Fix C++11 build failure by adding compatibility macro for relaxed constexpr (Fixes #1713) #1714

leetcodez · 2026-01-29T07:51:24Z

The Problem
I noticed that building CLAD with -std=c++11 fails right now. The issue is that ArrayRef.h and Differentiator.h are using some "relaxed constexpr" features (like loops, if statements, and multiple returns) that are only allowed in C++14 and newer.

The Fix
I added a simple compatibility macro called CLAD_CONSTEXPR_CXX14.

For C++14+, it acts just like constexpr.
For C++11, it falls back to inline. This keeps the code efficient but allows it to actually compile under the stricter C++11 rules.

What I Changed

Added the macro definition to Differentiator.h and ArrayRef.h.
Swapped out constexpr for the macro in the crashing functions (mostly in ArrayRef::operator= and slice).
Fixed a small issue where an auto return type was missing its trailing return type (another C++11 requirement).

Verification
I verified this locally by running clang++ -std=c++11 ... against a demo file, and it builds cleanly now without the constexpr errors.

…ssue vgvassilev#1713)

include/clad/Differentiator/ArrayRef.h

github-actions

clang-tidy made some suggestions

github-actions · 2026-01-29T07:59:51Z

include/clad/Differentiator/ArrayRef.h


-  constexpr CUDA_HOST_DEVICE array_ref<T>& operator=(const array_ref<T>& a) {
+  // FIXED: Loop inside constexpr crashes C++11
+  CLAD_CONSTEXPR_CXX14 CUDA_HOST_DEVICE array_ref<T>& operator=(const array_ref<T>& a) {


warning: operator=() does not handle self-assignment properly [bugprone-unhandled-self-assignment]

CLAD_CONSTEXPR_CXX14 CUDA_HOST_DEVICE array_ref<T>& operator=(const array_ref<T>& a) { ^

github-actions · 2026-01-29T07:59:51Z

include/clad/Differentiator/Differentiator.h


    /// Return the string representation for the generated derivative.
-    constexpr const char* getCode() const {
+    CLAD_CONSTEXPR_CXX14 const char* getCode() const {


warning: function 'getCode' should be marked [[nodiscard]] [modernize-use-nodiscard]

Suggested change

CLAD_CONSTEXPR_CXX14 const char* getCode() const {

[[nodiscard]] CLAD_CONSTEXPR_CXX14 const char* getCode() const {

Additional context

include/clad/Differentiator/Differentiator.h:36: expanded from macro 'CLAD_CONSTEXPR_CXX14'

#define CLAD_CONSTEXPR_CXX14 constexpr ^

github-actions · 2026-01-29T07:59:51Z

include/clad/Differentiator/Differentiator.h

 // Enable clad after the header was included.
 // FIXME: The header inclusion should be made automatic if the pragma is seen.
-#pragma clad ON
+#pragma clad ON


warning: unknown pragma ignored [clang-diagnostic-unknown-pragmas]

#pragma clad ON ^

codecov · 2026-01-29T08:09:55Z

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

github-actions

clang-tidy made some suggestions

There were too many comments to post at once. Showing the first 10 out of 91. Check the log or trigger a new build to see more.

github-actions · 2026-01-29T08:28:48Z

include/clad/Differentiator/ArrayRef.h


-  constexpr CUDA_HOST_DEVICE array_ref<T>& operator=(const array_ref<T>& a) {
+  CLAD_CONSTEXPR_CXX14 CUDA_HOST_DEVICE array_ref<T>&
+  operator=(const array_ref<T>& a) {


warning: operator=() does not handle self-assignment properly [bugprone-unhandled-self-assignment]

operator=(const array_ref<T>& a) { ^

github-actions · 2026-01-29T08:28:48Z

include/clad/Differentiator/ArrayRef.h

+template <> class array_ref<void> {
+private:
+  /// The pointer to the underlying array
+  void* m_arr = nullptr;


warning: invalid case style for private member 'm_arr' [readability-identifier-naming]

Suggested change

void* m_arr = nullptr;

void* m_Arr = nullptr;

include/clad/Differentiator/ArrayRef.h:243:

- : m_arr((void*)arr), m_size(size) {} + : m_Arr((void*)arr), m_size(size) {}

include/clad/Differentiator/ArrayRef.h:246:

- : m_arr(other.ptr()), m_size(other.size()) {} + : m_Arr(other.ptr()), m_size(other.size()) {}

include/clad/Differentiator/ArrayRef.h:250:

- return array_ref<T>((T*)(m_arr), m_size); + return array_ref<T>((T*)(m_Arr), m_size);

include/clad/Differentiator/ArrayRef.h:252:

- [[nodiscard]] constexpr CUDA_HOST_DEVICE void* ptr() const { return m_arr; } + [[nodiscard]] constexpr CUDA_HOST_DEVICE void* ptr() const { return m_Arr; }

github-actions · 2026-01-29T08:28:48Z

include/clad/Differentiator/ArrayRef.h

+  /// The pointer to the underlying array
+  void* m_arr = nullptr;
+  /// The size of the array
+  std::size_t m_size = 0;


warning: invalid case style for private member 'm_size' [readability-identifier-naming]

Suggested change

std::size_t m_size = 0;

std::size_t m_Size = 0;

include/clad/Differentiator/ArrayRef.h:243:

- : m_arr((void*)arr), m_size(size) {} + : m_arr((void*)arr), m_Size(size) {}

include/clad/Differentiator/ArrayRef.h:246:

- : m_arr(other.ptr()), m_size(other.size()) {} + : m_arr(other.ptr()), m_Size(other.size()) {}

include/clad/Differentiator/ArrayRef.h:250:

- return array_ref<T>((T*)(m_arr), m_size); + return array_ref<T>((T*)(m_arr), m_Size);

include/clad/Differentiator/ArrayRef.h:254:

- return m_size; + return m_Size;

github-actions · 2026-01-29T08:28:48Z

include/clad/Differentiator/ArrayRef.h

+  // delete the default constructor
+  array_ref() = delete;
+
+  template <typename T, class = typename std::enable_if<


warning: use c++14 style type templates [modernize-type-traits]

Suggested change

template <typename T, class = typename std::enable_if<

template <typename T, class = std::enable_if_t<

include/clad/Differentiator/ArrayRef.h:241:

- std::is_same<T, std::nullptr_t>::value>::type> + std::is_same<T, std::nullptr_t>::value>>

github-actions · 2026-01-29T08:28:48Z

include/clad/Differentiator/ArrayRef.h

+  array_ref() = delete;
+
+  template <typename T, class = typename std::enable_if<
+                            std::is_pointer<T>::value ||


warning: use c++17 style variable templates [modernize-type-traits]

Suggested change

std::is_pointer<T>::value ||

std::is_pointer_v<T> ||

github-actions · 2026-01-29T08:28:48Z

include/clad/Differentiator/ArrayRef.h

+
+  template <typename T, class = typename std::enable_if<
+                            std::is_pointer<T>::value ||
+                            std::is_same<T, std::nullptr_t>::value>::type>


warning: no header providing "std::nullptr_t" is directly included [misc-include-cleaner]

std::is_same<T, std::nullptr_t>::value>::type> ^

github-actions · 2026-01-29T08:28:48Z

include/clad/Differentiator/ArrayRef.h

+
+  template <typename T, class = typename std::enable_if<
+                            std::is_pointer<T>::value ||
+                            std::is_same<T, std::nullptr_t>::value>::type>


warning: use c++17 style variable templates [modernize-type-traits]

Suggested change

std::is_same<T, std::nullptr_t>::value>::type>

std::is_same_v<T, std::nullptr_t>>::type>

github-actions · 2026-01-29T08:28:48Z

include/clad/Differentiator/Differentiator.h

+/// A specialization for C arrays
+template <typename T, std::size_t N, std::size_t SBO_SIZE = 64,
+          std::size_t SLAB_SIZE = 1024>
+CUDA_HOST_DEVICE void pop(tape<T[N], SBO_SIZE, SLAB_SIZE>& to) {


warning: do not declare C-style arrays, use std::array<> instead [cppcoreguidelines-avoid-c-arrays]

CUDA_HOST_DEVICE void pop(tape<T[N], SBO_SIZE, SLAB_SIZE>& to) { ^

github-actions · 2026-01-29T08:28:48Z

include/clad/Differentiator/Differentiator.h

+/// A specialization for C arrays
+template <typename T, typename U, size_t N, std::size_t SBO_SIZE = 64,
+          std::size_t SLAB_SIZE = 1024>
+void push(tape<T[N], SBO_SIZE, SLAB_SIZE, /*is_multithreaded=*/true>& to,


warning: do not declare C-style arrays, use std::array<> instead [cppcoreguidelines-avoid-c-arrays]

void push(tape<T[N], SBO_SIZE, SLAB_SIZE, /*is_multithreaded=*/true>& to, ^

github-actions · 2026-01-29T08:28:48Z

include/clad/Differentiator/Differentiator.h

+/// A specialization for C arrays
+template <typename T, std::size_t N, std::size_t SBO_SIZE = 64,
+          std::size_t SLAB_SIZE = 1024>
+void pop(tape<T[N], SBO_SIZE, SLAB_SIZE, /*is_multithreaded=*/true>& to) {


warning: do not declare C-style arrays, use std::array<> instead [cppcoreguidelines-avoid-c-arrays]

void pop(tape<T[N], SBO_SIZE, SLAB_SIZE, /*is_multithreaded=*/true>& to) { ^

leetcodez added 2 commits January 9, 2026 13:01

[Docs] Add LLVM troubleshooting guide and improve WSL setup

eada890

Fix C++11 build failure by adding CLAD_CONSTEXPR_CXX14 macro (fixes i…

dfd50da

…ssue vgvassilev#1713)

vgvassilev force-pushed the fix/cpp11-constexpr-compat branch from dfd50da to deedef7 Compare January 29, 2026 07:54

vgvassilev reviewed Jan 29, 2026

View reviewed changes

include/clad/Differentiator/ArrayRef.h Outdated Show resolved Hide resolved

include/clad/Differentiator/ArrayRef.h Outdated Show resolved Hide resolved

github-actions bot reviewed Jan 29, 2026

View reviewed changes

Address review feedback: move macro to CladConfig.h and remove comments

7c134e7

leetcodez closed this Jan 29, 2026

leetcodez force-pushed the fix/cpp11-constexpr-compat branch from deedef7 to 7c134e7 Compare January 29, 2026 08:16

leetcodez reopened this Jan 29, 2026

github-actions bot reviewed Jan 29, 2026

View reviewed changes

	CLAD_CONSTEXPR_CXX14 const char* getCode() const {
	[[nodiscard]] CLAD_CONSTEXPR_CXX14 const char* getCode() const {

	template <typename T, class = typename std::enable_if<
	template <typename T, class = std::enable_if_t<

	std::is_same<T, std::nullptr_t>::value>::type>
	std::is_same_v<T, std::nullptr_t>>::type>

Fix C++11 build failure by adding compatibility macro for relaxed constexpr (Fixes #1713) #1714

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Fix C++11 build failure by adding compatibility macro for relaxed constexpr (Fixes #1713) #1714

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