Untangle most of the big import cycle (#7397)

Currently mypy has a 47 module import cycle, which is not great.

This untangles things, leaving the following cycles:
 * Size 2: mypy.build, mypy.semanal_main
 * Size 2: mypy.plugin, mypy.interpreted_plugin
 * Size 4: mypy.checker, mypy.checkexpr, mypy.checkmember, mypy.checkstrformat. The type checker itself. This could be untangled by having the sub-checkers operate on an interface of the checker, like the semantic analyzer does.
 * Size 5: mypy.nodes, mypy.types, mypy.type_visitor, mypy.visitors, mypy.strconv. This can't really be untangled.
 * Size 14: mypy.messages, mypy.expandtype, mypy.erasetype, mypy.typevars, mypy.maptype, mypy.typeops, mypy.sametypes, mypy.subtypes, mypy.constraints, mypy.applytype, mypy.join, mypy.meet, mypy.solve, mypy.infer. The "typeops tangle". Untangling a little further is probably possible but will require some thought and actual logic changes to code. Messages can definitely be removed.

The untangling done here is pretty simple and consists of two main parts:
 * Remove `mypy.types`'s dependency on the type ops tangle by moving all of its functions that depend on it into `mypy.typeops`.
 * Remove the dependency of the type ops tangle on the type checker by moving some functions from `mypy.checkmember` to `mypy.typeops`.

Fixes #93. Fixes #6329.

mypy/build.py

@@ -31,7 +31,7 @@
 from mypy.nodes import MypyFile, ImportBase, Import, ImportFrom, ImportAll, SymbolTable
 from mypy.semanal_pass1 import SemanticAnalyzerPreAnalysis
 from mypy.semanal import SemanticAnalyzer
-from mypy.semanal_main import semantic_analysis_for_scc
+import mypy.semanal_main
 from mypy.checker import TypeChecker
 from mypy.indirection import TypeIndirectionVisitor
 from mypy.errors import Errors, CompileError, ErrorInfo, report_internal_error
@@ -2961,7 +2961,7 @@ def process_stale_scc(graph: Graph, scc: List[str], manager: BuildManager) -> No
         # SemanticAnalyzerPass2.add_builtin_aliases for details.
         typing_mod = graph['typing'].tree
         assert typing_mod, "The typing module was not parsed"
-    semantic_analysis_for_scc(graph, scc, manager.errors)
+    mypy.semanal_main.semantic_analysis_for_scc(graph, scc, manager.errors)
 
     # Track what modules aren't yet done so we can finish them as soon
     # as possible, saving memory.

mypy/checker.py

@@ -34,7 +34,7 @@
     Type, AnyType, CallableType, FunctionLike, Overloaded, TupleType, TypedDictType,
     Instance, NoneType, strip_type, TypeType, TypeOfAny,
     UnionType, TypeVarId, TypeVarType, PartialType, DeletedType, UninhabitedType, TypeVarDef,
-    true_only, false_only, function_type, is_named_instance, union_items, TypeQuery, LiteralType,
+    function_type, is_named_instance, union_items, TypeQuery, LiteralType,
     is_optional, remove_optional, TypeTranslator, StarType, get_proper_type, ProperType,
     get_proper_types, is_literal_type
 )
@@ -45,8 +45,12 @@
 )
 import mypy.checkexpr
 from mypy.checkmember import (
-    map_type_from_supertype, bind_self, erase_to_bound, type_object_type,
-    analyze_descriptor_access,
+    analyze_descriptor_access, type_object_type,
+)
+from mypy.typeops import (
+    map_type_from_supertype, bind_self, erase_to_bound, make_simplified_union,
+    erase_def_to_union_or_bound, erase_to_union_or_bound,
+    true_only, false_only,
 )
 from mypy import message_registry
 from mypy.subtypes import (
@@ -1539,7 +1543,7 @@ def get_op_other_domain(self, tp: FunctionLike) -> Optional[Type]:
             raw_items = [self.get_op_other_domain(it) for it in tp.items()]
             items = [it for it in raw_items if it]
             if items:
-                return UnionType.make_simplified_union(items)
+                return make_simplified_union(items)
             return None
         else:
             assert False, "Need to check all FunctionLike subtypes here"
@@ -1973,7 +1977,7 @@ def check_assignment(self, lvalue: Lvalue, rvalue: Expression, infer_lvalue_type
                             if not self.current_node_deferred:
                                 # Partial type can't be final, so strip any literal values.
                                 rvalue_type = remove_instance_last_known_values(rvalue_type)
-                                inferred_type = UnionType.make_simplified_union(
+                                inferred_type = make_simplified_union(
                                     [rvalue_type, NoneType()])
                                 self.set_inferred_type(var, lvalue, inferred_type)
                             else:
@@ -2417,7 +2421,7 @@ def check_multi_assignment_from_union(self, lvalues: List[Expression], rvalue: E
                                             rv_type=item, undefined_rvalue=True)
                 for t, lv in zip(transposed, self.flatten_lvalues(lvalues)):
                     t.append(self.type_map.pop(lv, AnyType(TypeOfAny.special_form)))
-        union_types = tuple(UnionType.make_simplified_union(col) for col in transposed)
+        union_types = tuple(make_simplified_union(col) for col in transposed)
         for expr, items in assignments.items():
             # Bind a union of types collected in 'assignments' to every expression.
             if isinstance(expr, StarExpr):
@@ -2432,8 +2436,8 @@ def check_multi_assignment_from_union(self, lvalues: List[Expression], rvalue: E
 
             types, declared_types = zip(*clean_items)
             self.binder.assign_type(expr,
-                                    UnionType.make_simplified_union(list(types)),
-                                    UnionType.make_simplified_union(list(declared_types)),
+                                    make_simplified_union(list(types)),
+                                    make_simplified_union(list(declared_types)),
                                     False)
         for union, lv in zip(union_types, self.flatten_lvalues(lvalues)):
             # Properly store the inferred types.
@@ -2856,9 +2860,9 @@ def try_infer_partial_type_from_indexed_assignment(
                     # TODO: Don't infer things twice.
                     key_type = self.expr_checker.accept(lvalue.index)
                     value_type = self.expr_checker.accept(rvalue)
-                    full_key_type = UnionType.make_simplified_union(
+                    full_key_type = make_simplified_union(
                         [key_type, var.type.inner_types[0]])
-                    full_value_type = UnionType.make_simplified_union(
+                    full_value_type = make_simplified_union(
                         [value_type, var.type.inner_types[1]])
                     if (is_valid_inferred_type(full_key_type) and
                             is_valid_inferred_type(full_value_type)):
@@ -3176,7 +3180,7 @@ def check_except_handler_test(self, n: Expression) -> Type:
 
             all_types.append(exc_type)
 
-        return UnionType.make_simplified_union(all_types)
+        return make_simplified_union(all_types)
 
     def get_types_from_except_handler(self, typ: Type, n: Expression) -> List[Type]:
         """Helper for check_except_handler_test to retrieve handler types."""
@@ -3524,7 +3528,7 @@ def partition_by_callable(self, typ: Type,
             # do better.
             # If it is possible for the false branch to execute, return the original
             # type to avoid losing type information.
-            callables, uncallables = self.partition_by_callable(typ.erase_to_union_or_bound(),
+            callables, uncallables = self.partition_by_callable(erase_to_union_or_bound(typ),
                                                                 unsound_partition)
             uncallables = [typ] if len(uncallables) else []
             return callables, uncallables
@@ -4087,7 +4091,7 @@ def conditional_type_map(expr: Expression,
     if it was not the proposed type, if any. None means bot, {} means top"""
     if proposed_type_ranges:
         proposed_items = [type_range.item for type_range in proposed_type_ranges]
-        proposed_type = UnionType.make_simplified_union(proposed_items)
+        proposed_type = make_simplified_union(proposed_items)
         if current_type:
             if isinstance(proposed_type, AnyType):
                 # We don't really know much about the proposed type, so we shouldn't
@@ -4170,7 +4174,7 @@ def builtin_item_type(tp: Type) -> Optional[Type]:
                 return tp.args[0]
     elif isinstance(tp, TupleType) and all(not isinstance(it, AnyType)
                                            for it in get_proper_types(tp.items)):
-        return UnionType.make_simplified_union(tp.items)  # this type is not externally visible
+        return make_simplified_union(tp.items)  # this type is not externally visible
     elif isinstance(tp, TypedDictType):
         # TypedDict always has non-optional string keys. Find the key type from the Mapping
         # base class.
@@ -4221,7 +4225,7 @@ def or_conditional_maps(m1: TypeMap, m2: TypeMap) -> TypeMap:
     for n1 in m1:
         for n2 in m2:
             if literal_hash(n1) == literal_hash(n2):
-                result[n1] = UnionType.make_simplified_union([m1[n1], m2[n2]])
+                result[n1] = make_simplified_union([m1[n1], m2[n2]])
     return result
 
 
@@ -4435,7 +4439,7 @@ def overload_can_never_match(signature: CallableType, other: CallableType) -> bo
     # the below subtype check and (surprisingly?) `is_proper_subtype(Any, Any)`
     # returns `True`.
     # TODO: find a cleaner solution instead of this ad-hoc erasure.
-    exp_signature = expand_type(signature, {tvar.id: tvar.erase_to_union_or_bound()
+    exp_signature = expand_type(signature, {tvar.id: erase_def_to_union_or_bound(tvar)
                                 for tvar in signature.variables})
     assert isinstance(exp_signature, CallableType)
     return is_callable_compatible(exp_signature, other,
@@ -4689,7 +4693,7 @@ class Status(Enum):
 
     if isinstance(typ, UnionType):
         items = [try_expanding_enum_to_union(item, target_fullname) for item in typ.items]
-        return UnionType.make_simplified_union(items)
+        return make_simplified_union(items)
     elif isinstance(typ, Instance) and typ.type.is_enum and typ.type.fullname() == target_fullname:
         new_items = []
         for name, symbol in typ.type.names.items():
@@ -4704,7 +4708,7 @@ class Status(Enum):
         # only using CPython, but we might as well for the sake of full correctness.
         if sys.version_info < (3, 7):
             new_items.sort(key=lambda lit: lit.value)
-        return UnionType.make_simplified_union(new_items)
+        return make_simplified_union(new_items)
     else:
         return typ
 
@@ -4716,7 +4720,7 @@ def coerce_to_literal(typ: Type) -> ProperType:
     typ = get_proper_type(typ)
     if isinstance(typ, UnionType):
         new_items = [coerce_to_literal(item) for item in typ.items]
-        return UnionType.make_simplified_union(new_items)
+        return make_simplified_union(new_items)
     elif isinstance(typ, Instance) and typ.last_known_value:
         return typ.last_known_value
     else: