ambiguous_set_or_map_literal_both

The literal can't be either a map or a set because it contains at least one literal map entry or a spread operator spreading a 'Map', and at least one element which is neither of these.

Because map and set literals use the same delimiters ({ and }), the analyzer looks at the type arguments and the elements to determine which kind of literal you meant. When there are no type arguments, then the analyzer uses the types of the elements. If all of the elements are literal map entries and all of the spread operators are spreading a Map then it's a Map. If none of the elements are literal map entries and all of the spread operators are spreading an Iterable, then it's a Set. If neither of those is true then it's ambiguous.

The analyzer produces this diagnostic when at least one element is a literal map entry or a spread operator spreading a Map, and at least one element is neither of these, making it impossible for the analyzer to determine whether you are writing a map literal or a set literal.

The following code produces this diagnostic:

union(Map<String, String> a, List<String> b, Map<String, String> c) =>
    {...a, ...b, ...c};

The list b can only be spread into a set, and the maps a and c can only be spread into a map, and the literal can't be both.

There are two common ways to fix this problem. The first is to remove all of the spread elements of one kind or another, so that the elements are consistent. In this case, that likely means removing the list and deciding what to do about the now unused parameter:

union(Map<String, String> a, List<String> b, Map<String, String> c) =>
    {...a, ...c};

The second fix is to change the elements of one kind into elements that are consistent with the other elements. For example, you can add the elements of the list as keys that map to themselves:

union(Map<String, String> a, List<String> b, Map<String, String> c) =>
    {...a, for (String s in b) s: s, ...c};

Unless stated otherwise, the documentation on this site reflects Dart 3.8.1. Page last updated on 2025-05-08. View source or report an issue.