From 6062a751175824ff2ae700ce39d6228f36be4d92 Mon Sep 17 00:00:00 2001
From: Lee James O'Riordan <mlxd@users.noreply.github.com>
Date: Tue, 8 Oct 2024 16:48:15 -0400
Subject: [PATCH] Ensure inline templated func hit with arith types only (#936)

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------------------------------------------------------------------------------------------------------------

**Context:** Closes
https://github.com/PennyLaneAI/pennylane-lightning/issues/935

**Description of the Change:** Adds concept restrictions to the
`ConstMult` function definition for scalar-time-complex only
instantiations.

**Benefits:** Removes the given build failure.

**Possible Drawbacks:**

**Related GitHub Issues:**

---------

Co-authored-by: ringo-but-quantum <github-ringo-but-quantum@xanadu.ai>
Co-authored-by: Ali Asadi <10773383+maliasadi@users.noreply.github.com>
---
 .github/CHANGELOG.md                        | 3 +++
 pennylane_lightning/core/_version.py        | 2 +-
 pennylane_lightning/core/src/utils/Util.hpp | 2 ++
 3 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/.github/CHANGELOG.md b/.github/CHANGELOG.md
index 27ecd62a7f..95e18a73f2 100644
--- a/.github/CHANGELOG.md
+++ b/.github/CHANGELOG.md
@@ -110,6 +110,9 @@
 
 ### Bug fixes
 
+* Add concept restriction to ensure `ConstMult` inline function only hit with arithmetic-values times complex values. Fixes build failures with the test suite when enabling OpenMP, and disabling BLAS and Python under clang.
+  [(#936)](https://github.com/PennyLaneAI/pennylane-lightning/pull/936)
+
 * Bug fix for `applyMatrix` in `lightning.tensor`. Matrix operator data is not stored in the `cuGateCache` object to support `TensorProd` obs with multiple `Hermitian` obs.
   [(#932)](https://github.com/PennyLaneAI/pennylane-lightning/pull/932)
 
diff --git a/pennylane_lightning/core/_version.py b/pennylane_lightning/core/_version.py
index c6e0b5050a..ab5a1f0f5d 100644
--- a/pennylane_lightning/core/_version.py
+++ b/pennylane_lightning/core/_version.py
@@ -16,4 +16,4 @@
    Version number (major.minor.patch[-label])
 """
 
-__version__ = "0.39.0-dev40"
+__version__ = "0.39.0-dev41"
diff --git a/pennylane_lightning/core/src/utils/Util.hpp b/pennylane_lightning/core/src/utils/Util.hpp
index e0d3a1170e..5544c96ba9 100644
--- a/pennylane_lightning/core/src/utils/Util.hpp
+++ b/pennylane_lightning/core/src/utils/Util.hpp
@@ -26,6 +26,7 @@
 #include <set>
 #include <type_traits> // is_same_v
 #include <vector>
+#include <concepts> // integral, floating_point
 
 #include "Error.hpp"
 #include "TypeTraits.hpp" // remove_complex_t
@@ -41,6 +42,7 @@ namespace Pennylane::Util {
  * @return constexpr std::complex<T>
  */
 template <class T, class U = T>
+    requires std::integral<U> || std::floating_point<U>
 inline static constexpr auto ConstMult(U a, std::complex<T> b)
     -> std::complex<T> {
     return {a * b.real(), a * b.imag()};