Cannot use bigint with any in certain arithmetic expressions

[abacabadabacaba]

TypeScript Version: 4.2.0-dev.20201130

Search Terms: is:issue bigint

Code

var x: any, y: bigint
-x + y
++x + y
x++ + y
~x + y
x - x + y
x * x + y

Expected behavior:

The code should compile without errors, because x could be a bigint.

Actual behavior:

Each of the expressions fails to typecheck: error TS2365: Operator '+' cannot be applied to types 'number' and 'bigint'..

Playground Link: link

Related Issues: none.

[RyanCavanaugh]

This was intentional since, unlike other operands, operations like 80n + 1 throw rather than produce possibly-useful values.

[abacabadabacaba]

@RyanCavanaugh I disagree. Since x is any, it could be a bigint, in which case all of the expressions are perfectly valid. The problem is that if the type of x is any, the type of -x, x - x, etc. is deduced to be number, which is wrong because it could be bigint.

[abacabadabacaba]

Another example:

var x: any = 0n
var y = -x

Now, the type of y is number, but the value is 0n, a bigint.

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.

[prettydiff]

I am getting this error: Operator '+' cannot be applied to types 'BigInt' and 'BigInt'

TypeScript version 4.1.3.

I can understand throwing an error for applying arithmetic to mixed types since BigInt and number are not compatible, but this is a BigInt plus a BigInt. For some background the operation is question is:

details.size = details.size + size;
// details.size is defined as a BigInt
// size comes from "size" property of Node type BigIntStats

EDIT: The problem is that @types/node was out of date. I updated to version 14.14.20 and it reports BigIntStats.size as type bigint all lowercase which is not compatible with language standard type BigInt.

EDIT2: Seems I needed to read the documentation:
https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-2.html#bigint
BigInt support in TypeScript introduces a new primitive type called the bigint (all lowercase).

[fwienber]

Another example:

var x: any = 0n
var y = -x

Now, the type of y is number, but the value is 0n, a bigint.

This issue has been marked 'Working as Intended' ...

I can understand the design decision to not allow "potentially mixed computations", but this last example is clearly wrong typing, they can't be serious this is "working as intended". The original argument was to "throw rather than produce possibly-useful values" (@RyanCavanaugh), but this example shows that there are cases where the typing is too "optimistic".
It seems the unary - operator suffers the same problem as update expressions (++, --), see #49558. I could include your example in that issue as it seems to be a closely related problem.