Fix crash on partial type inference within a lambda (#14087)

Fixes #9654

Seems to be quite straightforward. Erased types should never be stored
on variables, it is just a temporary thing for nested generic calls.

Author: ilevkivskyi

mypy/checker.py

@@ -178,6 +178,7 @@
     AnyType,
     CallableType,
     DeletedType,
+    ErasedType,
     FunctionLike,
     Instance,
     LiteralType,
@@ -7040,18 +7041,26 @@ def is_valid_inferred_type(typ: Type, is_lvalue_final: bool = False) -> bool:
         return is_lvalue_final
     elif isinstance(proper_type, UninhabitedType):
         return False
-    return not typ.accept(NothingSeeker())
+    return not typ.accept(InvalidInferredTypes())
 
 
-class NothingSeeker(TypeQuery[bool]):
-    """Find any <nothing> types resulting from failed (ambiguous) type inference."""
+class InvalidInferredTypes(TypeQuery[bool]):
+    """Find type components that are not valid for an inferred type.
+
+    These include <Erased> type, and any <nothing> types resulting from failed
+    (ambiguous) type inference.
+    """
 
     def __init__(self) -> None:
         super().__init__(any)
 
     def visit_uninhabited_type(self, t: UninhabitedType) -> bool:
         return t.ambiguous
 
+    def visit_erased_type(self, t: ErasedType) -> bool:
+        # This can happen inside a lambda.
+        return True
+
 
 class SetNothingToAny(TypeTranslator):
     """Replace all ambiguous <nothing> types with Any (to avoid spurious extra errors)."""

test-data/unit/check-incremental.test

@@ -6286,3 +6286,29 @@ class C: ...
 [out]
 [out2]
 [out3]
+
+[case testNoCrashOnPartialLambdaInference]
+import m
+[file m.py]
+from typing import TypeVar, Callable
+
+V = TypeVar("V")
+def apply(val: V, func: Callable[[V], None]) -> None:
+    return func(val)
+
+xs = []
+apply(0, lambda a: xs.append(a))
+[file m.py.2]
+from typing import TypeVar, Callable
+
+V = TypeVar("V")
+def apply(val: V, func: Callable[[V], None]) -> None:
+    return func(val)
+
+xs = []
+apply(0, lambda a: xs.append(a))
+reveal_type(xs)
+[builtins fixtures/list.pyi]
+[out]
+[out2]
+tmp/m.py:9: note: Revealed type is "builtins.list[builtins.int]"