fix(cplusplus): forward-declare recursive unions instead of self-referential aliases

[schani]

The bug

Issue #273 tracks "Detect and properly handle recursive types" across
quicktype's backends. The concrete, still-reproducing report (2021 comment,
confirmed on current master) is in the C++ backend: a recursive union that
refers back to itself through a container (e.g. std::vector) was emitted
as a plain type alias:

using RecursiveThing = std::variant<std::vector<RecursiveThing>, std::string>;

This does not compile with g++ -std=c++17:

error: 'RecursiveThing' was not declared in this scope

because a using alias's name is not in scope on its own right-hand side —
unlike a struct/class, whose name is visible immediately after its
opening brace.

Root cause

CPlusPlusRenderer.isImplicitCycleBreaker treated array/map container
types as unconditional cycle breakers, so a union reachable only through a
container was never flagged as needing forward declaration/heap indirection
the way recursive classes already are. emitUnionTypedefs then always
emitted named unions as a bare using alias, which is only valid when the
union's own name doesn't appear inside its own definition.

The fix

• A container (array/map) is now only treated as an implicit cycle
  breaker when none of its children are (or contain) a union — since
  breaking the cycle for a union alias doesn't actually work.
• Added isRecursiveUnion to detect a named union that reaches itself
  through its members.
• emitUnionTypedefs now emits such recursive unions as a forward-declared
  struct X : std::variant<...> wrapper (with inherited constructors/
  assignment) instead of an invalid self-referential using alias, mirroring
  how recursive classes are already handled.
• Added from_json/to_json free-function forwarding for the wrapper struct
  so (de)serialization still goes through the underlying variant's
  adl_serializer.
• canBreakCycles now also recognizes UnionType (previously only
  ClassType) as able to break declaration cycles.
• Overrode CPlusPlusRenderer.emitIncludes's <optional> inclusion to also
  cover cases (haveNamedUnions && !boost) that only arise via wrapper
  structs.

Test coverage

test/inputs/schema/recursive-union-flattening.schema already exercised
this exact bug shape and was listed in CPlusPlusLanguage.skipSchema in
test/languages.ts with the comment "Recursive top-level unions produce
aliases that can refer to later aliases." That schema is now un-skipped and
serves as the regression test — it was verified to fail (bad/non-compiling
output) before the fix and to pass after.

Verification performed locally

• npm run build passes.
• Manually reproduced the original bug report's repro command
  (--lang c++ --code-format with-struct --namespace schema --src-lang schema --no-boost) against a minimal recursive schema; confirmed the invalid
  using alias before the fix, and a compiling forward-declared struct
  wrapper after. Compiled the generated header with g++ -std=c++17 and
  additionally wrote a small round-trip driver confirming
  parse→serialize produces the original JSON.
• QUICKTEST=true FIXTURE=schema-cplusplus script/test: all 70/70 samples
  pass, including the newly un-skipped recursive-union-flattening.schema.
• QUICKTEST=true FIXTURE=cplusplus script/test: all samples pass except
  two pre-existing, unrelated failures caused by Boost headers not being
  installed in this sandbox (boost: true renderer-option cases), which are
  not touched by this change.
• npm run test:unit: 170/170 tests pass.
• CI will additionally validate this across the full (non-QUICKTEST) fixture
  matrix.

Fixes #273

🤖 Generated with Claude Code

[github-actions]

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[schani]

This is odd. It includes <optional> in many cases where it's not needed. And then in some cases where it changes something else it doesn't include <optional>.

[github-actions]

Generated-output differences

5 files differ — 4 modified, 1 new, 0 deleted
495 changed lines — +476 / −19

Open the generated-output report →