[PATCH v3 01/16] libcamera: matrix: Replace SFINAE with static_asserts

[Stefan Klug]

SFINAE is difficult to read and not needed in these cases. Replace it
with static_asserts. The idea came from [1] where it is stated:

"The use of enable_if seems misguided to me. SFINAE is useful for the
situation where we consider multiple candidates for something (overloads
or class template specializations) and try to choose the correct one,
without causing compilation to fail."

[1]: https://stackoverflow.com/questions/62109526/c-friend-template-that-use-sfinae

Signed-off-by: Stefan Klug <stefan.klug at ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>

---

Changes in v2:
- Added this patch

Changes in v3:
- Left SFINAE in place for the operators as static asserts could cause
  issues there
---
 include/libcamera/internal/matrix.h | 19 +++++--------------
 1 file changed, 5 insertions(+), 14 deletions(-)

diff --git a/include/libcamera/internal/matrix.h b/include/libcamera/internal/matrix.h
index a055e6926c94..b9c3d41ef855 100644
--- a/include/libcamera/internal/matrix.h
+++ b/include/libcamera/internal/matrix.h
@@ -19,14 +19,11 @@ namespace libcamera {
 
 LOG_DECLARE_CATEGORY(Matrix)
 
-#ifndef __DOXYGEN__
-template<typename T, unsigned int Rows, unsigned int Cols,
-	 std::enable_if_t<std::is_arithmetic_v<T>> * = nullptr>
-#else
 template<typename T, unsigned int Rows, unsigned int Cols>
-#endif /* __DOXYGEN__ */
 class Matrix
 {
+	static_assert(std::is_arithmetic_v<T>, "Matrix type must be arithmetic");
+
 public:
 	Matrix()
 	{
@@ -123,16 +120,10 @@ Matrix<U, Rows, Cols> operator*(const Matrix<U, Rows, Cols> &m, T d)
 	return d * m;
 }
 
-#ifndef __DOXYGEN__
-template<typename T,
-	 unsigned int R1, unsigned int C1,
-	 unsigned int R2, unsigned int C2,
-	 std::enable_if_t<C1 == R2> * = nullptr>
-#else
-template<typename T, unsigned int R1, unsigned int C1, unsigned int R2, unsigned in C2>
-#endif /* __DOXYGEN__ */
-Matrix<T, R1, C2> operator*(const Matrix<T, R1, C1> &m1, const Matrix<T, R2, C2> &m2)
+template<typename T, unsigned int R1, unsigned int C1, unsigned int R2, unsigned int C2>
+constexpr Matrix<T, R1, C2> operator*(const Matrix<T, R1, C1> &m1, const Matrix<T, R2, C2> &m2)
 {
+	static_assert(C1 == R2, "Matrix dimensions must match for multiplication");
 	Matrix<T, R1, C2> result;
 
 	for (unsigned int i = 0; i < R1; i++) {
-- 
2.43.0