Add new open project for consteval support in clad

Author: vaithak

_data/openprojectlist.yml

@@ -838,6 +838,61 @@
       * Solve any general-purpose issues that come up with Clad during the process.
       * Prepare demos and carry out development needed for lossy compression. 
 
+- name: "Add support for consteval and constexpr functions in clad"
+  description: |
+    In mathematics and computer algebra, automatic differentiation (AD) is
+    a set of techniques to numerically evaluate the derivative of a function
+    specified by a computer program. Automatic differentiation is an alternative
+    technique to Symbolic differentiation and Numerical differentiation (the
+    method of finite differences). Clad is based on Clang which provides the
+    necessary facilities for code transformation. The AD library can differentiate
+    non-trivial functions, to find a partial derivative for trivial cases and has
+    good unit test coverage.
+
+    C++ provides the specifiers consteval and constexpr to allow compile time evaluation
+    of functions. `constexpr` declares a possibility, i.e the function will be
+    evaluated at compile time if possible, else at runtime; whereas `consteval` makes it
+    mandatory, i.e every call to the function must produce a compile-time constant.
+
+    The aim of this project is to ensure that same semantics are followed by the generated
+    derivative function, i.e if the primal function is evaluated at compile time
+    (because of constexpr or consteval specifier), then the generated derivative code
+    should also have the same specifier to be evaluatable at compile time.
+
+    This will enable clad to demonstrate the benefits of doing automatic differentiation
+    directly on C++ frontend to utilize the benefits of clang's infrastructure.
+
+    After successful completion of the project the code snippet should work
+    as expected:
+    ```cpp
+    #include <cstdio>
+    #include "clad/Differentiator/Differentiator.h"
+    
+    constexpr double sq(double x) { return x*x; }
+
+    consteval double fn(double x, double y, double z) {
+      double res = sq(x) + sq(y) + sq(z);
+      return res;
+    }
+
+    int main() {
+      auto d_fn = clad::gradient(fn);
+      double dx = 0, dy = 0, dz = 0;
+      d_fn.execute(3, 4, 5, &dx, &dy, &dz);
+      printf("Gradient vector: [%.2f, %.2f, %.2f]", dx, dy, dz);
+      return 0;
+    }
+    ```
+  tasks: |
+    The project consists of the following tasks:
+      * Add support for differentiation with respect to consteval and constexpr
+        functions in the forward mode.
+      * Add support for differentiation with respect to consteval and constexpr
+        functions in the reverse mode.
+      * Extend the unit test coverage.
+      * Develop tutorials and documentation.
+      * Present the work at the relevant meetings and conferences.
+
 - name: "Improve robustness of dictionary to module lookups in ROOT"
   description: |
     The LHC smashes groups of protons together at close to the speed of light: