Prioritize attribute in from/import statement (#15041)

This tweaks the new semantics from #15026 a bit when a symbol could be
interpreted both as an attribute and a submodule of a package. For
`from...import`, we should actually prioritize the attribute, because of
how the statement itself is implemented [1].

> 1. check if the imported module has an attribute by that name
> 2. if not, attempt to import a submodule with that name and then check
the imported module again for that attribute

[1] https://docs.python.org/3/reference/simple_stmts.html#the-import-statement

Author: dcreager

crates/red_knot_python_semantic/resources/mdtest/import/conflicts.md

@@ -0,0 +1,75 @@
+# Conflicting attributes and submodules
+
+## Via import
+
+```py
+import a.b
+
+reveal_type(a.b)  # revealed: <module 'a.b'>
+```
+
+```py path=a/__init__.py
+b = 42
+```
+
+```py path=a/b.py
+```
+
+## Via from/import
+
+```py
+from a import b
+
+reveal_type(b)  # revealed: Literal[42]
+```
+
+```py path=a/__init__.py
+b = 42
+```
+
+```py path=a/b.py
+```
+
+## Via both
+
+```py
+import a.b
+from a import b
+
+reveal_type(b)  # revealed: <module 'a.b'>
+reveal_type(a.b)  # revealed: <module 'a.b'>
+```
+
+```py path=a/__init__.py
+b = 42
+```
+
+```py path=a/b.py
+```
+
+## Via both (backwards)
+
+In this test, we infer a different type for `b` than the runtime behavior of the Python interpreter.
+The interpreter will not load the submodule `a.b` during the `from a import b` statement, since `a`
+contains a non-module attribute named `b`. (See the [definition][from-import] of a `from...import`
+statement for details.) However, because our import tracking is flow-insensitive, we will see that
+`a.b` is imported somewhere in the file, and therefore assume that the `from...import` statement
+sees the submodule as the value of `b` instead of the integer.
+
+```py
+from a import b
+import a.b
+
+# Python would say `Literal[42]` for `b`
+reveal_type(b)  # revealed: <module 'a.b'>
+reveal_type(a.b)  # revealed: <module 'a.b'>
+```
+
+```py path=a/__init__.py
+b = 42
+```
+
+```py path=a/b.py
+```
+
+[from-import]: https://docs.python.org/3/reference/simple_stmts.html#the-import-statement

crates/red_knot_python_semantic/src/types/infer.rs

@@ -2307,11 +2307,26 @@ impl<'db> TypeInferenceBuilder<'db> {
             asname: _,
         } = alias;
 
-        // Check if the symbol being imported is a submodule.  This won't get handled by the
-        // `Type::member` call below because it relies on the semantic index's `imported_modules`
-        // set.  The semantic index does not include information about `from...import` statements
-        // because there are two things it cannot determine while only inspecting the content of
-        // the current file:
+        // First try loading the requested attribute from the module.
+        if let Symbol::Type(ty, boundness) = module_ty.member(self.db, name) {
+            if boundness == Boundness::PossiblyUnbound {
+                // TODO: Consider loading _both_ the attribute and any submodule and unioning them
+                // together if the attribute exists but is possibly-unbound.
+                self.diagnostics.add_lint(
+                    &POSSIBLY_UNBOUND_IMPORT,
+                    AnyNodeRef::Alias(alias),
+                    format_args!("Member `{name}` of module `{module_name}` is possibly unbound",),
+                );
+            }
+            self.add_declaration_with_binding(alias.into(), definition, ty, ty);
+            return;
+        };
+
+        // If the module doesn't bind the symbol, check if it's a submodule.  This won't get
+        // handled by the `Type::member` call because it relies on the semantic index's
+        // `imported_modules` set.  The semantic index does not include information about
+        // `from...import` statements because there are two things it cannot determine while only
+        // inspecting the content of the current file:
         //
         //   - whether the imported symbol is an attribute or submodule
         //   - whether the containing file is in a module or a package (needed to correctly resolve
@@ -2336,26 +2351,12 @@ impl<'db> TypeInferenceBuilder<'db> {
             }
         }
 
-        // Otherwise load the requested attribute from the module.
-        let Symbol::Type(ty, boundness) = module_ty.member(self.db, name) else {
-            self.diagnostics.add_lint(
-                &UNRESOLVED_IMPORT,
-                AnyNodeRef::Alias(alias),
-                format_args!("Module `{module_name}` has no member `{name}`",),
-            );
-            self.add_unknown_declaration_with_binding(alias.into(), definition);
-            return;
-        };
-
-        if boundness == Boundness::PossiblyUnbound {
-            self.diagnostics.add_lint(
-                &POSSIBLY_UNBOUND_IMPORT,
-                AnyNodeRef::Alias(alias),
-                format_args!("Member `{name}` of module `{module_name}` is possibly unbound",),
-            );
-        }
-
-        self.add_declaration_with_binding(alias.into(), definition, ty, ty);
+        self.diagnostics.add_lint(
+            &UNRESOLVED_IMPORT,
+            AnyNodeRef::Alias(alias),
+            format_args!("Module `{module_name}` has no member `{name}`",),
+        );
+        self.add_unknown_declaration_with_binding(alias.into(), definition);
     }
 
     fn infer_return_statement(&mut self, ret: &ast::StmtReturn) {