Remove definition of ExpressionRef::ExpressionRef(double&);

Due to the removed definition, calling this method will result
in a compile-time error instead of a run-time error.

Change-Id: Iac2514c05d79a66fcbad124587d08eb9309aa6a5

include/ceres/internal/expression_ref.h

diff --git a/include/ceres/internal/expression_ref.h b/include/ceres/internal/expression_ref.h
index 3d376ee..a1afd14 100644
--- a/include/ceres/internal/expression_ref.h
+++ b/include/ceres/internal/expression_ref.h
@@ -56,10 +56,14 @@
   // must work for T = Jet<ExpressionRef>.
   ExpressionRef(double compile_time_constant);
 
-  // By adding this constructor (which always throws an error) we can detect
-  // invalid usage of ExpressionRef. ExpressionRef can only be created from
-  // constexpr doubles.
-  ExpressionRef(double& test);
+  // By adding this deleted constructor we can detect invalid usage of
+  // ExpressionRef. ExpressionRef must only be created from constexpr doubles.
+  //
+  // If you get a compile error here, you have probably written something like:
+  //   T x = local_variable_;
+  // Change this into:
+  //   T x = CERES_LOCAL_VARIABLE(local_variable_);
+  ExpressionRef(double&) = delete;
 
   // Create an ASSIGNMENT expression from other to this.
   //

internal/ceres/expression_ref.cc

diff --git a/internal/ceres/expression_ref.cc b/internal/ceres/expression_ref.cc
index 9aa25d4..e9cda42 100644
--- a/internal/ceres/expression_ref.cc
+++ b/internal/ceres/expression_ref.cc
@@ -44,11 +44,6 @@
   id = Expression::CreateCompileTimeConstant(compile_time_constant);
 }
 
-ExpressionRef::ExpressionRef(double& test) {
-  CHECK(false)
-      << "ExpressionRefs can only be created from compile-time constants.";
-}
-
 ExpressionRef::ExpressionRef(const ExpressionRef& other) { *this = other; }
 
 ExpressionRef& ExpressionRef::operator=(const ExpressionRef& other) {