Allow redefinition of non-physical vars in subclasses and private fields satisfying public interfaces

[Copilot]

Two related OOP visibility restrictions in the Haxe type checker were unnecessarily strict given that Haxe's private is semantically protected (and emitted as public at runtime).

Changes

src/typing/typeloadFields.ml — Non-physical var redefinition (#12521)

Redefining a variable in a subclass was unconditionally rejected. Now only physical vars (actual memory slots) are rejected; non-physical properties (accessor-only, no @:isVar) can be redefined:

abstract class Dispatcher {
    public var scheduler(get, never):Float;  // non-physical
    abstract function get_scheduler():Float;
}

class ConcreteDispatcher extends Dispatcher {
    @:isVar public var scheduler(get, null):Int;  // was: error, now: allowed

    function get_scheduler() return scheduler;
}

Access narrowing and visibility reduction remain errors (e.g. get → private get, public parent → private child).

src/typing/typeloadCheck.ml — Private field satisfying public interface (#12268)

A private field can now implement a public interface requirement on non-extern classes. Since private in Haxe is protected, the field is accessible via the interface type (a "view" pattern) and is public at runtime anyway. For extern classes, where private may map to a truly-private target-language construct, the error is retained.

interface I { function f():Int; }

class C implements I {
    public function new() {}
    function f():Int return 42;  // was: error, now: allowed
}

var i:I = new C();
trace(i.f());  // 42

src/generators/cpp/gen/cppCppia.ml — CPPIA duplicate member var fix

When a child class redefines a parent's non-physical property, the CPPIA runtime was producing "duplicate member var" errors because both parent and child emitted metadata for the same field name. This caused GetFieldByLinkage to fail and return 0/null instead of the actual value.

Two cases are now handled:

• Non-physical → non-physical (e.g. widening write access): the child's var is skipped entirely in CPPIA metadata. The parent's IaAccessCall definition stays active and the overridden accessor methods handle dispatch naturally.
• Non-physical → physical (e.g. @:isVar redefinition): the physical backing slot is emitted under a _hxb_<name> name to avoid the parent collision. A per-class backing_field_remap in the script_writer transparently redirects all IaFThisInst accesses within the class to the backing name, while external access continues through the parent's accessor dispatch to the overridden getter/setter.

Tests

• tests/misc/eval/projects/Issue12521/ — compile-fail test covering allowed and disallowed redefinition cases
• tests/unit/src/unit/TestRedefinition.hx — runtime tests for redefinition and access widening, including on CPPIA (no target guard required after the compiler-side fix)
• tests/unit/src/unit/issues/Issue12268.hx — runtime test verifying dispatch through the interface type works correctly

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[Simn]

@copilot Yes indeed, you were supposed to check why that is and how we can fix it!

[Copilot]

Investigated and fixed in 736c7ad.

Root cause: Both the parent (Dispatcher.scheduler as non-physical/accessor-based) and child (ConcreteDispatcher.scheduler as physical @:isVar) were emitting CPPIA metadata for the same field name. The CPPIA runtime combines parent+child fields when loading a class, sees the same name twice → "duplicate member var" → field lookup via GetFieldByLinkage fails → returns 0/null.

Fix in cppCppia.ml:

• Non-physical → non-physical (e.g., ChildWidenedNonPhysical.name): Skip the child's var in CPPIA entirely. The parent's IaAccessCall definition stays active; overridden get_name()/set_name() methods handle dispatch.
• Non-physical → physical (e.g., ConcreteDispatcher.scheduler, ChildWidened.name): Emit the physical slot under a _hxb_<name> backing name (no parent collision). All IaFThisInst accesses to the original field name within the class's methods are remapped to the backing name. External access still goes through the parent's IaAccessCall → overridden getter/setter → backing slot.

The #if !cppia guard is removed — tests now run on CPPIA too.

[Simn]

Yeah but this still doesn't work my friend...