Only apply indexed access write simplifications to types that arise from mutation

[Jack Williams (jack-williams)]

This is an attempt at alleviate some additional errors introduced by #30769. The write simplifications introduced there were applied whenever a type was related to an indexed access type, irrespective of whether that type denoted a real update or just a value of lookup-type.

This PR only applies the write simplifications to indexed access types that arise through assignments to element access expressions.

Fixes #31833
Fixes #32816

A related digression: there are code paths that do not get write-simplified (with and without this PR) which introduce unsoundness. Is this by design?

const foo = { x: { a: true }, y: { b: 42 } };
type Foo = typeof foo;

const bar = { x: { c: false }, y: { d: 100 } };
type Bar = typeof bar;

function example(arg: Foo | Bar, k: "x" | "y") {
    // The union Foo | Bar is not simplified
    // to an intersection under indexed access
    // even though arg[k] conceptually has the
    // type: (Foo | Bar)["x" | "y"]
    arg[k] = { a: true, b: 100 } // no error, but what if arg is Bar?
}

[Wesley Wigham (weswigham)]

there are code paths that do not get write-simplified (with and without this PR) which introduce unsoundness. Is this by design?

Yeah; element accesses are generally slightly unsound.

This approach looks fine to me, but we'll need to sync it and run the extended test suites on it. Also ping Anders Hejlsberg (@ahejlsberg) since I imagine he'll be interested in it.

[Jack Williams (jack-williams)]

This approach looks fine to me, but we'll need to sync it and run the extended test suites on it. Also ping Anders Hejlsberg (@ahejlsberg) since I imagine he'll be interested in it.

Wesley Wigham (@weswigham), will do! FWIW, very few people seem to have stumbled upon this issue so maybe it isn't worth the additional complexity.

[Nathan Shively-Sanders (sandersn)]

Unfortunately, we never finished reviewing this PR. It is pretty old now, so I'm going to close it to reduce the number of open PRs.