An executable and the JIT say what an exception nothing caught threw (Windows and Linux) - #833

An uncaught exception used to end a program without saying what was thrown.

• Windows: the C runtime's terminate() called abort(), which exited silently with 3, or 0xC0000409 with --opt.
• Linux: libstdc++ printed terminate called after throwing an instance of 'char const*' and the program died of SIGABRT. Under the JIT, LLVM's crash handler reported it as a crash of the compiler.

Either way the program's earlier output, still in stdout's buffer, was lost. That is why JSON.stringify({x: 1}) looked like a crash (fixed in #832). The Windows JIT already reported error: uncaught exception: <what>.

function main() { print("before"); throw new TypeError("boom"); }

Now, as an executable or under the JIT, on Windows and Linux:

• before on stdout
• error: uncaught exception: TypeError: boom on stderr (the JIT adds its tslang: prefix)
• exit code 1

Change

• TypeScriptAsyncRuntime/UncaughtException.cpp (new): its initializer sets the handler.

  • Windows: an unhandled-exception filter in front of the C runtime's, which acts on C++ throws only.
  • Linux: a std::terminate handler. terminate() called with no exception (e.g. a joinable std::thread destroyed) goes to the previous handler, as before.
  • On an uncaught throw, the handler flushes stdout, describes the value on stderr and exits with 1 (TerminateProcess / _exit), as the JIT does.

• tslang/exe.cpp: every executable is linked with -include:__tslang_report_uncaught (Windows) or -Wl,-u,__tslang_report_uncaught (Linux), placed before the runtime archive. The archive is checked for the symbol first, so a runtime older than this links as before. Android is unchanged.

• tslang/jit.cpp: on Linux the JIT sets the same terminate handler around the entry call.

• TypeScript/UncaughtException.h (new): the code that describes the value, which both use. It describes:

  • a string
  • a number, as the program prints it
  • a bool, an int or a bigint
  • a class, by its name
  • an Error, with its message

  On Windows it reads the exception record, as jit.cpp did; that code is moved here unchanged. On Linux it reads the current exception through std::current_exception() and its type_info name (PKc, d, P7Failure, …). An Error is found by walking the class type_info's base chain. tslang is built without RTTI, so the Itanium layouts are read directly.

Tests

• uncaught-exe.cmake builds the four uncaught/*.ts programs with --emit=exe, under gc, rc, none and own, with and without --opt: 32 tests. Each checks exit code 1, before on stdout and the description on stderr.
• These, and the existing JIT uncaught tests, now also run on Linux.

Verified locally

• Windows x64:
  • ctest: 7024/7024 passed.
  • DefaultLib tests.ps1 passes for all four models, release and debug, compiled and JIT. This run predates the Linux commit, which leaves the Windows code as it was.
• Linux (WSL, Ubuntu 26.04):
  • The 44 uncaught tests pass.
  • With the DefaultLib, throw new TypeError("boom") prints error: uncaught exception: TypeError: boom.
  • Full ctest: 7004/7005. The one failure is test-compile-shared-import-chain-rtti-skipped, a type-checking error ("Class member 'reports' can't be found") in a test unrelated to this change. It passed in this PR's earlier Linux CI run, which had the same compiler front end, and looks specific to the reused local WSL build tree. Linux CI will confirm.

Not covered

• Android keeps libc++'s default handler.
• Programs that test-runner links itself. test-runner links its own object files, so they do not take the runtime file. Only tslang --emit=exe does.

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