Misleading error message when incompatible return type and `iter`

Bug Report

I was adding type hints to an __iter__ method which returns an Iterator from a tuple with different types. MRE is as follows:

def func(cls: type, **options: Any) -> Iterator[Union[type, Dict[str, Any]]]:
    as_tuple = (cls, options)
    return iter(as_tuple)

As you can see Iterator[Union[type, Dict[str, Any]]] is wrong with mypy because of how it handles unions vs joins (Iterator[object] works).

But the problem I had is the error message returned by this snippet:

error: Argument 1 to "iter" has incompatible type "tuple[type, dict[str, Any]]"; expected "SupportsIter[Iterator[type | dict[str, Any]]]"  [arg-type]

While when using:

def func(cls: type, **options: Any) -> Iterator[Union[type, Dict[str, Any]]]:
    as_tuple = (cls, options)
    rv = iter(as_tuple)
    return rv

We get a way better error message:

error: Incompatible return value type (got "Iterator[object]", expected "Iterator[type | dict[str, Any]]")  [return-value]

I don't know if this is specific to this iter use case, but never encountered it before.

• Mypy version used: master
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.11

Start with the two minimal reproductions in the issue and compare diagnostics for direct iter(as_tuple) versus assigning its result before returning. Trace the iter call's type-checking path and add a regression test covering the reported tuple and expected return annotation. Done when the direct form reports the useful incompatible-return-value context shown by the two-step form without regressing existing iter diagnostics.