C++ error with no verbose output

```
python3.7: /root/miniconda3/conda-bld/llvmdev_1531160641630/work/lib/Support/Error.cpp:116: void llvm::report_fatal_error(llvm::Error, bool): Assertion `Err && "report_fatal_error called with success value"' failed.

Process finished with exit code 134 (interrupted by signal 6: SIGABRT)
```
Sadly due to lack of verbosity I have no clue what part of my code causes this.

Start by reproducing the reported Python 3.7 failure and tracing the llvm::report_fatal_error assertion in Error.cpp. Done means identifying the failing code path and determining how to expose a useful diagnostic instead of only the assertion message.

Written by the indexing model from the issue text.