Fix new/free mismatch when differentiating delegating constructors.

For a delegating constructor initializer, DifferentiateCtorInit replaced
`_this`'s initializer -- set by BuildThisExpr to malloc and paired with
free(_this) -- with an allocating `new ClassTy(args)`, so the new-ed
pointer was then handed to free(). Valgrind reports the mismatched
deallocation in Gradient/Constructors.C.

Emit a placement new into the malloc'd `_this` instead, as the
base-initializer path already does, keeping allocation and deallocation
malloc/free.

Author: vgvassilev

lib/Differentiator/ReverseModeVisitor.cpp

@@ -734,11 +734,12 @@ Expr* ReverseModeVisitor::getStdInitListSizeExpr(const Expr* E) {
                                             initDiff.getExpr(), baseTSI,
                                             {placementArg});
         } else if (CI->isDelegatingInitializer()) {
-          auto* thisDRE = cast<DeclRefExpr>(thisExpr);
-          auto* thisVD = cast<VarDecl>(thisDRE->getDecl());
-          Expr* newInit = utils::BuildCXXNewExpr(m_Sema, baseTy, nullptr,
-                                                 initDiff.getExpr(), baseTSI);
-          SetDeclInit(thisVD, newInit);
+          // Placement-new into the malloc'd `_this` (paired with free(_this)),
+          // as the base-initializer path above does. An allocating
+          // `new ClassTy(args)` here would be freed with free() -- a mismatch.
+          initCall =
+              utils::BuildCXXNewExpr(m_Sema, baseTy, /*arraySize=*/nullptr,
+                                     initDiff.getExpr(), baseTSI, {thisExpr});
         }
       }
       CompoundStmt* block = endBlock(direction::reverse);

test/Gradient/Constructors.C

@@ -2,8 +2,6 @@
 // RUN: ./Constructors.out | %filecheck_exec %s
 // RUN: %cladclang -Xclang -plugin-arg-clad -Xclang -disable-tbr -Xclang -plugin-arg-clad -Xclang -enable-va %s -I%S/../../include -oConstructors.out
 // RUN: ./Constructors.out | %filecheck_exec %s
-// FIXME: real new/free mismatch for delegating constructors; drop when fixed.
-// XFAIL: valgrind
 
 #include "clad/Differentiator/Differentiator.h"
 #include "clad/Differentiator/STLBuiltins.h"
@@ -368,15 +366,17 @@ double fn6(double x, double y) {
 } // x^2 * y
 
 // CHECK: static clad::ValueAndAdjoint<argByValWrapper, argByValWrapper> constructor_reverse_forw(clad::Tag<argByValWrapper>, double v, double u, double _d_v, double _d_u) {
-// CHECK-NEXT:     argByValWrapper *_this = new argByValWrapper(v);
+// CHECK-NEXT:     argByValWrapper *_this = (argByValWrapper *)malloc(sizeof(argByValWrapper));
 // CHECK-NEXT:     argByValWrapper *_d_this = (argByValWrapper *)malloc(sizeof(argByValWrapper));
 // CHECK-NEXT:     memset(_d_this, 0, sizeof(argByValWrapper));
+// CHECK-NEXT:     new (_this) argByValWrapper(v);
 // CHECK-NEXT:     _this->z = _this->y * u;
 // CHECK-NEXT:     return {*_this, *_d_this};
 // CHECK-NEXT: }
 
 // CHECK:  static void constructor_pullback(double v, double u, argByValWrapper *_d_this, double *_d_v, double *_d_u) {
-// CHECK-NEXT:      argByValWrapper *_this = new argByValWrapper(v);
+// CHECK-NEXT:      argByValWrapper *_this = (argByValWrapper *)malloc(sizeof(argByValWrapper));
+// CHECK-NEXT:      new (_this) argByValWrapper(v);
 // CHECK-NEXT:      _this->z = _this->y * u;
 // CHECK-NEXT:      {
 // CHECK-NEXT:          double _r_d0 = _d_this->z;