[Type] ndarray typing 3: Add cook_dtype calls at C++ boundaries

[hughperkins]

…refactor)

Add cook_dtype() calls at all points where dtype values are passed to C++ code. Make PyQuadrants.default_fp/ip/up into properties that always store DataTypeCxx. Rename shadowed dtype var in create_field_member. All changes are behavioral no-ops with current code, preparing for a future refactor of primitive dtypes into Python classes.

Issue: #

Brief Summary

copilot:summary

Walkthrough

copilot:walkthrough

[hughperkins]

Opus 4.6 review:

PR Review: hp/typing-t4-3-cook-dtype

Branch: hp/typing-t4-3-cook-dtype (parent: hp/typing-t4-2-ndarray-subscript)

Summary

This PR adds cook_dtype() calls at C++ boundaries (expr, impl, matrix, sparse_matrix, sparse_solver) and refactors default_fp/default_ip/default_up in PyQuadrants into properties with backing _default_* attributes. It is a preparatory refactor for a future PR that will convert dtypes to Python classes; with current DataTypeCxx instances, cook_dtype() is effectively a no-op (identity for DataTypeCxx subclasses).

Issues Found

1. Bug fix (positive finding): create_field_member variable shadowing

The rename from dtype to checkbit_dtype in the adjoint checkbit block fixes a real bug. In the original code:

if prog.config().debug:
    dtype = u8
    if prog.config().arch == _qd_core.vulkan:
        dtype = i32
    x_grad_checkbit.ptr = _qd_core.expr_field(..., cook_dtype(dtype))
# ...
x_dual.ptr = _qd_core.expr_field(x_dual.ptr, dtype)  # BUG: dtype was overwritten to u8/i32!

The inner block overwrote the dtype parameter, so x_dual was incorrectly created with u8 or i32 instead of the field’s actual dtype when prog.config().debug was True. The rename correctly isolates the checkbit dtype.

2. Minor: Redundant cook_dtype in make_constant_expr

In make_constant_expr, when dtype is not None, dtype is already cooked at the top. For float/int branches, constant_dtype is then cooked again. Since cook_dtype is idempotent for DataTypeCxx, this is harmless but redundant. Consider removing the second cook_dtype(constant_dtype) for the dtype is not None path, or leave as-is for consistency and future-proofing.

Correctness

• No-op behavior: cook_dtype() returns dtype unchanged when type(dtype) is a subclass of _qd_core.DataTypeCxx. Current primitives (f32, i32, u1, etc.) are such instances, so behavior is unchanged.
• Property refactor: The default_fp/default_ip/default_up properties correctly preserve read/write behavior. The setters apply cook_dtype(value), so future Python dtype classes can be normalized when assigned.
• set_default_ip: Uses self.default_ip = ip and self.default_up = u32 if ip == i32 else u64, which works with current DataTypeCxx values. A future Python dtype class would need == to compare correctly with i32/i64 for this logic to remain correct.

Edge Cases

• cook_dtype(None): Would raise ValueError. Call sites guard with if dtype is not None where appropriate (e.g. make_constant_expr).
• qd.default_fp = float: cook_dtype(float) returns impl.get_runtime().default_fp, i.e. the current value. Assigning float is effectively a no-op and may be surprising; acceptable for a preparatory refactor.
• SharedArray / expr_init_shared_array: element_type is now cooked before passing to C++. Correct and consistent with other boundaries.

Style

• Consistent use of cook_dtype() at C++ boundaries.
• Clear variable naming (checkbit_dtype, dtype_cxx in sparse modules).
• Imports and formatting are consistent with the rest of the codebase.

Suggestions for Improvement

1. Document the bug fix: Add a brief comment near the checkbit_dtype rename noting that it fixes the previous shadowing of dtype, which caused x_dual to get the wrong dtype in debug mode.
2. Optional: Add a test that exercises create_field_member with debug=True and needs_dual=True to lock in the fix and guard against regressions.
3. Future PR: When introducing Python dtype classes, ensure set_default_ip’s ip == i32 comparison works for both DataTypeCxx and the new Python dtype wrappers (e.g. via cook_dtype(ip) == cook_dtype(i32) or an explicit equality method).

Verdict

The PR is correct and behaves as a no-op for current dtypes. It also fixes a real bug in create_field_member when debug mode is enabled. Tests pass. Ready to merge with optional minor suggestions above.