Fix pointer type mismatch 1804 (#1805)

This fixes the const pointer mismatch issue that was unveiled due to the fixes in #1749. Previously the fallback for an inactive const parameter output a const double * type which resulted in a type mismatch.
This PR adds a check for const pointees and remove their constness before returning the double *nullptr.

Fixes #1804

Co-authored-by: Shubham Shukla <shubham@localhost>

Author: Fermats-Last-Theorem

lib/Differentiator/BaseForwardModeVisitor.cpp

@@ -1215,8 +1215,15 @@ StmtDiff BaseForwardModeVisitor::VisitCallExpr(const CallExpr* CE) {
           }
         }
       }
-      if (!dArg)
-        dArg = getZeroInit(arg->getType());
+      if (!dArg) {
+        QualType zeroTy = arg->getType();
+        if (zeroTy->isPointerType()) {
+          QualType pointeeTy = zeroTy->getPointeeType();
+          if (pointeeTy.isConstQualified())
+            zeroTy = m_Context.getPointerType(pointeeTy.getUnqualifiedType());
+        }
+        dArg = getZeroInit(zeroTy);
+      }
       // pointer/array arguments are dynamically synthesized above
       diffArgs.push_back(dArg);
     }

test/Regressions/issue-1804.cpp

@@ -0,0 +1,18 @@
+// RUN: %cladclang -c -std=c++17 -I%S/../../include %s
+
+#include "clad/Differentiator/Differentiator.h"
+
+double f_inner(double *params, double const *obs) {
+   double arg = (obs[0] - params[1]) / params[2];
+   return arg * arg;
+}
+
+double f_outer(double *params, double const *obs) {
+   return f_inner(params, obs) + params[0];
+}
+
+void check() {
+   auto hess = clad::hessian(f_outer, "params[0:2]");
+}
+
+// CHECK: f_inner_pushforward_pullback(params, obs, (double[3]){1., 0., 0.}, nullptr, _d_t0, _d_params, (double[3]){0., 0., 0.}, nullptr);