From 333d0603a1e171d6af7e31a21c8f85a9d718ccdf Mon Sep 17 00:00:00 2001
From: John Elliott <54456354+PhysicistJohn@users.noreply.github.com>
Date: Tue, 11 Aug 2026 21:27:38 -0700
Subject: [PATCH] Preserve lanes in complex-like conversions

---
 mlx/types/complex.h   | 41 ++++++++++++++++++++++----
 tests/array_tests.cpp | 67 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 102 insertions(+), 6 deletions(-)

diff --git a/mlx/types/complex.h b/mlx/types/complex.h
index 51101cc97b..8c8cd66fb5 100644
--- a/mlx/types/complex.h
+++ b/mlx/types/complex.h
@@ -2,6 +2,9 @@
 
 #pragma once
 #include <complex>
+#include <concepts>
+#include <type_traits>
+
 #include "mlx/types/half_types.h"
 
 namespace mlx::core {
@@ -9,6 +12,22 @@ namespace mlx::core {
 struct complex64_t;
 struct complex128_t;
 
+namespace detail {
+
+template <typename T>
+concept complex_like = requires(T& value) {
+  value.real();
+  value.imag();
+};
+
+template <typename T, typename Scalar>
+concept convertible_complex_like = complex_like<T> && requires(const T& value) {
+  { value.real() } -> std::convertible_to<Scalar>;
+  { value.imag() } -> std::convertible_to<Scalar>;
+};
+
+} // namespace detail
+
 template <typename T>
 inline constexpr bool can_convert_to_complex128 =
     !std::is_same_v<T, complex128_t> && std::is_convertible_v<T, double>;
@@ -18,9 +37,14 @@ struct complex128_t : public std::complex<double> {
   complex128_t(double v, double u) : std::complex<double>(v, u) {};
   complex128_t(std::complex<double> v) : std::complex<double>(v) {};
 
-  template <
-      typename T,
-      typename = typename std::enable_if<can_convert_to_complex128<T>>::type>
+  template <typename T>
+    requires(
+        !std::same_as<std::remove_cvref_t<T>, complex128_t> &&
+        detail::convertible_complex_like<T, double>)
+  complex128_t(const T& x) : std::complex<double>(x.real(), x.imag()){};
+
+  template <typename T>
+    requires(can_convert_to_complex128<T> && !detail::complex_like<T>)
   complex128_t(T x) : std::complex<double>(x){};
 
   operator float() const {
@@ -37,9 +61,14 @@ struct complex64_t : public std::complex<float> {
   complex64_t(float v, float u) : std::complex<float>(v, u) {};
   complex64_t(std::complex<float> v) : std::complex<float>(v) {};
 
-  template <
-      typename T,
-      typename = typename std::enable_if<can_convert_to_complex64<T>>::type>
+  template <typename T>
+    requires(
+        !std::same_as<std::remove_cvref_t<T>, complex64_t> &&
+        detail::convertible_complex_like<T, float>)
+  complex64_t(const T& x) : std::complex<float>(x.real(), x.imag()){};
+
+  template <typename T>
+    requires(can_convert_to_complex64<T> && !detail::complex_like<T>)
   complex64_t(T x) : std::complex<float>(x){};
 
   operator float() const {
diff --git a/tests/array_tests.cpp b/tests/array_tests.cpp
index 68a4bed3a8..9b870b5c00 100644
--- a/tests/array_tests.cpp
+++ b/tests/array_tests.cpp
@@ -10,6 +10,30 @@
 
 using namespace mlx::core;
 
+namespace {
+
+struct ComplexLike {
+  double real() const {
+    return 1.25;
+  }
+  double imag() const {
+    return -2.5;
+  }
+  operator float() const {
+    return 99.0f;
+  }
+};
+
+struct NonConvertibleLane {};
+
+struct UnsupportedComplexLike {
+  NonConvertibleLane real() const;
+  NonConvertibleLane imag() const;
+  operator float() const;
+};
+
+} // namespace
+
 TEST_CASE("test array basics") {
   // Scalar
   array x(1.0);
@@ -117,6 +141,49 @@ TEST_CASE("test array basics") {
   }
 }
 
+TEST_CASE("test complex-like conversions") {
+  static_assert(can_convert_to_complex64<ComplexLike>);
+  static_assert(can_convert_to_complex128<ComplexLike>);
+  static_assert(can_convert_to_complex64<UnsupportedComplexLike>);
+  static_assert(can_convert_to_complex128<UnsupportedComplexLike>);
+  static_assert(can_convert_to_complex64<complex128_t>);
+  static_assert(can_convert_to_complex128<complex64_t>);
+  static_assert(!can_convert_to_complex64<complex64_t>);
+  static_assert(!can_convert_to_complex128<complex128_t>);
+  static_assert(!std::constructible_from<complex64_t, UnsupportedComplexLike>);
+  static_assert(!std::constructible_from<complex128_t, UnsupportedComplexLike>);
+  static_assert(sizeof(complex64_t) == sizeof(std::complex<float>));
+  static_assert(alignof(complex64_t) == alignof(std::complex<float>));
+  static_assert(sizeof(complex128_t) == sizeof(std::complex<double>));
+  static_assert(alignof(complex128_t) == alignof(std::complex<double>));
+
+  const complex64_t custom64 = ComplexLike{};
+  CHECK_EQ(custom64.real(), 1.25f);
+  CHECK_EQ(custom64.imag(), -2.5f);
+  const complex128_t custom128 = ComplexLike{};
+  CHECK_EQ(custom128.real(), 1.25);
+  CHECK_EQ(custom128.imag(), -2.5);
+
+  const array custom_array(ComplexLike{}, complex64);
+  CHECK_EQ(custom_array.dtype(), complex64);
+  CHECK_EQ(custom_array.item<complex64_t>(), complex64_t{1.25f, -2.5f});
+
+  const complex64_t from128 = complex128_t{3.5, -4.25};
+  const complex128_t from64 = complex64_t{5.5f, -6.75f};
+  CHECK_EQ(from128, complex64_t{3.5f, -4.25f});
+  CHECK_EQ(from64, complex128_t{5.5, -6.75});
+
+  const complex64_t cross64 = std::complex<double>{7.0, -8.0};
+  const complex128_t cross128 = std::complex<float>{9.0f, -10.0f};
+  CHECK_EQ(cross64, complex64_t{7.0f, -8.0f});
+  CHECK_EQ(cross128, complex128_t{9.0, -10.0});
+
+  const complex64_t scalar64 = 11;
+  const complex128_t scalar128 = 12;
+  CHECK_EQ(scalar64, complex64_t{11.0f, 0.0f});
+  CHECK_EQ(scalar128, complex128_t{12.0, 0.0});
+}
+
 TEST_CASE("test array types") {
 #define basic_dtype_test(T, mlx_type) \
   T val = 42;                         \