bug #1281: fix AutoDiffScalar's make_coherent for nested expression of constant ADs.

unsupported/Eigen/src/AutoDiff/AutoDiffScalar.h

diff --git a/unsupported/Eigen/src/AutoDiff/AutoDiffScalar.h b/unsupported/Eigen/src/AutoDiff/AutoDiffScalar.h
index 2db9147..88edd6b 100755
--- a/unsupported/Eigen/src/AutoDiff/AutoDiffScalar.h
+++ b/unsupported/Eigen/src/AutoDiff/AutoDiffScalar.h
@@ -453,6 +453,24 @@
   void operator+() const;
 };
 
+template<typename BinOp, typename A, typename B, typename RefType>
+void make_coherent_expression(CwiseBinaryOp<BinOp,A,B> xpr, const RefType &ref)
+{
+  make_coherent(xpr.const_cast_derived().lhs(), ref);
+  make_coherent(xpr.const_cast_derived().rhs(), ref);
+}
+
+template<typename UnaryOp, typename A, typename RefType>
+void make_coherent_expression(const CwiseUnaryOp<UnaryOp,A> &xpr, const RefType &ref)
+{
+  make_coherent(xpr.nestedExpression().const_cast_derived(), ref);
+}
+
+// needed for compilation only
+template<typename UnaryOp, typename A, typename RefType>
+void make_coherent_expression(const CwiseNullaryOp<UnaryOp,A> &, const RefType &)
+{}
+
 template<typename A_Scalar, int A_Rows, int A_Cols, int A_Options, int A_MaxRows, int A_MaxCols, typename B>
 struct make_coherent_impl<Matrix<A_Scalar, A_Rows, A_Cols, A_Options, A_MaxRows, A_MaxCols>, B> {
   typedef Matrix<A_Scalar, A_Rows, A_Cols, A_Options, A_MaxRows, A_MaxCols> A;
@@ -462,6 +480,10 @@
       a.resize(b.size());
       a.setZero();
     }
+    else if (B::SizeAtCompileTime==Dynamic && a.size()!=0 && b.size()==0)
+    {
+      make_coherent_expression(b,a);
+    }
   }
 };
 
@@ -474,13 +496,17 @@
       b.resize(a.size());
       b.setZero();
     }
+    else if (A::SizeAtCompileTime==Dynamic && b.size()!=0 && a.size()==0)
+    {
+      make_coherent_expression(a,b);
+    }
   }
 };
 
 template<typename A_Scalar, int A_Rows, int A_Cols, int A_Options, int A_MaxRows, int A_MaxCols,
          typename B_Scalar, int B_Rows, int B_Cols, int B_Options, int B_MaxRows, int B_MaxCols>
 struct make_coherent_impl<Matrix<A_Scalar, A_Rows, A_Cols, A_Options, A_MaxRows, A_MaxCols>,
-                             Matrix<B_Scalar, B_Rows, B_Cols, B_Options, B_MaxRows, B_MaxCols> > {
+                          Matrix<B_Scalar, B_Rows, B_Cols, B_Options, B_MaxRows, B_MaxCols> > {
   typedef Matrix<A_Scalar, A_Rows, A_Cols, A_Options, A_MaxRows, A_MaxCols> A;
   typedef Matrix<B_Scalar, B_Rows, B_Cols, B_Options, B_MaxRows, B_MaxCols> B;
   static void run(A& a, B& b) {

unsupported/test/autodiff.cpp

diff --git a/unsupported/test/autodiff.cpp b/unsupported/test/autodiff.cpp
index bafea6a..2cea56b 100644
--- a/unsupported/test/autodiff.cpp
+++ b/unsupported/test/autodiff.cpp
@@ -352,6 +352,21 @@
   return v2(0).value();
 }
 
+// check with expressions on constants
+double bug_1281() {
+  int n = 2;
+  typedef AutoDiffScalar<VectorXd> AD;
+  const AD c = 1.;
+  AD x0(2,n,0);
+  AD y1 = (AD(c)+AD(c))*x0;
+  y1 = x0 * (AD(c)+AD(c));
+  AD y2 = (-AD(c))+x0;
+  y2 = x0+(-AD(c));
+  AD y3 = (AD(c)*(-AD(c))+AD(c))*x0;
+  y3 = x0 * (AD(c)*(-AD(c))+AD(c));
+  return (y1+y2+y3).value();
+}
+
 #endif
 
 EIGEN_DECLARE_TEST(autodiff)
@@ -367,5 +382,6 @@
   CALL_SUBTEST_5( bug_1223() );
   CALL_SUBTEST_5( bug_1260() );
   CALL_SUBTEST_5( bug_1261() );
+  CALL_SUBTEST_5( bug_1281() );
 }