Codegen fails for fn with if when both branches return string of same length

[timmw]

Issue Description

I haven't narrowed down the exact commit but this is a regression that was introduced recently.

The following code fails codegen:

((fn [] (if true "a" "b")))

Full error:

user=> ((fn [] (if true "a" "b")))
In file included from <<< inputs >>>:1:
/tmp/jank-repl-HwoOqH:1:17: error: brackets are not allowed here; to declare an
      array, place the brackets after the name
    1 | const char[2] v5{ };
      |           ~~~   ^
      |                 [2]
/tmp/jank-repl-HwoOqH:1:4: error: cannot assign to variable 'v5' with
      const-qualified type 'const char[2]'
    1 | v5 = std::move(v7);
      | ~~ ^
/tmp/jank-repl-HwoOqH:1:15: note: variable 'v5' declared const here
    1 | const char[2] v5{ };
      | ~~~~~~~~~~~~~~^~~~~
/tmp/jank-repl-HwoOqH:1:4: error: cannot assign to variable 'v5' with
      const-qualified type 'const char[2]'
    1 | v5 = std::move(v10);
      | ~~ ^
/tmp/jank-repl-HwoOqH:1:15: note: variable 'v5' declared const here
    1 | const char[2] v5{ };
      | ~~~~~~~~~~~~~~^~~~~
error: Parsing failed.
─ codegen/internal-failure ─────────────────────────────────────────────────────────────────────────
error: Unable to compile C++ source.

─────┬──────────────────────────────────────────────────────────────────────────────────────────────
     │ /tmp/jank-repl-HwoOqH
─────┼──────────────────────────────────────────────────────────────────────────────────────────────
  1  │ ((fn [] (if true "a" "b")))
     │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^ Found here.
─────┼──────────────────────────────────────────────────────────────────────────────────────────────
  🗎  │ https://book.jank-lang.org/reference/error/codegen/internal-failure.html
─────┴──────────────────────────────────────────────────────────────────────────────────────────────

However these are fine:

(if true "a" "b")

(fn [] (if true "a" "b")) ; fails when called, not when defined

((fn [] (if true "a" "bb"))

((fn [] (cond true "a" :else "b")))

Reproduction Steps

Run the following in repl:

((fn [] (if true "a" "b")))

jank health check

─ system ──────────────────────────────────────────────────────────────────────
─ ✅ operating system: linux
─ ✅ default triple: x86_64-unknown-linux-gnu

─ jank install ────────────────────────────────────────────────────────────────
─ ✅ jank version: jank-0.1-alpha
─ ✅ jank cmake build type: Release
─ ✅ jank resource dir: ../lib/jank/0.1
─ ✅ jank resolved resource dir: /home/tim/.local/bin/../lib/jank/0.1 (found)
─ ✅ jank user cache dir: /home/tim/.cache/jank/x86_64-unknown-linux-gnu-f3d5d4bf0d952cd60c105ecd8c4ecbab1de6a8cb5d2278f3abe20df6353238e4 (found)

─ clang install ───────────────────────────────────────────────────────────────
─ ✅ configured clang path: /nix/store/yjvbvb8rm3pbws3hap8csf62jf9cgvfc-clang-wrapper-23.0.0-unstable-2026-06-21/bin/clang++ (found)
─ ✅ configured clang resource dir: /nix/store/yjvbvb8rm3pbws3hap8csf62jf9cgvfc-clang-wrapper-23.0.0-unstable-2026-06-21/resource-root (found)

─ jank runtime ────────────────────────────────────────────────────────────────
─ ✅ jank runtime initialized
─ ✅ jank pch path: /home/tim/.cache/jank/x86_64-unknown-linux-gnu-f3d5d4bf0d952cd60c105ecd8c4ecbab1de6a8cb5d2278f3abe20df6353238e4 (found)
─ ✅ jank can jit compile c++

Stack trace

No response

OS Version Information

Fedora 43

I acknowledge that:

• I have searched the jank repository (both open and closed Discussions and Issues) and confirm this is not a duplicate of an existing issue or discussion.
• I have checked the "Preview" tab on all text fields to ensure that everything looks right, and have wrapped all configuration and code in code blocks with a group of three backticks (```) on separate lines.

[jeaye]

Great repro! I know what's going on here and I'll get it fixed and tested. Thank you!