Exceptions not cleared if except control flow is interrupted

[yglukhov]

Nim Version

Nim Compiler Version 2.3.1 [Linux: amd64]
Compiled at 2025-07-07
Copyright (c) 2006-2025 by Andreas Rumpf

git hash: fbdc9a4c19aafc25937aaa51f5c1f01084094688
active boot switches: -d:release

Description

proc foo() =
  try:
    try:
      raise newException(Exception, "1")
    except:
      echo "EXCEPT1: ", getCurrentExceptionMsg()
      return # Prevents exception clearing
      # raise newException(Exception, "2") # also prevents clearing
    finally:
      echo "FINALLY: ", getCurrentExceptionMsg()
  except:
    echo "EXCEPT2:", getCurrentExceptionMsg()
  finally:
    echo "FINALLY2: ", getCurrentExceptionMsg()

  echo "LAST: ", getCurrentExceptionMsg()

foo()

Current Output

EXCEPT1: 1
FINALLY1: 1
FINALLY2: 1

Expected Output

EXCEPT1: 1
FINALLY1:
FINALLY2:

Known Workarounds

No response

Additional Information

While playing with closureiter transformation I realized I'm not accounting for this case, so I went to see how it's handled in regular code, and it turned out it isn't.

[yglukhov]

For the reference, here's an equivalent c++ program that shows consistent exception stack behavior. Interestingly enough, in c++ there's no current exception in finallies.

#include <iostream>
#include <exception>
#include <string>

int getCurrentException() {
  std::exception_ptr ex = std::current_exception();
  int result = 0;
  if (ex) {
    try {
        std::rethrow_exception(ex);
    }
    catch (int e) {
        result = e;
    }
  }
  return result;
}

class Finally {
  public:
    Finally(std::string name) : mName(name) {}
    ~Finally() {
      std::cout << mName << ": " << getCurrentException() << std::endl;
    }
  private:
    std::string mName;
};

void foo() {
  Finally f1("FINALLY2");
  try {
    Finally f1("FINALLY1");
    try {
      throw 1;
    }
    catch (...) {
      std::cout << "EXCEPT1: " << getCurrentException() << std::endl;
      // return;
      throw 2;
    }
  }
  catch (...) {
    std::cout << "EXCEPT2: " << getCurrentException() << std::endl;
  }
  std::cout << "LAST: " << getCurrentException() << std::endl;
}

int main() {
  foo();
  return 0;
}

outputs:

EXCEPT1: 1
FINALLY1: 0
EXCEPT2: 2
LAST: 0
FINALLY2: 0

[yglukhov]

#25037 same cause seemingly