[NopeCharon]

Summary

A caret or tilde requirement whose version leaves segments unwritten read them as zeros: ^0 and ^0.0 were bounded at <0.0.1, ~1 at <1.1 and ~0 at <0.1. A dependency "^0" against an index with 0.3.0 and 0.4.0 failed with constraint '^0' matches none of: [0.3.0, 0.4.0], and "~1" never reached 1.1.0.

matches now follows Cargo's partial requirements, which mcpp.xpkg_version already follows for the caret. A segment that is not written is a wildcard:

• caret: the bound bumps the leftmost non-zero segment, looking no further than the patch and stopping at the last segment written. ^0 → <1.0.0, ^0.0 → <0.1.0; ^0.0.0 stays <0.0.1.
• tilde: with no minor written, the bound holds the major. ~1 → <2.0.0; ~1.2 and ~1.2.3 stay <1.3.0.

The grammar notes at the top of version_req.cppm and the constraint list in docs/05-dependencies.md (both languages) name the two forms.

Closes #864.

Criteria

• VersionReq.PartialCaretBoundsFollowTheWrittenSegments and VersionReq.PartialTildeWithOnlyAMajorHoldsTheMajor (tests/unit/test_version_req.cpp). On aarch64 Linux with the bootstrap 2026.10.29.1 binary, mcpp test test_version_req fails both before the change (matches("^0.0", 0.0.7), matches("~1", 1.1.0), matches("~0", 0.4.0) are false) and passes all 31 after it.
• The full unit suite (mcpp test, 156 passed, 0 failed) and mcpp test -p versioning pass on aarch64 Linux. Other hosts are left to CI.
• End to end, with the e2e 169 layout (a project [indices] path index publishing acme.gadget 0.3.0 and 0.4.0): the released binary refuses gadget = "^0" and "~0" with "matches none of". The binary built from this branch resolves both to 0.4.0, builds, runs the program and writes version = "0.4.0" to mcpp.lock.

Intersections

New rule or feature	Invariant it crosses	Test at the crossing
Caret bound stops at the last written segment	The leftmost-nonzero rule for fully written versions, including four-segment date versions (^2026.7.27.3 → <2027)	MatchCaretBare, MatchCaretZeroMajor, CaretUpperBoundZeroesTheFourthSegment, and the ^0.0.0 / ^0.0.3 / ^0.5 cases in the new test
Tilde with only a major holds the major	~X.Y[.Z] holds the minor; date versions zero the tail	MatchTilde, TildeUpperBoundZeroesTheFourthSegment, the ~1.2 case in the new test
Either bound	A pre-release is visible only when the requirement names one at the same numbers; the bound is a release	RangesDoNotSeePrereleasesUnlessAskedFor, CaretUpperBoundDoesNotLeakIntoTheNextMajorsPrerelease

Compatibility

Only requirements spelled ^0, ^0.0, ~N (one segment) change. Each now admits the versions Cargo admits, a superset of what it admitted before. A manifest that failed to resolve with one of them now resolves. One that resolved now selects the highest match in the wider range, and the lock records it. All other requirement spellings, and bare versions (exact addresses), are unchanged.

Checks before merging

• bash .github/tools/check_docs_style.sh, check_docs_structure.sh and check_version_pins.sh pass.
• python3 .github/tools/check_workflow_assertions.py passes (a step asserts what its name says).
• No commit on the branch carries an attribution trailer:
  git log origin/main..HEAD -i --grep='Co-Authored-By' prints nothing.
• The squash merge is given an explicit subject and body, so GitHub does not
  compose one from the branch's commits. Suggested subject: fix: a partial caret or tilde requirement treats unwritten segments as wildcards.