Generics is not supported with some operators

• Are you reporting a bug, or opening a feature request?
  Bug
• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

Supporting operators with union does not work with generics.

S = TypeVar('S', bound=Union[int, float])
T = TypeVar('T', bound=Union[int, float])

def some_function(a:S, b:T) -> S:
    // do some operation and returns the type based on first parameter.
     c = a + b
    // if there is an addition involved in the computation, then we would get the below error. 
    // some_computed_variable would be finally computed and has the value based on first parameter type.
    return some_computed_variable

• What is the actual behavior/output?
  error: Unsupported operand types for + ("S" and "T")

  We can use alias (Union[int, float] as type) instead of generics (S) as a workaround but it loses generic support.

• What is the behavior/output you expect?
  Should not report error

• What are the versions of mypy and Python you are using?
  Python: 3.6 mypy: 0.650

• What are the mypy flags you are using? (For example --strict-optional)
  --ignore-missing-imports

Start by reproducing the generic operator case from the issue with a current mypy checkout, then review the supporting-operators-with-union change in PR #5545. Trace how bounded type variables are checked for addition and add coverage showing compatible generic operands are accepted without losing the return type based on the first parameter.