Conditionals inside Group { … } crash the Swift type checker

[leogdion]

Repro

Minimal:

import SyntaxKit

let _ = Group {
    if true {
        Struct("A") { Variable(.let, name: "x", type: "Int") }
    }
}

Compile (or interpret via swift Repro.swift) against SyntaxKit:

error: failed to produce diagnostic for expression; please submit a bug report (https://swift.org/contributing/#reporting-bugs)
let _ = Group {
        ^

Expected

The CodeBlockBuilderResult declares both forks of buildEither and buildOptional (Sources/SyntaxKit/CodeBlocks/CodeBlockBuilderResult.swift:46-58), so the API surface advertises support for if/else inside Group { … }. The expectation is that the conditional contributes (or not) a Struct to the group like any other CodeBlock.

Actual

The Swift compiler bails on type inference with error: failed to produce diagnostic for expression — a type-checker timeout / failed solver, not a useful diagnostic. The body compiles fine when the if is removed.

Notes

• Reproduces on Swift 6.x / macOS, both in swift script mode and swiftc compile mode.
• The variadic buildBlock(_ components: any CodeBlock...) overload combined with buildEither overload resolution is the most likely culprit — both EmptyCodeBlock-typed branches and concrete CodeBlock branches need to unify through the existential any CodeBlock.
• No test under Tests/SyntaxKitTests/Unit/ exercises an if inside Group { … }, which is why this hasn't been caught.

Surfaced from

Issue #154 — POC step 1 for a SyntaxKit-driven codegen CLI. See Docs/research/poc-step1-results.md §4. Not blocking that work, but blocks users who want to write conditional codegen.