Type validated by Dictionary is different than returned type

[Hoxmot]

Tested on:

• @tsconfig/node18: 2.0.1
• node: 18.15.0
• runtypes: 6.7.0
• typescript: 5.1.3

The type returned by validation with Dictionary differs from the type accepted by the type validator. This can lead to problems in runtime, even when we pass the type validation. I noticed it's occurring when the key in the dictionary is Union.

Here's a simple example:

const Key = Union(Literal("a"), Literal("b"));
const Dict = Dictionary(Number, Key);

const example = {
  a: 42
};
const checked = Dict.check(example);  // Runtime: OK
// Compile: OK
// Runtime: Cannot read properties of undefined (reading 'toString')
console.log(checked.b.toString(8));

It looks like the validation performed by Dictionary is different than the type it says it returns. Indeed, if we try to create this object and during the declaration, we mark it's of this type, we fail at compilation.

type Dict = Static<typeof Dict>;

// Compile: Property 'b' is missing in type '{ a: number; }' but required in type '{ a: number; b: number; }'
const typedExample: Dict = {
  a: 42
};

The object which goes through the validation should be the same as the type. It'll reduce the number of possible problems.

In my opinion, the behaviour should be that the validation checks all the required keys for existence, so in the example Dict.check(example) should fail.