Fixed bug that results in a false negative if a | union operator cr…

[erictraut]

…eates a union of generic types. These types should be specialized with default type arguments. This addresses #9415.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

ibis (https://github.com/ibis-project/ibis)
+           "Value" is not assignable to "StringValue"
-             "Value" is not assignable to "StringValue"
+       Type "dict[str, Deferred]" is not assignable to type "(Value) -> Value"
-         Type parameter "_KT@Mapping" is invariant, but "str" is not the same as "str | None"
+         Type parameter "_KT@Mapping" is invariant, but "str" is not the same as "str | None" (reportArgumentType)
-       Type "dict[str, Deferred]" is not assignable to type "(Value) -> Value" (reportArgumentType)
+       Type "dict[str, Deferred]" is not assignable to type "(Value) -> Value"
-         Type parameter "_KT@Mapping" is invariant, but "str" is not the same as "str | None"
+         Type parameter "_KT@Mapping" is invariant, but "str" is not the same as "str | None" (reportArgumentType)
-       Type "dict[str, Deferred]" is not assignable to type "(Value) -> Value" (reportArgumentType)
+         Type "Value" is not assignable to type "(Value) -> Value"
-         "Value" is not assignable to "Mapping[str | None, Deferred | ((Value) -> Value)]"
+         "Value" is not assignable to "Mapping[str | None, Deferred | ((Value) -> Value)]" (reportArgumentType)
-         Type "Value" is not assignable to type "(Value) -> Value" (reportArgumentType)
+       Type "Value" is not assignable to type "(Value) -> Value"
-       "Value" is not assignable to "Mapping[str | None, Deferred | ((Value) -> Value)]"
+       "Value" is not assignable to "Mapping[str | None, Deferred | ((Value) -> Value)]" (reportArgumentType)
-       Type "Value" is not assignable to type "(Value) -> Value" (reportArgumentType)
-       Type "Value" is not assignable to type "(Value, Value) -> Value"
+       Type "Value" is not assignable to type "(Value) -> Value"
-       Type "Value" is not assignable to type "(Value) -> Value" (reportArgumentType)
+       Type "Value" is not assignable to type "(Value, Value) -> Value" (reportArgumentType)
-       Type "Value" is not assignable to type "(Value, IntegerValue) -> (bool | BooleanValue)"
+       Type "Value" is not assignable to type "(Value) -> (bool | BooleanValue)"
-       Type "Value" is not assignable to type "(Value) -> (bool | BooleanValue)" (reportArgumentType)
+       Type "Value" is not assignable to type "(Value, IntegerValue) -> (bool | BooleanValue)" (reportArgumentType)

mitmproxy (https://github.com/mitmproxy/mitmproxy)
-         "None" is not assignable to "WebSocketMessage"
+         "None" is not assignable to "UDPMessage"
-         "None" is not assignable to "UDPMessage" (reportArgumentType)
+         "None" is not assignable to "WebSocketMessage" (reportArgumentType)
-         "None" is not assignable to "WebSocketMessage"
+         "None" is not assignable to "UDPMessage"
-         "None" is not assignable to "UDPMessage" (reportArgumentType)
+         "None" is not assignable to "WebSocketMessage" (reportArgumentType)
-         "None" is not assignable to "WebSocketMessage"
+         "None" is not assignable to "UDPMessage"
-         "None" is not assignable to "UDPMessage" (reportArgumentType)
+         "None" is not assignable to "WebSocketMessage" (reportArgumentType)