             |
+| ``!=1.7.*`` | ``1.7``         | False             |
+| ``!=1.7*``  | ``1.7*``        | True              |
+
+Using ``==``, ``<``, ``<=``, ``>``, ``>=`` comparison operator, ``*`` suffix is always parsed
+with the version, and ``.*`` always dropped (with a warning but users may not see it, for instance
+if the spec is used in a recipe).
+
+| VersionSpec | Version operand | ``1.7.0alpha`` matches |
+|-------------|-----------------|------------------------|
+| ``>=1.7.*`` | ``1.7``         | False                  |
+| ``>=1.7*``  | ``1.7*``        | True                   |
+
+Core maintainers from [mamba](github.com/mamba-org/mamba),
+[rattler](github.com/mamba-org/rattler), and [conda-forge](https://conda-forge.org/) failed to
+properly predict which version would match the aforementionned version specs.
+regular users would be all the more surprised of the difference, especially when ``*`` is not a
+usual version character (_e.g._ not in Python [PEP440](https://peps.python.org/pep-0440/)).
+
+### Why was the ``*`` character introduced
+_Note this is a best effort explanation done by the author of this CEP._
+
+Conda versions can have arbitrarily many ``.`` separated segements, each containing arbitrarily
+many alternation of strings and numbers.
+For instance ``1.1 == 1.1.0 == 1.1.0.0 == ...``, and ``1.1 < 1.1p < 1.1p1 < 1.1p1p < ...1``.
+In a finite semantic verisioning scheme, such as Python PEP440, when asking for a spec
+"_matching all the versions starting with 1.1" (_i.e._ ``=1.1*`` in conda), one could use
+``>=1.1.0a0`` and ``<1.2.0a0`` to represent the spec as an interval.
+This is a convenient representation that eases working with specs, especially to compute set
+intersections.
+
+However, in conda we have ``1.1.0a0 > 1.1.0a0a0 > 1.1.0a0a0a0 > ...``, there is no smallest
+version that can be used as a lower bound for such an interval.
+As a comparison, this would be like searching for a decimal to represent the lower bound of the
+interval ``>π`` (the mathematical constant).
+Hence, the ``*`` character, conveniently not used in practice in versions, was used to represent
+this "version" that does not exist but that is useful for representing intervals
+(``>1.1*,<1.2*`` in the above example).
+We refer to this "imaginary" version as the _``1.1`` infimum_ in the rest of this proposal.
+
+But that special version should always have stayed an implementation detail and never leak into
+the set of valid versions, as allowing so breaks it purpose when considering the spec ``1.1*.0*``.
+The special character ``*`` should also have been limited to a single appearance per version,
+otherwise we can repeat the infinitly decreasing sequence ``1.1* > 1.1*.0* > 1.1*.0*.0* > ...``
+of versions in that start with ``1.1``.
+
+
+## Rationale
+We searched through the ``repodata.json`` of all subdirs of the ``anaconda``, ``conda-forge``,
+and ``bioconda`` channels and found that:
+ - No ``"version"`` field of any package contains a ``*``
+ - Occurences of ``>1.1*``, ``>1.1.*``, ``>=1.1*``, ``>=1.1.*``
+ - Few occurences of ``<1.1*`` (on bioconda), ``<1.1.*``, no occurence of ``<=1.1*`` and ``<=1.1.*``
+ - Single occurence of ``==2.6*`` (dep on Python bioconda neve matching) and no occurence of ``=1.1*``
+
+
+## Specification
+### [V1] Version
+Given that no version currently contains the ``*`` character, we recommend to simply forbid ``*``
+inside a version.
+ - The public API of ``Version`` must fail to create/parse a version from string contaning ``*``.
+ - ``Version`` must not serialize to any string containing a ``*``.
+ - ``repodata.json`` and the like must not contain a ``*`` in any of the ``"version"`` fields.
+
+Note that ``*`` can still be used internally to represent these special infimum "versions".
+
+### VersionSpec
+#### [VS1] Equality and startswith
+For equality (``==`` and no prefix operators) and starts with (``=`` operator) we advocate to always
+interpret ``*`` as a glob expression.
+From the motivation section, this would change the behaviour of ``=1.7*`` and ``==1.7*``.
+Since no version where found containing a ``*``, the only occurence of such version spec are
+most likely a mistake.
+
+We further argue to differentiate between ``.*`` and ``*`` suffixes to make their meaning
+closer to glob expresions.
+- ``1.7*`` should match versions starting with ``1.7``, which would _include_ ``1.7alpha``;
+- ``1.7.*`` should match versionw starting with ``1.7.``, which would _exclude_ ``1.7alpha``.
+
+Currently, in ``conda``, only ``=1.7`` matches ``1.7alpha``.
+
+#### [VS2] Relational comparison operators ``>``, ``>=``, ``<``, ``<=``
+This is the most delicate part to handle since ``>1.7*`` is frequently used to mean greater than the
+``1.7`` infimum (could also be writen as ``=1.7|>=1.7``, that is, mathching some versions smaller
+than ``1.7.0``, such as ``1.7alpha``).
+
+##### [VS2A] Replace the ``*``
+``conda`` is already ignoring ``.*`` suffixes on these operators.
+We could extend that to the ``*`` suffix, making the breaking change that ``>1.7*`` and the like
+would no longer match pre and alpha releases.
+In conda-forge, pre and alpha releases are kept in separate channels, so the impacts are not as
+important.
+
+However, this solution has the downside of not long being able to represent version infima.
+This could be reintroduced with a new operator, such as ``>inf(1.7)`` or ``>-1.7``.
+Conda-forge also has a
+[repodata patching tool](https://github.com/conda-forge/conda-forge-repodata-patches-feedstock)
+for globally migrating package metadata.
+
+
+##### [VS2B] Reinterpret the ``*``
+Alternatively, we could reinterpret ``>1.7*`` as a comparison on the set of versions starting with
+``1.7``.
+
+``>=1.7*`` could be understood as _greater than at least one version in the ``1.7`` set_.
+This has no breakage with the current use of ``>=1.7*``.
+With a symetric meaning, ``<=1.7*`` would be a breaking change, but this is seldom used (not used
+in the ``repodata.json`` surveyed).
+
+As for ``>1.7*``, it could become equivalent to ``>=1.7*`` in order to minimize breakage, or
+be reinterpreted to _greater than all versions in the ``1.7`` set_ (hence equivalent to ``>=1.8*``).
+The latter is (subjectively) more intuitive.
+For ``<1.7*`` the breakage is reversed, but this is again not used as often.
+
+As for the equality operator in [VS1], we suggest ``>1.7*`` and ``1.7.*`` to have different meanings.
+
+## Copyright
+All CEPs are explicitly [CC0 1.0 Universal](https://creativecommons.org/publicdomain/zero/1.0/).