First part of response to comments (bigger things)

mypy/checkexpr.py

@@ -6,7 +6,9 @@
     Type, AnyType, CallableType, Overloaded, NoneTyp, Void, TypeVarDef,
     TupleType, Instance, TypeVarId, TypeVarType, ErasedType, UnionType,
     PartialType, DeletedType, UnboundType, UninhabitedType, TypeType,
-    true_only, false_only, is_named_instance, function_type
+    true_only, false_only, is_named_instance, function_type,
+    get_typ_args, set_typ_args
+
 )
 from mypy.nodes import (
     NameExpr, RefExpr, Var, FuncDef, OverloadedFuncDef, TypeInfo, CallExpr,
@@ -1378,18 +1380,12 @@ def visit_type_application(self, tapp: TypeApplication) -> Type:
     def visit_type_alias_expr(self, alias: TypeAliasExpr) -> Type:
         """ Get type of a type alias (could be generic) in a runtime expression."""
         item = alias.type
-        if (isinstance(item, (Instance, TupleType, UnionType, CallableType))
-           and not alias.runtime):
+        if not alias.runtime:
             item = self.replace_tvars_any(item)
         if isinstance(item, Instance):
             tp = type_object_type(item.type, self.named_type)
         else:
-            # TODO: Better error reporting: need to find line for
-            # unsubscribed generic aliases, that are invalid at runtime.
-            if alias.line > 0:
-                self.chk.fail('Invalid type alias in runtime expression: {}'
-                              .format(item), alias)
-            return AnyType()
+            return alias.fback
         if isinstance(tp, CallableType):
             if len(tp.variables) != len(item.args):
                 self.msg.incompatible_type_application(len(tp.variables),
@@ -1411,11 +1407,7 @@ def replace_tvars_any(self, tp: Type) -> Type:
         a runtime expression. Basically, this function finishes what could not be done
         in similar funtion from typeanal.py.
         """
-        if not isinstance(tp, (Instance, UnionType, TupleType, CallableType)):
-            return tp
-        typ_args = (tp.args if isinstance(tp, Instance) else
-                    tp.items if not isinstance(tp, CallableType) else
-                    tp.arg_types + [tp.ret_type])
+        typ_args = get_typ_args(tp)
         new_args = typ_args[:]
         for i, arg in enumerate(typ_args):
             if isinstance(arg, UnboundType):
@@ -1428,14 +1420,7 @@ def replace_tvars_any(self, tp: Type) -> Type:
                     new_args[i] = AnyType()
             else:
                 new_args[i] = self.replace_tvars_any(arg)
-        if isinstance(tp, Instance):
-            return Instance(tp.type, new_args, tp.line)
-        if isinstance(tp, TupleType):
-            return tp.copy_modified(items=new_args)
-        if isinstance(tp, UnionType):
-            return UnionType.make_union(new_args, tp.line)
-        if isinstance(tp, CallableType):
-            return tp.copy_modified(arg_types=new_args[:-1], ret_type=new_args[-1])
+        return set_typ_args(tp, new_args)
 
     def visit_list_expr(self, e: ListExpr) -> Type:
         """Type check a list expression [...]."""

mypy/nodes.py

@@ -1732,13 +1732,13 @@ class TypeAliasExpr(Expression):
     """Type alias expression (rvalue)."""
 
     type = None  # type: mypy.types.Type
-    line = None  # type: int
+    fback = None  # type: mypy.types.Type
     runtime = False  # type: bool
 
-    def __init__(self, type: 'mypy.types.Type', line: int = -1,
+    def __init__(self, type: 'mypy.types.Type', fback: 'mypy.types.Type' = None,
                  runtime: bool = False) -> None:
         self.type = type
-        self.line = line
+        self.fback = fback
         self.runtime = runtime
 
     def accept(self, visitor: NodeVisitor[T]) -> T:

mypy/semanal.py

@@ -1175,7 +1175,7 @@ def visit_assignment_stmt(self, s: AssignmentStmt) -> None:
                     node.kind = TYPE_ALIAS
                     node.type_override = res
                     if isinstance(s.rvalue, IndexExpr):
-                        s.rvalue.analyzed = TypeAliasExpr(res)
+                        s.rvalue.analyzed = TypeAliasExpr(res, fback=self.alias_fallback(res))
         if s.type:
             # Store type into nodes.
             for lvalue in s.lvalues:
@@ -1213,6 +1213,20 @@ def analyze_simple_literal_type(self, rvalue: Expression) -> Optional[Type]:
             return self.named_type_or_none('builtins.unicode')
         return None
 
+    def alias_fallback(self, tp: Type) -> Instance:
+        """Make a dummy Instance with no methods. It is used as a fallback type
+        to detect errors for non-Instance aliases (i.e. Unions, Tuples, Callables).
+        """
+
+        kind = (' to Callable' if isinstance(tp, CallableType) else
+                ' to Tuple' if isinstance(tp, TupleType) else
+                ' to Union' if isinstance(tp, UnionType) else '')
+        cdef = ClassDef('Type alias{}'.format(kind), Block([]))
+        fb_info = TypeInfo(SymbolTable(), cdef, self.cur_mod_id)
+        fb_info.bases = [self.object_type()]
+        fb_info.mro = [fb_info, self.object_type().type]
+        return Instance(fb_info, [])
+
     def check_and_set_up_type_alias(self, s: AssignmentStmt) -> None:
         """Check if assignment creates a type alias and set it up as needed."""
         # For now, type aliases only work at the top level of a module.
@@ -2370,7 +2384,7 @@ def visit_index_expr(self, expr: IndexExpr) -> None:
                                      self.lookup_qualified,
                                      self.lookup_fully_qualified,
                                      self.fail)
-            expr.analyzed = TypeAliasExpr(res, line=expr.line, runtime=True)
+            expr.analyzed = TypeAliasExpr(res, fback=self.alias_fallback(res), runtime=True)
         elif refers_to_class_or_function(expr.base):
             # Special form -- type application.
             # Translate index to an unanalyzed type.
@@ -3061,6 +3075,8 @@ def visit_decorator(self, dec: Decorator) -> None:
 
     def visit_assignment_stmt(self, s: AssignmentStmt) -> None:
         self.analyze(s.type)
+        if isinstance(s.rvalue, IndexExpr) and isinstance(s.rvalue.analyzed, TypeAliasExpr):
+            self.analyze(s.rvalue.analyzed.type)
         super().visit_assignment_stmt(s)
 
     def visit_cast_expr(self, e: CastExpr) -> None:

mypy/typeanal.py

@@ -5,7 +5,7 @@
 from mypy.types import (
     Type, UnboundType, TypeVarType, TupleType, UnionType, Instance,
     AnyType, CallableType, Void, NoneTyp, DeletedType, TypeList, TypeVarDef, TypeVisitor,
-    StarType, PartialType, EllipsisType, UninhabitedType, TypeType
+    StarType, PartialType, EllipsisType, UninhabitedType, TypeType, get_typ_args, set_typ_args
 )
 from mypy.nodes import (
     BOUND_TVAR, UNBOUND_TVAR, TYPE_ALIAS, UNBOUND_IMPORTED,
@@ -209,11 +209,7 @@ def get_type_var_names(self, tp: Type) -> List[str]:
         in order of textual appearance (recursively, if needed).
         """
         tvars = []  # type: List[str]
-        if not isinstance(tp, (Instance, UnionType, TupleType, CallableType)):
-            return tvars
-        typ_args = (tp.args if isinstance(tp, Instance) else
-                    tp.items if not isinstance(tp, CallableType) else
-                    tp.arg_types + [tp.ret_type])
+        typ_args = get_typ_args(tp)
         for arg in typ_args:
             tvar = self.get_tvar_name(arg)
             if tvar:
@@ -243,11 +239,7 @@ def replace_alias_tvars(self, tp: Type, vars: List[str], subs: List[Type]) -> Ty
         """ Replace type variables in a generic type alias tp with substitutions subs.
         Length of subs should be already checked.
         """
-        if not isinstance(tp, (Instance, UnionType, TupleType, CallableType)) or not subs:
-            return tp
-        typ_args = (tp.args if isinstance(tp, Instance) else
-                    tp.items if not isinstance(tp, CallableType) else
-                    tp.arg_types + [tp.ret_type])
+        typ_args = get_typ_args(tp)
         new_args = typ_args[:]
         for i, arg in enumerate(typ_args):
             tvar = self.get_tvar_name(arg)
@@ -257,15 +249,7 @@ def replace_alias_tvars(self, tp: Type, vars: List[str], subs: List[Type]) -> Ty
             else:
                 # ...recursively, if needed.
                 new_args[i] = self.replace_alias_tvars(arg, vars, subs)
-        # Create a copy with type vars replaced.
-        if isinstance(tp, Instance):
-            return Instance(tp.type, new_args, tp.line)
-        if isinstance(tp, TupleType):
-            return tp.copy_modified(items=new_args)
-        if isinstance(tp, UnionType):
-            return UnionType.make_union(new_args, tp.line)
-        if isinstance(tp, CallableType):
-            return tp.copy_modified(arg_types=new_args[:-1], ret_type=new_args[-1])
+        return set_typ_args(tp, new_args)
 
     def visit_any(self, t: AnyType) -> Type:
         return t

mypy/types.py

@@ -1524,3 +1524,22 @@ def function_type(func: mypy.nodes.FuncBase, fallback: Instance) -> FunctionLike
             name,
             implicit=True,
         )
+
+def get_typ_args(tp: Type) -> List[Type]:
+    if not isinstance(tp, (Instance, UnionType, TupleType, CallableType)):
+        return []
+    typ_args = (tp.args if isinstance(tp, Instance) else
+                tp.items if not isinstance(tp, CallableType) else
+                tp.arg_types + [tp.ret_type])
+    return typ_args
+
+def set_typ_args(tp: Type, args: List[Type]) -> Type:
+    if isinstance(tp, Instance):
+        return Instance(tp.type, new_args, tp.line)
+    if isinstance(tp, TupleType):
+        return tp.copy_modified(items=new_args)
+    if isinstance(tp, UnionType):
+        return UnionType.make_union(new_args, tp.line)
+    if isinstance(tp, CallableType):
+        return tp.copy_modified(arg_types=new_args[:-1], ret_type=new_args[-1])
+    return tp