This uses the refactoring and support for secondary diagnostic messages
to improve the diagnostic for "invalid argument type." The main
improvement here is that we show where the function being called is
defined, and annotate the span corresponding to the invalid parameter.
## Summary
I noticed that the diagnostic range in specific unpacking assignments is
wrong. For this example
```py
a, b = 1
```
we previously got (see first commit):
```
error: lint:not-iterable
--> /src/mdtest_snippet.py:1:1
|
1 | a, b = 1
| ^^^^ Object of type `Literal[1]` is not iterable
|
```
and with this change, we get:
```
error: lint:not-iterable
--> /src/mdtest_snippet.py:1:8
|
1 | a, b = 1
| ^ Object of type `Literal[1]` is not iterable
|
```
## Test Plan
New snapshot tests.
This causes the diagnostic to highlight the actual unresovable import
instead of the entire `from ... import ...` statement.
While we're here, we expand the test coverage to cover all of the
possible ways that an `import` or a `from ... import` can fail.
Some considerations:
* The first commit in this PR adds a regression test for the current
behavior.
* This creates a new `mdtest/diagnostics` directory. Are folks cool
with this? I guess the idea is to put tests more devoted to diagnostics
than semantics in this directory. (Although I'm guessing there will
be some overlap.)
Fixes#15866
