[red-knot] Rewrite Type::try_iterate() to improve type inference an…

…d diagnostic messages (#16321)

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_Bad_`__getitem__`_method.snap

@@ -0,0 +1,52 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - Bad `__getitem__` method
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | class Iterable:
+ 4 |     # invalid because it will implicitly be passed an `int`
+ 5 |     # by the interpreter
+ 6 |     def __getitem__(self, key: str) -> int:
+ 7 |         return 42
+ 8 | 
+ 9 | # error: [not-iterable]
+10 | for x in Iterable():
+11 |     reveal_type(x)  # revealed: int
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:10:10
+   |
+ 9 | # error: [not-iterable]
+10 | for x in Iterable():
+   |          ^^^^^^^^^^ Object of type `Iterable` is not iterable because it has no `__iter__` method and its `__getitem__` method has an incorrect signature for the old-style iteration protocol (expected a signature at least as permissive as `def __getitem__(self, key: int): ...`)
+11 |     reveal_type(x)  # revealed: int
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:11:5
+   |
+ 9 | # error: [not-iterable]
+10 | for x in Iterable():
+11 |     reveal_type(x)  # revealed: int
+   |     -------------- info: Revealed type is `int`
+   |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_Invalid_iterable.snap

@@ -0,0 +1,32 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - Invalid iterable
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | nonsense = 123
+2 | for x in nonsense:  # error: [not-iterable]
+3 |     pass
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+ --> /src/mdtest_snippet.py:2:10
+  |
+1 | nonsense = 123
+2 | for x in nonsense:  # error: [not-iterable]
+  |          ^^^^^^^^ Object of type `Literal[123]` is not iterable because it doesn't have an `__iter__` method or a `__getitem__` method
+3 |     pass
+  |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_New_over_old_style_iteration_protocol.snap

@@ -0,0 +1,37 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - New over old style iteration protocol
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | class NotIterable:
+2 |     def __getitem__(self, key: int) -> int:
+3 |         return 42
+4 |     __iter__: None = None
+5 | 
+6 | for x in NotIterable():  # error: [not-iterable]
+7 |     pass
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+ --> /src/mdtest_snippet.py:6:10
+  |
+4 |     __iter__: None = None
+5 |
+6 | for x in NotIterable():  # error: [not-iterable]
+  |          ^^^^^^^^^^^^^ Object of type `NotIterable` is not iterable because its `__iter__` attribute has type `None`, which is not callable
+7 |     pass
+  |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_No_`__iter__`_method_and_`__getitem__`_is_not_callable.snap

@@ -0,0 +1,49 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - No `__iter__` method and `__getitem__` is not callable
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | from typing_extensions import reveal_type
+2 | 
+3 | class Bad:
+4 |     __getitem__: None = None
+5 | 
+6 | # error: [not-iterable]
+7 | for x in Bad():
+8 |     reveal_type(x)  # revealed: Unknown
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+ --> /src/mdtest_snippet.py:7:10
+  |
+6 | # error: [not-iterable]
+7 | for x in Bad():
+  |          ^^^^^ Object of type `Bad` is not iterable because it has no `__iter__` method and its `__getitem__` attribute has type `None`, which is not callable
+8 |     reveal_type(x)  # revealed: Unknown
+  |
+
+```
+
+```
+info: revealed-type
+ --> /src/mdtest_snippet.py:8:5
+  |
+6 | # error: [not-iterable]
+7 | for x in Bad():
+8 |     reveal_type(x)  # revealed: Unknown
+  |     -------------- info: Revealed type is `Unknown`
+  |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_Possibly-not-callable_`__getitem__`_method.snap

@@ -0,0 +1,98 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - Possibly-not-callable `__getitem__` method
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | def _(flag: bool):
+ 4 |     class CustomCallable:
+ 5 |         if flag:
+ 6 |             def __call__(self, *args, **kwargs) -> int:
+ 7 |                 return 42
+ 8 |         else:
+ 9 |             __call__: None = None
+10 | 
+11 |     class Iterable1:
+12 |         __getitem__: CustomCallable = CustomCallable()
+13 | 
+14 |     class Iterable2:
+15 |         if flag:
+16 |             def __getitem__(self, key: int) -> int:
+17 |                 return 42
+18 |         else:
+19 |             __getitem__: None = None
+20 | 
+21 |     # error: [not-iterable]
+22 |     for x in Iterable1():
+23 |         # TODO... `int` might be ideal here?
+24 |         reveal_type(x)  # revealed: int | Unknown
+25 | 
+26 |     # error: [not-iterable]
+27 |     for y in Iterable2():
+28 |         # TODO... `int` might be ideal here?
+29 |         reveal_type(y)  # revealed: int | Unknown
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:22:14
+   |
+21 |     # error: [not-iterable]
+22 |     for x in Iterable1():
+   |              ^^^^^^^^^^^ Object of type `Iterable1` may not be iterable because it has no `__iter__` method and its `__getitem__` attribute (with type `CustomCallable`) may not be callable
+23 |         # TODO... `int` might be ideal here?
+24 |         reveal_type(x)  # revealed: int | Unknown
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:24:9
+   |
+22 |     for x in Iterable1():
+23 |         # TODO... `int` might be ideal here?
+24 |         reveal_type(x)  # revealed: int | Unknown
+   |         -------------- info: Revealed type is `int | Unknown`
+25 |
+26 |     # error: [not-iterable]
+   |
+
+```
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:27:14
+   |
+26 |     # error: [not-iterable]
+27 |     for y in Iterable2():
+   |              ^^^^^^^^^^^ Object of type `Iterable2` may not be iterable because it has no `__iter__` method and its `__getitem__` attribute (with type `Literal[__getitem__] | None`) may not be callable
+28 |         # TODO... `int` might be ideal here?
+29 |         reveal_type(y)  # revealed: int | Unknown
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:29:9
+   |
+27 |     for y in Iterable2():
+28 |         # TODO... `int` might be ideal here?
+29 |         reveal_type(y)  # revealed: int | Unknown
+   |         -------------- info: Revealed type is `int | Unknown`
+   |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_Possibly_invalid_`__getitem__`_methods.snap

@@ -0,0 +1,94 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - Possibly invalid `__getitem__` methods
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | def _(flag: bool):
+ 4 |     class Iterable1:
+ 5 |         if flag:
+ 6 |             def __getitem__(self, item: int) -> str:
+ 7 |                 return "foo"
+ 8 |         else:
+ 9 |             __getitem__: None = None
+10 | 
+11 |     class Iterable2:
+12 |         if flag:
+13 |             def __getitem__(self, item: int) -> str:
+14 |                 return "foo"
+15 |         else:
+16 |             def __getitem__(self, item: str) -> int:
+17 |                 return "foo"
+18 | 
+19 |     # error: [not-iterable]
+20 |     for x in Iterable1():
+21 |         # TODO: `str` might be better
+22 |         reveal_type(x)  # revealed: str | Unknown
+23 | 
+24 |     # error: [not-iterable]
+25 |     for y in Iterable2():
+26 |         reveal_type(y)  # revealed: str | int
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:20:14
+   |
+19 |     # error: [not-iterable]
+20 |     for x in Iterable1():
+   |              ^^^^^^^^^^^ Object of type `Iterable1` may not be iterable because it has no `__iter__` method and its `__getitem__` attribute (with type `Literal[__getitem__] | None`) may not be callable
+21 |         # TODO: `str` might be better
+22 |         reveal_type(x)  # revealed: str | Unknown
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:22:9
+   |
+20 |     for x in Iterable1():
+21 |         # TODO: `str` might be better
+22 |         reveal_type(x)  # revealed: str | Unknown
+   |         -------------- info: Revealed type is `str | Unknown`
+23 |
+24 |     # error: [not-iterable]
+   |
+
+```
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:25:14
+   |
+24 |     # error: [not-iterable]
+25 |     for y in Iterable2():
+   |              ^^^^^^^^^^^ Object of type `Iterable2` may not be iterable because it has no `__iter__` method and its `__getitem__` method (with type `Literal[__getitem__, __getitem__]`) may have an incorrect signature for the old-style iteration protocol (expected a signature at least as permissive as `def __getitem__(self, key: int): ...`)
+26 |         reveal_type(y)  # revealed: str | int
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:26:9
+   |
+24 |     # error: [not-iterable]
+25 |     for y in Iterable2():
+26 |         reveal_type(y)  # revealed: str | int
+   |         -------------- info: Revealed type is `str | int`
+   |
+
+```