Luau Type Solver Fails to Properly Cast Literal Types as Keys in Primitive Dictionary

Bug Reports Studio Bugs
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#1

The Luau type solver does not correctly register literal types when used as table keys.

Specifically, when defining a table with {[Literals]: someType}, where Literals is a union of string literals, the type inference does not properly recognise Literals as valid keys.

This leads to an inability to assign a type to dictionary values without overwriting any existing dictionary keys.
This issue also occurs in --!strict, --!nonstrict and persist even with the new beta luau type solver

Reproduction

type Literals = 'exampleKey' | 'someOtherKey'

local Dictionary: {[Literals]: someRandomType}

Additionally, this bug prevents defining a generic dictionary type with literal string keys, making the following type impossible

type someDict<UnionOfLiteralStrings> = {[UnionOfLiteralStrings]: someRandomType}

Expected behavior

I expected Literals to be properly recognised as Literals and not as a string

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Help with Typed Luau and Dictionaries
#2

Hello! Thank you for the report. Reading over Help with Typed Luau and Dictionaries - #26 by MinecoIII2, it sounds like the bug is:

--!strict

export type Argument = {
	description: string,
	parse: () -> (),
}

type ArgumentType = "player" | "color" | "speed"

local Args:{[ArgumentType]: Argument} = {
	["player"] = {
		description = "",
		parse = function() end,
	},

	["color"] = {
		description = "",
		parse = function() end,
	},

	["speed"] = {
		description = "",
		parse = function() end,
	},
}

local x = Args.| -- expect "player", "color", and "speed" to show up as autocomplete options here

return Args

If so, that sounds pretty reasonable to me.

#3

Thank you for your response! That is indeed the bug, yes. To clarify, are you saying that this behavior is expected and not a bug?

#4

To clarify, are you saying that this behavior is expected and not a bug?

I would describe this as a bug, and the specific case you cite should probably be fixed.

2 Likes