[ty] Respect the gradual guarantee when reporting errors in resolving MROs

crates/ty_python_semantic/resources/mdtest/mro.md

@@ -391,28 +391,35 @@ class E(
 
 ## `__bases__` lists with duplicate `Unknown` bases
 
+We do not emit errors on classes where multiple bases are inferred as `Unknown`, `Todo` or `Any`.
+Usually having duplicate bases in a bases list like this would cause us to emit a diagnostic;
+however, for gradual types this would break the
+[gradual guarantee](https://typing.python.org/en/latest/spec/concepts.html#the-gradual-guarantee):
+the dynamic base can usually be materialised to a type that would lead to a resolvable MRO.
+
 ```py
-# error: [unresolved-import]
-from does_not_exist import unknown_object_1, unknown_object_2
-
-reveal_type(unknown_object_1)  # revealed: Unknown
-reveal_type(unknown_object_2)  # revealed: Unknown
-
-# We *should* emit an error here to warn the user that we have no idea
-# what the MRO of this class should really be.
-# However, we don't complain about "duplicate base classes" here,
-# even though two classes are both inferred as being `Unknown`.
-#
-# (TODO: should we revisit this? Does it violate the gradual guarantee?
-# Should we just silently infer `[Foo, Unknown, object]` as the MRO here
-# without emitting any error at all? Not sure...)
-#
-# error: [inconsistent-mro] "Cannot create a consistent method resolution order (MRO) for class `Foo` with bases list `[Unknown, Unknown]`"
-class Foo(unknown_object_1, unknown_object_2): ...
+from unresolvable_module import UnknownBase1, UnknownBase2  # error: [unresolved-import]
+
+reveal_type(UnknownBase1)  # revealed: Unknown
+reveal_type(UnknownBase2)  # revealed: Unknown
+
+# no error here -- we respect the gradual guarantee:
+class Foo(UnknownBase1, UnknownBase2): ...
 
 reveal_type(Foo.__mro__)  # revealed: tuple[<class 'Foo'>, Unknown, <class 'object'>]
 ```
 
+However, if there are duplicate class elements, we do emit an error, even if there are also multiple
+dynamic members. The following class definition will definitely fail, no matter what the dynamic
+bases materialize to:
+
+```py
+# error: [duplicate-base] "Duplicate base class `Foo`"
+class Bar(UnknownBase1, Foo, UnknownBase2, Foo): ...
+
+reveal_type(Bar.__mro__)  # revealed: tuple[<class 'Bar'>, Unknown, <class 'object'>]
+```
+
 ## Unrelated objects inferred as `Any`/`Unknown` do not have special `__mro__` attributes
 
 ```py

crates/ty_python_semantic/src/types/mro.rs

@@ -153,43 +153,63 @@ impl<'db> Mro<'db> {
                         .collect(),
                 );
 
-                c3_merge(seqs).ok_or_else(|| {
-                    let duplicate_bases: Box<[DuplicateBaseError<'db>]> = {
-                        let mut base_to_indices: FxHashMap<ClassType<'db>, Vec<usize>> =
-                            FxHashMap::default();
-
-                        for (index, base) in valid_bases
-                            .iter()
-                            .enumerate()
-                            .filter_map(|(index, base)| Some((index, base.into_class()?)))
-                        {
-                            base_to_indices.entry(base).or_default().push(index);
-                        }
+                if let Some(mro) = c3_merge(seqs) {
+                    return Ok(mro);
+                }
+
+                let mut duplicate_dynamic_bases = false;
+
+                let duplicate_bases: Vec<DuplicateBaseError<'db>> = {
+                    let mut base_to_indices: FxHashMap<ClassBase<'db>, Vec<usize>> =
+                        FxHashMap::default();
 
-                        base_to_indices
-                            .iter()
-                            .filter_map(|(base, indices)| {
-                                let (first_index, later_indices) = indices.split_first()?;
-                                if later_indices.is_empty() {
-                                    return None;
-                                }
-                                Some(DuplicateBaseError {
-                                    duplicate_base: base.class_literal(db).0,
+                    for (index, base) in valid_bases.iter().enumerate() {
+                        base_to_indices.entry(*base).or_default().push(index);
+                    }
+
+                    let mut errors = vec![];
+
+                    for (base, indices) in base_to_indices {
+                        let Some((first_index, later_indices)) = indices.split_first() else {
+                            continue;
+                        };
+                        if later_indices.is_empty() {
+                            continue;
+                        }
+                        match base {
+                            ClassBase::Class(class) => {
+                                errors.push(DuplicateBaseError {
+                                    duplicate_base: class.class_literal(db).0,
                                     first_index: *first_index,
                                     later_indices: later_indices.iter().copied().collect(),
-                                })
-                            })
-                            .collect()
-                    };
-
-                    if duplicate_bases.is_empty() {
-                        MroErrorKind::UnresolvableMro {
-                            bases_list: valid_bases.into_boxed_slice(),
+                                });
+                            }
+                            // TODO these should also be reported as duplicate bases
+                            // rather than using the less specific `inconsistent-mro` error
+                            ClassBase::Generic(_) | ClassBase::Protocol => continue,
+                            ClassBase::Dynamic(_) => duplicate_dynamic_bases = true,
                         }
+                    }
+
+                    errors
+                };
+
+                if duplicate_bases.is_empty() {
+                    if duplicate_dynamic_bases {
+                        Ok(Mro::from_error(
+                            db,
+                            class.apply_optional_specialization(db, specialization),
+                        ))
                     } else {
-                        MroErrorKind::DuplicateBases(duplicate_bases)
+                        Err(MroErrorKind::UnresolvableMro {
+                            bases_list: valid_bases.into_boxed_slice(),
+                        })
                     }
-                })
+                } else {
+                    Err(MroErrorKind::DuplicateBases(
+                        duplicate_bases.into_boxed_slice(),
+                    ))
+                }
             }
         }
     }