Type inference of local variable assigned in different branches uses type of assignment in first branch

[syastrov]

• Are you reporting a bug, or opening a feature request?
  It's a matter of opinion.

• Please insert below the code you are checking with mypy.

def foo(a: bool) -> None:
  if a:
    b = "blah"
  else:
    b = None
  return None

• What is the actual behavior/output?

$ mypy a.py
a.py:5: error: Incompatible types in assignment (expression has type "None", variable has type "str")

• What is the behavior/output you expect?

Since there is no annotation on b, mypy should infer that the variable b has the broadest type that matches the possible assignments: Union[str, None] (aka Optional[str]).
I didn't expect that mypy would just infer the type of the variable based on the first assignment.
FWIW: PyCharm's type inference infers the type Optional[str]

If this is not possible, then perhaps mypy could give a more meaningful error and/or hint (e.g. "error: variable b is assigned a different type based on different branches" + "hint: provide a type annotation for b: Optional[str]").

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

Python 3.7.4 / mypy 0.750+dev.3fc382321e4531450e91c55fddb6d4ed54611d5e
(git master)

• What are the mypy flags you are using? (For example --strict-optional)
  No flags specified, but it also occurs with --strict-optional

Related to #4061

[ilevkivskyi]

Duplicate of #2611