`if`/`else` inside `Group { … }` builder triggers Swift type-checker crash

[leogdion]

Repro

import SyntaxKit

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

Fails to compile with:

error: failed to produce diagnostic for expression; please submit a bug report
let _ = Group {
        ^

Confirmed against Apple Swift version 6.3.2 (swiftlang-6.3.2.1.108) on arm64-apple-macosx26.0 against SyntaxKit at e5b1b73 (research/swift-manifest-codegen).

What should happen

CodeBlockBuilderResult declares buildEither(first:), buildEither(second:), buildOptional, and buildArray (Sources/SyntaxKit/CodeBlocks/CodeBlockBuilderResult.swift:46-71), so the API surface says if/else inside the builder is supported. The Swift type-checker should either accept the expression or emit a real diagnostic.

Probable cause

The type-checker is bailing out on overload resolution. CodeBlockBuilderResult has two buildBlock variants — (any CodeBlock...) -> [any CodeBlock] and ([any CodeBlock]...) -> [any CodeBlock] — combined with buildEither / buildOptional / buildArray over any CodeBlock existentials. The combination is likely exhausting the type-checker's effort budget and triggering the "failed to produce diagnostic" fallback rather than a real error.

Impact

Users can't write conditional codegen at the top level of a builder closure, even though the API documents it as supported. Affects anyone trying to emit conditional structures (platform gates, debug-only fields, optional members from runtime config, etc.). No unit tests under Tests/SyntaxKitTests/Unit/ exercise an if inside Group { … }, which is why this hasn't been caught.

Workaround

Hoist the conditional into a normal function that returns any CodeBlock, then call the function from inside the builder — the type-checker never sees the if at the builder level.

func extraField(_ include: Bool) -> any CodeBlock {
    if include { Variable(.let, name: "debug", type: "Bool") }
    else { EmptyCodeBlock() }
}

Group {
    Variable(.let, name: "name", type: "String")
    extraField(true)
}

References

• `Docs/research/poc-step1-results.md` §4 — original discovery during the skitrun POC.
• `Docs/research/codegen-cli-design.md` §7 — tracked as an open SyntaxKit bug, independent of the skitrun CLI design.
• PR skit: SyntaxKit codegen CLI for v0.0.5 #156 (skitrun POC) — surfaces this for users writing input files to the codegen CLI.

[leogdion]

Workaround caveat (from v0.0.5 probes)

The workaround documented in the issue body ("hoist the conditional into a helper function") only works if the helper uses plain Swift if/else to return one of two CodeBlocks. Wrapping the helper's body in Group { if … } still trips the type-checker crash inside the helper:

// ❌ Still crashes — the inner Group hits the same overload-resolution issue.
public func optionalDebugField(_ include: Bool) -> any CodeBlock {
    Group {
        if include {
            Variable(.let, name: "debug", type: "Bool")
        }
    }
}

// ✅ Works — plain Swift control flow, no result builder.
public func optionalDebugField(_ include: Bool) -> any CodeBlock {
    if include {
        return Variable(.let, name: "debug", type: "Bool")
    } else {
        return Group {}  // empty Group as the "no-op" CodeBlock; there's no
                          // public EmptyCodeBlock type to return otherwise.
    }
}

The second form renders cleanly through skit run. Also worth noting: there's no public empty-CodeBlock type, so the false branch has to return an empty Group { }. A public EmptyCodeBlock (or making CodeBlock? valid in builders) would be a small ergonomic win.