Fix the problem with ignored potential type alias (#2419)

Fixes #2416.

This is a minimalistic fix (basically just restoring status quo for invalid aliases).

Author: ilevkivskyi

mypy/checkexpr.py

@@ -1378,6 +1378,9 @@ 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."""
+        if isinstance(alias.type, Instance) and alias.type.invalid:
+            # An invalid alias, error already has been reported
+            return AnyType()
         item = alias.type
         if not alias.in_runtime:
             # We don't replace TypeVar's with Any for alias used as Alias[T](42).

mypy/typeanal.py

@@ -416,6 +416,7 @@ def visit_instance(self, t: Instance) -> None:
             # otherwise the type checker may crash as it expects
             # things to be right.
             t.args = [AnyType() for _ in info.type_vars]
+            t.invalid = True
         elif info.defn.type_vars:
             # Check type argument values.
             for (i, arg), TypeVar in zip(enumerate(t.args), info.defn.type_vars):

mypy/types.py

@@ -416,6 +416,7 @@ class Instance(Type):
     type = None  # type: mypy.nodes.TypeInfo
     args = None  # type: List[Type]
     erased = False  # True if result of type variable substitution
+    invalid = False  # True if recovered after incorrect number of type arguments error
 
     def __init__(self, typ: mypy.nodes.TypeInfo, args: List[Type],
                  line: int = -1, column: int = -1, erased: bool = False) -> None:

test-data/unit/check-generics.test

@@ -970,6 +970,13 @@ main:9:7: error: Type argument 1 of "A" has incompatible value "str"
 main:13: error: Type argument 1 of "A" has incompatible value "str"
 main:13: error: Type argument 2 of "A" has incompatible value "str"
 
+[case testGenericTypeAliasesIgnoredPotentialAlias]
+class A: ...
+Bad = A[int] # type: ignore
+
+reveal_type(Bad) # E: Revealed type is 'Any'
+[out]
+
 
 -- Multiple assignment with lists
 -- ------------------------------