[p-linnane]

With only the Command Line Tools installed, MacOS::Xcode.version is inferred from clang. Xcode 26.4 to 26.6 and 27 all report clang 21.0.0, so CLT 26.4 to 26.6 installs were reported as Xcode 27.0, including on Intel where Xcode 27 isn't available. That value ends up as built_on.xcode in install receipts and SBOMs.

The full clang build does differ: 2100.0.x for 26.4, 2100.1.x for 26.5 and 26.6 and 2100.3.x for 27. Pass it through and map it to the newest matching Xcode, as the existing table does, so 26.5 and 26.6 both map to 26.6. DevelopmentTools.clang_build_version is unchanged.

I checked the CLT 26.5 and 26.6 packages from Apple's Software Update catalog (both clang-2100.1.1.101) and an installed CLT 27.0 (clang-2100.3.34.2). CLT 26.4.1 is no longer in the catalog, so 2100.0.x relies on Xcode 26.4.1's clang-2100.0.123.102.

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Claude Code (Opus 5.5) drafted the implementation and tests and GPT-6 Astra reviewed them; I reviewed the diff, verified the new tests fail without the change and pass with it, and ran brew lgtm --online plus targeted specs.

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[Copilot]

Copilot review overview

🟢 Approval recommended

No unresolved review issues were identified.

Review effort: Lite
Findings: None

What changed in this PR

Updates Xcode version inference to use full Apple Clang build identifiers, distinguishing Xcode 26.4, 26.6, and 27.

Changes:

• Passes full Clang build information through detection.
• Maps builds to the corresponding Xcode versions.
• Adds coverage for relevant CLT builds.

File	Description
Library/​Homebrew/​test/​os/​mac/​xcode_spec.rb	Tests compiler-build mappings.
Library/​Homebrew/​os/​mac/​xcode.rb	Infers Xcode versions from full Clang builds.

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[MikeMcQuaid]

@p-linnane Thanks! We should probably think about this "Xcode detected from Clang version" logic; maybe we can remove it entirely in favour of just using Xcode and CLT versions now?