fixes ReraiseDefect after typeless except: + finally: (cpp backend)

[puffball1567]

Bug

A bare except: followed by a finally: block raises a spurious ReraiseDefect: no exception to reraise when compiled with nim cpp:

proc test() =
  try:
    raise newException(CatchableError, "x")
  except:
    discard
  finally:
    echo "finally"

test()
echo "after"

Expected output:

finally
after

Actual output:

finally
fatal.nim(53)            sysFatal
Error: unhandled exception: no exception to reraise [ReraiseDefect]

This reproduces on every memory manager (--mm:arc, --mm:orc, --mm:refc).

Root cause

genTryCpp emits try { ... } catch (Exception* T_) { ... } followed by a finally block that ends with if (T_) std::rethrow_exception(T_);. In the typed except branches the codegen explicitly sets T_ = nullptr; once the exception is handled, so the rethrow check in the finally is a no-op.

The typeless except: branch (the if t[i].len == 1 arm) emitted only popCurrentException() and forgot to clear T_. After the handler body finished, T_ still pointed at the original exception, so the trailing if (T_) std::rethrow_exception(T_); rethrew it. By that point Nim's current-exception stack had already been popped, and the rethrow surfaced as ReraiseDefect.

Fix

Emit T_ = nullptr; at the start of the typeless except: handler body, mirroring what is already done for the typed branches. This is the same one-line treatment that fixed the analogous typed-except case for #5871.

Tests

Adds tests/exception/treraise_typeless_except_finally.nim, exercising the bug pattern on --mm:arc, --mm:orc, and --mm:refc.

Locally:

• tests/exception/ — 43 PASS, 0 FAIL, 3 SKIP
• new test passes on all three memory managers

Backport

Tagged [backport] in the commit message — the same bug exists in version-2-2 and the fix applies cleanly there.

Related

Independent of, but in the same family as, #25775 (also currently open). Both are silent-finally / cpp-backend exception handling fixes; they touch different lines of genTryCpp and don't conflict.