Ensure inline templated func hit with arith types only (#936)

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

**Context:** Closes
#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 <[email protected]>
Co-authored-by: Ali Asadi <[email protected]>

.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)
 

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"

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()};