[Anders Hejlsberg (ahejlsberg)]

With this PR we expand on #33821 to reflect more effects of optional chaining in control flow analysis. Some examples:

declare function assert(x: unknown): asserts x;
declare function assertNonNull<T>(x: T): asserts x is NonNullable<T>;

type Thing = { foo?: string | number, bar(): number };

function foo(o: Thing | undefined, value: number) {
    if (o?.foo === value) {
        o;  // Thing
        o.foo;  // number
    }
    if (typeof o?.foo === "string") {
        o;  // Thing
        o.foo;  // string
    }
    switch (o?.foo) {
        case value:
            o;  // Thing
            o.foo;  // number
    }
    switch (typeof o?.foo) {
        case "string":
            o;  // Thing
            o.foo;  // string
    }
    if (!!true) {
        assert(typeof o?.foo === "string");
        o;  // Thing
        o.foo;  // string
    }
    if (!!true) {
        assertNonNull(o?.foo);
        o;  // Thing
        o.foo;  // string | number
    }
}

The complete list of optional chaining constructs with control flow effects in --strictNullChecks mode is now:

• in code guarded by o?.foo === value, where the type of value doesn't include undefined, we remove undefined and null from the type of o in the true branch, and
• in code guarded by o?.foo !== value, where the type of value doesn't include undefined, we remove undefined and null from the type of o in the false branch, and
• in code guarded by o?.foo == value, where the type of value doesn't include undefined or null, we remove undefined and null from the type of o in the true branch, and
• in code guarded by o?.foo != value, where the type of value doesn't include undefined or null, we remove undefined and null from the type of o in the false branch, and
• in code guarded by o?.foo === undefined, o?.foo == undefined or o?.foo == null we remove undefined and null from the type of o in the false branch, and
• in code guarded by o?.foo !== undefined, o?.foo != undefined or o?.foo != null we remove undefined and null from the type of o in the true branch, and
• in code guarded by typeof o?.foo === "xxx" or typeof o?.foo == "xxx", where "xxx" is not "undefined", we remove undefined and null from the type of o in the true branch, and
• in code guarded by typeof o?.foo !== "xxx" or typeof o?.foo != "xxx", where "xxx" is not "undefined", we remove undefined and null from the type of o in the false branch, and
• in code guarded by typeof o?.foo === "undefined" or typeof o?.foo == "undefined" we remove undefined and null from the type of o in the false branch, and
• in code guarded by typeof o?.foo !== "undefined" or typeof o?.foo != "undefined" we remove undefined and null from the type of o in the true branch, and
• in code guarded by o?.foo instanceof xxx we remove undefined and null from the type of o in the true branch, and
• in a case xxx block of a switch (o?.foo) statement we remove undefined and null from the type of o provided the type of xxx doesn't include undefined, and
• in a case xxx block of a switch (typeof o?.foo) statement we remove undefined and null from the type of o provided xxx isn't "undefined", and
• in code following a top-level call expression assertIsXXX(o?.foo), where assertIsXXX is declared with an asserts x is XXX type predicate and type XXX doesn't include undefined, we remove undefined and null from the type of o, and
• in code following a top-level call expression assert(xxx), where assert is declared with an asserts x type predicate, we reflect the false branch effects of the logical expression xxx, including effects from optional property access chains.

Fixes #34570.