[feature request] Support type predicate getters for control flow narrowing

[js2me]

Summary

• Allow type predicates in getter return types, e.g. get hasUser(): this is { user: string }
• Narrow the receiver in truthiness checks, e.g. if (session.hasUser) { ... }, matching the behavior of type guard methods like hasUser(): this is T
• Extend control flow analysis to resolve type predicates from property access to getters

Motivation

Type guard methods already support this is predicates when called (if (session.hasUser())), but the getter form (if (session.hasUser)) was rejected or did not narrow types. This PR adds parity for the getter syntax.

Example

class Session {
    user: string | null = null;

    get hasUser(): this is { user: string } {
        return this.user !== null;
    }
}

const session = new Session();

if (session.hasUser) {
    session.user.toUpperCase(); // ok: `user` is `string`
}

[Copilot]

Pull request overview

Adds support for control-flow narrowing based on type predicate return types declared on getters (e.g. get hasUser(): this is ...), aligning getter truthiness checks with existing type-guard method behavior.

Changes:

• Extends control flow narrowing to resolve and apply type predicates from property/element access when the accessed member is a getter.
• Allows type predicate nodes in get-accessor return type positions during checking.
• Adds a new compiler test and reference baselines validating narrowing behavior for getter predicates.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
testdata/tests/cases/compiler/typeGuardGetterTypePredicate.ts	New regression test covering getter-based type predicate narrowing (and parity with method predicates).
testdata/baselines/reference/compiler/typeGuardGetterTypePredicate.types	New .types baseline validating the narrowed receiver type in truthiness checks.
testdata/baselines/reference/compiler/typeGuardGetterTypePredicate.symbols	New .symbols baseline for the added test.
internal/checker/flow.go	Implements predicate lookup for getter property access and integrates it into truthiness-based narrowing.
internal/checker/checker.go	Permits type predicate nodes in get-accessor return type positions.

[js2me]

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