fix: match version specs with compiler pragma semantics

[charles-cooper]

Fixes #6.

Problem

Source-version inference selected the prerelease target 0.5.0a3 as the source validation compiler for a ^0.4.2 pragma, so the source compile failed with:

vyper.exceptions.VersionException: Version specification "~=0.4.2" is not compatible with compiler version "0.5.0a3"

Root cause

known_versions_satisfying() expanded ^0.4.2 to >=0.4.2, <0.5.0 and evaluated the clauses with raw PEP 440 ordering comparisons. Under PEP 440 ordering, 0.5.0a3 < 0.5.0 is true, so the 0.5.0 alphas leaked through the caret's upper bound and compiler_version_for_source_validation() picked the newest one.

Vyper itself checks pragmas differently (vyper.ast.pre_parser.validate_version_pragma): it normalizes ^X.Y.Z to ~=X.Y.Z and bare versions to ==X.Y.Z, then tests SpecifierSet.contains(compiler_version, prereleases=True). PEP 440 specifier semantics reject a prerelease at an exclusive ordered bound (<0.5.0 never admits 0.5.0a3), so vyupgrade's matcher was strictly more permissive than the compiler it invokes.

Fix

known_versions_satisfying() now mirrors the compiler's check exactly: the spec is normalized the same way and candidates are tested with SpecifierSet.contains(..., prereleases=True). The old clause-based matcher is kept only as a fallback for specs SpecifierSet cannot parse. With this, membership in the candidate list is equivalent to "this compiler accepts this pragma", so this class of divergence cannot recur.

For the issue's repro, source validation now picks 0.4.3:

Repro.vy
  VY001 modernized version pragma (line 1)
  source compile (0.4.3): passed
  target compile: passed
  ABI unchanged: True

This also corrects compiler_version_for_spec("<0.5.0"), which previously resolved to 0.5.0a3 (same bug class).

Test updates

Two existing assertions used >=0.5.0a1,<0.5.0 as a broad alpha pragma, but the real compiler rejects that spec (0.5.0aN is a prerelease of the excluded bound 0.5.0 — verified against vyper 0.5.0a3). They now use >=0.5.0a1,<0.6.0, the form the CLI fixtures already use, plus an explicit assertion that >=0.5.0a1,<0.5.0 matches nothing. A new regression test covers the ^0.4.2 / 0.5.0a3 scenario and legacy ^0.1.0b14 caret pragmas.

ruff check, the full unit suite (484 passed), and tests/test_cli_integration.py (real compilers) all pass.

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https://claude.ai/code/session_01JtAsisni8yijKyn2e29q1R

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