INTERNAL ERROR on: errors.py, line 268, in _add_error_info, assert file not in self.flushed_files

[koterpillar]

If MYPYPATH is set to the directory I'm checking, once mypy finds an error it crashes with INTERNAL ERROR.

Here are the files to reproduce:

scripts/__init__.py: empty file
scripts/a.py:

"asdf" + 10

scripts/b.py:

import a

Command line:

MYPYPATH=scripts mypy scripts

• What is the actual behavior/output?

  scripts/a.py:1: error: Unsupported operand types for + ("str" and "int")
  scripts/a.py:1: error: INTERNAL ERROR -- please report a bug at https://github.com/python/mypy/issues version: 0.670
  scripts/a.py:1: : note: please use --show-traceback to print a traceback when reporting a bug
  

• What is the behavior/output you expect?

  The error (unsupported operand types) should be reported, but mypy shouldn't crash afterwards.

• What are the versions of mypy and Python you are using?

  Python 3.7.2
  mypy 0.670

• Do you see the same issue after installing mypy from Git master?

  Yes (version 0.680+dev.ce78d65d68cdd6899e7bc5c25e9f583d76520c50)

• If mypy crashed with a traceback, please paste
  the full traceback below.

Traceback (most recent call last):
  File "/xxxx/.virtualenvs/xxxx/bin/mypy", line 10, in <module>
    sys.exit(console_entry())
  File "/xxxx/.virtualenvs/xxxx/lib/python3.7/site-packages/mypy/__main__.py", line 7, in console_entry
    main(None)
  File "/xxxx/.virtualenvs/xxxx/lib/python3.7/site-packages/mypy/main.py", line 91, in main
    res = build.build(sources, options, None, flush_errors, fscache)
  File "/xxxx/.virtualenvs/xxxx/lib/python3.7/site-packages/mypy/build.py", line 162, in build
    result = _build(sources, options, alt_lib_path, flush_errors, fscache)
  File "/xxxx/.virtualenvs/xxxx/lib/python3.7/site-packages/mypy/build.py", line 217, in _build
    graph = dispatch(sources, manager)
  File "/xxxx/.virtualenvs/xxxx/lib/python3.7/site-packages/mypy/build.py", line 2360, in dispatch
    process_graph(graph, manager)
  File "/xxxx/.virtualenvs/xxxx/lib/python3.7/site-packages/mypy/build.py", line 2660, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/xxxx/.virtualenvs/xxxx/lib/python3.7/site-packages/mypy/build.py", line 2767, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/xxxx/.virtualenvs/xxxx/lib/python3.7/site-packages/mypy/build.py", line 1919, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "/xxxx/.virtualenvs/xxxx/lib/python3.7/site-packages/mypy/checker.py", line 282, in check_first_pass
    self.accept(d)
  File "/xxxx/.virtualenvs/xxxx/lib/python3.7/site-packages/mypy/checker.py", line 393, in accept
    stmt.accept(self)
  File "/xxxx/.virtualenvs/xxxx/lib/python3.7/site-packages/mypy/nodes.py", line 926, in accept
    return visitor.visit_expression_stmt(self)
  File "/xxxx/.virtualenvs/xxxx/lib/python3.7/site-packages/mypy/checker.py", line 2635, in visit_expression_stmt
    self.expr_checker.accept(s.expr, allow_none_return=True, always_allow_any=True)
  File "/xxxx/.virtualenvs/xxxx/lib/python3.7/site-packages/mypy/checkexpr.py", line 3188, in accept
    typ = node.accept(self)
  File "/xxxx/.virtualenvs/xxxx/lib/python3.7/site-packages/mypy/nodes.py", line 1627, in accept
    return visitor.visit_op_expr(self)
  File "/xxxx/.virtualenvs/xxxx/lib/python3.7/site-packages/mypy/checkexpr.py", line 1884, in visit_op_expr
    allow_reverse=True)
  File "/xxxx/.virtualenvs/xxxx/lib/python3.7/site-packages/mypy/checkexpr.py", line 2270, in check_op
    self.msg.add_errors(msg)
  File "/xxxx/.virtualenvs/xxxx/lib/python3.7/site-packages/mypy/messages.py", line 99, in add_errors
    self.errors.add_error_info(info)
  File "/xxxx/.virtualenvs/xxxx/lib/python3.7/site-packages/mypy/errors.py", line 286, in add_error_info
    self._add_error_info(file, info)
  File "/xxxx/.virtualenvs/xxxx/lib/python3.7/site-packages/mypy/errors.py", line 268, in _add_error_info
    assert file not in self.flushed_files
AssertionError:

[msullivan]

The fact that we crash instead of producing an error message is a bug here, but the issue is that the file a.py is being picked twice with different module names, since it appears both on the command line and in mypy_path.

(Dupe of #4881)