v2026.3.31 — Law 10, Match Overhaul, Dependency Fixes

v2026.3.31 — Law 10: Struct Initialization Completeness

Released: March 31, 2026

A session that started with a confused AI agent banging its head on match syntax ended with a new language axiom. Three features landed, each born from a real bug.

What Changed

Law 10: ..defaults Struct Initialization Completeness

Zig silently accepts missing fields when defaults exist. Rust forbids defaults entirely. Janus splits the difference: defaults exist, but omission requires explicit opt-in.

struct Config {
    host: str = "localhost",
    port: u16 = 8080,
    tls: bool = false,
}

// All fields listed — works
let c1 = Config { host: "prod", port: 443, tls: true }

// Explicit leniency — accept defaults for unlisted fields
let c2 = Config { host: "prod", ..defaults }

// COMPILE ERROR — missing 'tls', no ..defaults
let c3 = Config { host: "prod", port: 443 }

When a field is added to a struct:

• Callsites with ..defaults compile fine (they opted in)
• Callsites listing all fields get a compile error (must add the new field)
• The right split emerges automatically

Grammar: ..defaults uses the existing .. range token followed by the defaults identifier. Parser, lowering, and LLVM emission all updated.

else as Match Fallback

_ in match is dead. else is the only wildcard. One symbol, one meaning:

Symbol	Purpose	Context
else	Match fallback	match x { else => ... }
_	Discard binding	let _ = expr, catch |_|, tuple destructuring

match status {
    .Ok => handle_success(),
    .Retry => handle_retry(),
    else => handle_failure(),
}

.Variant Patterns in Match

Type-inferred variant patterns are now working. The compiler knows the enum type from the match subject — repeating it on every arm is noise:

enum Color { Red, Green, Blue }

func describe(c: Color) do
    match c {
        .Red => println("warm"),
        .Green => println("natural"),
        .Blue => println("cool"),
    }
end

Dependency Cache Fixes

• CachedImport.exported_error_variants was missing from initialization in 2 sites — silently relying on struct defaults (ironic, given Law 10)
• Diamond import replay debug output now includes error variant counts
• Removed unsafe struct default on CachedImport field — forces explicit initialization

LLVM Codegen Fixes

• emitUnionTagCheck: Fixed i1-to-i64 zero-extension for tag comparison results
• emitBinaryOp: Added type coercion when operand widths differ

Specs Updated

• SPEC-017 (Grammar): Law 10 added, wildcard_pattern now 'else' only, variant patterns documented
• SPEC-018 (:core Profile): Match examples use => (not ->), else (not _), .Variant patterns added
• LANGUAGE-REFERENCE: All match and struct init sections updated

Files Changed

Area	Files	Lines
Parser	janus_parser.zig	+32 (..defaults + else wildcard)
AST	core.zig	+3 (default_spread node kind)
Lowering	lower.zig	+195 (Law 10 enforcement, variant patterns, dep cache)
LLVM	llvm_emitter.zig	+20 (tag check zext, binop coercion)
Tests	Various	All match/struct tests pass

Migration

• Replace _ => with else => in all match expressions
• Struct initializers that omit fields now require ..defaults
• Enum match patterns can use .Variant instead of EnumType.Variant