There are some additional subtleties here. To understand this behavior, you need to first understandbidirectional type inference.

Normally, a static type checker infers the type of expression{'': ''} asdict[str, str]. This type is not assignable toJSON due to invariance. However, with bidirectional type inference, a type checker can take into account additional "inference context", sometimes referred to as an "expected type". In the statementb: JSON = {'': ''}, the "expected type" isJSON, which influences the inference of{'': ''}.

Bidirectional type inference can be applied to a variety of expression forms including literal container expressions (lists, sets, dicts, tuples), as well as comprehensions, assignments, unary operators, lambdas, and call expressions. In general, it cannot be applied to binary operators like| because these do not map to simple calls. The| operator maps to some combination of calls to__or__ or__ror__ depending on the types of the two operands. This can get very complicated if the operand types are unions. For this reason, type checkers do not generally apply bidirectional type inference for binary operators (or augmented assignments or ternary operators) and instead evaluate the LHS and RHS operands without the benefit of an "expected type". The operator is then evaluated using the resulting types.

In your examplec: JSON = {'': ''} | {}, the LHS (without the benefit of bidirectional type inference) is evaluated asdict[str, str]. By default, pyright's default inference rules infer the RHS asdict[Unknown, Unknown], but in this case, pyright applies a limited form of bidirectional type inference and leverages the LHS type as the "expected type". It therefore evaluates the RHS type asdict[str, str]. It then evaluates the| operator, which in this case maps to the__or__ magic method and produces a result ofdict[str, str]. This isn't assignable toJSON, so a type violation is reported.

Extending your example,d: JSON = b | {} type checks without error. In this case, the normal type evaluation rules (without the benefit of bidirectional type inference) evaluate the type ofb | {} asdict[str, AnyJSON], which is assignable toJSON.