[ty] No union with Unknown for module-global symbols (#20664)

## Summary

Quoting from the newly added comment:

Module-level globals can be mutated externally. A `MY_CONSTANT = 1`
global might be changed to `"some string"` from code outside of the
module that we're looking at, and so from a gradual-guarantee
perspective, it makes sense to infer a type of `Literal[1] | Unknown`
for global symbols. This allows the code that does the mutation to type
check correctly, and for code that uses the global, it accurately
reflects the lack of knowledge about the type.

External modifications (or modifications through `global` statements)
that would require a wider type are relatively rare. From a practical
perspective, we can therefore achieve a better user experience by
trusting the inferred type. Users who need the external mutation to work
can always annotate the global with the wider type. And everyone else
benefits from more precise type inference.

I initially implemented this by applying literal promotion to the type
of the unannotated module globals (as suggested in
astral-sh/ty#1069), but the ecosystem impact
showed a lot of problems (#20643).
I fixed/patched some of these problems, but this PR seems like a good
first step, and it seems sensible to apply the literal promotion change
in a second step that can be evaluated separately.

closes astral-sh/ty#1069

## Ecosystem impact

This seems like an (unexpectedly large) net positive with 650 fewer
diagnostics overall.. even though this change will certainly catch more
true positives.

* There are 666 removed `type-assertion-failure` diagnostics, where we
were previously used the correct type already, but removing the
`Unknown` now leads to an "exact" match.
* 1464 of the 1805 total new diagnostics are `unresolved-attribute`
errors, most (1365) of which were previously
`possibly-missing-attribute` errors. So they could also be counted as
"changed" diagnostics.
* For code that uses constants like
  ```py
  IS_PYTHON_AT_LEAST_3_10 = sys.version_info >= (3, 10)
  ```
where we would have previously inferred a type of `Literal[True/False] |
Unknown`, removing the `Unknown` now allows us to do reachability
analysis on branches that use these constants, and so we get a lot of
favorable ecosystem changes because of that.
* There is code like the following, where we previously emitted
`conflicting-argument-forms` diagnostics on calls to the aliased
`assert_type`, because its type was `Unknown | def …` (and the call to
`Unknown` "used" the type form argument in a non type-form way):
  ```py
  if sys.version_info >= (3, 11):
      import typing
  
      assert_type = typing.assert_type
  else:
      import typing_extensions
  
      assert_type = typing_extensions.assert_type
  ```
* ~100 new `invalid-argument-type` false positives, due to missing
`**kwargs` support (astral-sh/ty#247)

## Typing conformance

```diff
+protocols_modules.py:25:1: error[invalid-assignment] Object of type `<module '_protocols_modules1'>` is not assignable to `Options1`
```

This diagnostic should apparently not be there, but it looks like we
also fail other tests in that file, so it seems to be a limitation that
was previously hidden by `Unknown` somehow.

## Test Plan

Updated tests and relatively thorough ecosystem analysis.

Author: sharkdp

crates/ty_python_semantic/resources/mdtest/del.md

@@ -108,7 +108,7 @@ def foo():
     global x
     def bar():
         # allowed, refers to `x` in the global scope
-        reveal_type(x)  # revealed: Unknown | Literal[1]
+        reveal_type(x)  # revealed: Literal[1]
     bar()
     del x  # allowed, deletes `x` in the global scope (though we don't track that)
 ```

crates/ty_python_semantic/resources/mdtest/import/conditional.md

@@ -25,8 +25,8 @@ reveal_type(y)
 # error: [possibly-missing-import] "Member `y` of module `maybe_unbound` may be missing"
 from maybe_unbound import x, y
 
-reveal_type(x)  # revealed: Unknown | Literal[3]
-reveal_type(y)  # revealed: Unknown | Literal[3]
+reveal_type(x)  # revealed: Literal[3]
+reveal_type(y)  # revealed: Literal[3]
 ```
 
 ## Maybe unbound annotated
@@ -56,7 +56,7 @@ Importing an annotated name prefers the declared type over the inferred type:
 # error: [possibly-missing-import] "Member `y` of module `maybe_unbound_annotated` may be missing"
 from maybe_unbound_annotated import x, y
 
-reveal_type(x)  # revealed: Unknown | Literal[3]
+reveal_type(x)  # revealed: Literal[3]
 reveal_type(y)  # revealed: int
 ```
 

crates/ty_python_semantic/resources/mdtest/import/dunder_all.md

@@ -783,8 +783,8 @@ class A: ...
 ```py
 from subexporter import *
 
-# TODO: Should we avoid including `Unknown` for this case?
-reveal_type(__all__)  # revealed: Unknown | list[Unknown | str]
+# TODO: we could potentially infer `list[str] | tuple[str, ...]` here
+reveal_type(__all__)  # revealed: list[Unknown | str]
 
 __all__.append("B")
 

crates/ty_python_semantic/resources/mdtest/import/module_getattr.md

@@ -40,7 +40,7 @@ def __getattr__(name: str) -> int:
 import mixed_module
 
 # Explicit attribute should take precedence
-reveal_type(mixed_module.explicit_attr)  # revealed: Unknown | Literal["explicit"]
+reveal_type(mixed_module.explicit_attr)  # revealed: Literal["explicit"]
 
 # `__getattr__` should handle unknown attributes
 reveal_type(mixed_module.dynamic_attr)  # revealed: str

crates/ty_python_semantic/resources/mdtest/import/namespace.md

@@ -103,7 +103,7 @@ x = "namespace"
 ```py
 from foo import x
 
-reveal_type(x)  # revealed: Unknown | Literal["module"]
+reveal_type(x)  # revealed: Literal["module"]
 
 import foo.bar  # error: [unresolved-import]
 ```

crates/ty_python_semantic/resources/mdtest/import/star.md

@@ -144,8 +144,8 @@ X = (Y := 3) + 4
 ```py
 from exporter import *
 
-reveal_type(X)  # revealed: Unknown | Literal[7]
-reveal_type(Y)  # revealed: Unknown | Literal[3]
+reveal_type(X)  # revealed: Literal[7]
+reveal_type(Y)  # revealed: Literal[3]
 ```
 
 ### Global-scope symbols defined in many other ways
@@ -781,9 +781,9 @@ else:
 from exporter import *
 
 # error: [possibly-unresolved-reference]
-reveal_type(A)  # revealed: Unknown | Literal[1]
+reveal_type(A)  # revealed: Literal[1]
 
-reveal_type(B)  # revealed: Unknown | Literal[2, 3]
+reveal_type(B)  # revealed: Literal[2, 3]
 ```
 
 ### Reachability constraints in the importing module
@@ -804,7 +804,7 @@ if coinflip():
     from exporter import *
 
 # error: [possibly-unresolved-reference]
-reveal_type(A)  # revealed: Unknown | Literal[1]
+reveal_type(A)  # revealed: Literal[1]
 ```
 
 ### Reachability constraints in the exporting module *and* the importing module

crates/ty_python_semantic/resources/mdtest/narrow/assignment.md

@@ -34,7 +34,7 @@ class _:
 [reveal_type(a.z) for _ in range(1)]  # revealed: Literal[0]
 
 def _():
-    reveal_type(a.x)  # revealed: Unknown | int | None
+    reveal_type(a.x)  # revealed: int | None
     reveal_type(a.y)  # revealed: Unknown | None
     reveal_type(a.z)  # revealed: Unknown | None
 
@@ -75,7 +75,7 @@ class _:
 
     if cond():
         a = A()
-    reveal_type(a.x)  # revealed: int | None | Unknown
+    reveal_type(a.x)  # revealed: int | None
     reveal_type(a.y)  # revealed: Unknown | None
     reveal_type(a.z)  # revealed: Unknown | None
 
@@ -295,10 +295,10 @@ class C:
 
 def _():
     # error: [possibly-missing-attribute]
-    reveal_type(b.a.x[0])  # revealed: Unknown | int | None
+    reveal_type(b.a.x[0])  # revealed: int | None
     # error: [possibly-missing-attribute]
-    reveal_type(b.a.x)  # revealed: Unknown | list[int | None]
-    reveal_type(b.a)  # revealed: Unknown | A | None
+    reveal_type(b.a.x)  # revealed: list[int | None]
+    reveal_type(b.a)  # revealed: A | None
 ```
 
 ## Invalid assignments are not used for narrowing

crates/ty_python_semantic/resources/mdtest/narrow/complex_target.md

@@ -167,11 +167,11 @@ if c.x is not None:
 
 if c.x is not None:
     def _():
-        reveal_type(c.x)  # revealed: Unknown | int | None
+        reveal_type(c.x)  # revealed: int | None
 
 def _():
     if c.x is not None:
-        reveal_type(c.x)  # revealed: (Unknown & ~None) | int
+        reveal_type(c.x)  # revealed: int
 ```
 
 ## Subscript narrowing

crates/ty_python_semantic/resources/mdtest/narrow/conditionals/nested.md

@@ -86,7 +86,7 @@ class B:
     reveal_type(a.x)  # revealed: Literal["a"]
 
 def f():
-    reveal_type(a.x)  # revealed: Unknown | str | None
+    reveal_type(a.x)  # revealed: str | None
 
 [reveal_type(a.x) for _ in range(1)]  # revealed: Literal["a"]
 
@@ -96,7 +96,7 @@ class C:
     reveal_type(a.x)  # revealed: str | None
 
 def g():
-    reveal_type(a.x)  # revealed: Unknown | str | None
+    reveal_type(a.x)  # revealed: str | None
 
 [reveal_type(a.x) for _ in range(1)]  # revealed: str | None
 
@@ -109,7 +109,7 @@ class D:
     reveal_type(a.x)  # revealed: Literal["a"]
 
 def h():
-    reveal_type(a.x)  # revealed: Unknown | str | None
+    reveal_type(a.x)  # revealed: str | None
 
 # TODO: should be `str | None`
 [reveal_type(a.x) for _ in range(1)]  # revealed: Literal["a"]
@@ -190,7 +190,7 @@ def f(x: str | None):
             reveal_type(g)  # revealed: str
 
         if a.x is not None:
-            reveal_type(a.x)  # revealed: (Unknown & ~None) | str
+            reveal_type(a.x)  # revealed: str
 
         if l[0] is not None:
             reveal_type(l[0])  # revealed: str
@@ -206,7 +206,7 @@ def f(x: str | None):
             reveal_type(g)  # revealed: str
 
         if a.x is not None:
-            reveal_type(a.x)  # revealed: (Unknown & ~None) | str
+            reveal_type(a.x)  # revealed: str
 
         if l[0] is not None:
             reveal_type(l[0])  # revealed: str
@@ -382,12 +382,12 @@ def f():
     if a.x is not None:
         def _():
             # Lazy nested scope narrowing is not performed on attributes/subscripts because it's difficult to track their changes.
-            reveal_type(a.x)  # revealed: Unknown | str | None
+            reveal_type(a.x)  # revealed: str | None
 
         class D:
-            reveal_type(a.x)  # revealed: (Unknown & ~None) | str
+            reveal_type(a.x)  # revealed: str
 
-        [reveal_type(a.x) for _ in range(1)]  # revealed: (Unknown & ~None) | str
+        [reveal_type(a.x) for _ in range(1)]  # revealed: str
 
     if l[0] is not None:
         def _():
@@ -473,11 +473,11 @@ def f():
         if a.x is not None:
             def _():
                 if a.x != 1:
-                    reveal_type(a.x)  # revealed: (Unknown & ~Literal[1]) | str | None
+                    reveal_type(a.x)  # revealed: str | None
 
             class D:
                 if a.x != 1:
-                    reveal_type(a.x)  # revealed: (Unknown & ~Literal[1] & ~None) | str
+                    reveal_type(a.x)  # revealed: str
 
         if l[0] is not None:
             def _():

crates/ty_python_semantic/resources/mdtest/public_types.md

@@ -263,7 +263,7 @@ if flag():
     x = 1
 
     def f() -> None:
-        reveal_type(x)  # revealed: Unknown | Literal[1, 2]
+        reveal_type(x)  # revealed: Literal[1, 2]
     # Function only used inside this branch
     f()