Gurobi: global=-1 needs acc:_expr=0 #237

PL approximation only works for general constraints

doc/source/modeling-expressions.rst

@@ -642,11 +642,12 @@ these functions.
 Gurobi 12 defaults to proper nonlinear handling of these functions.
 Alternatively it allows their piecewise-linear approximation
 as part of preprocessing. Gurobi :ref:`option <solver-options>` ``alg:global``
-(``pre:funcnonlinear``) can be used to apply piecewise-linear approximation:
+(``pre:funcnonlinear``) can be used to apply piecewise-linear approximation,
+after disabling expression trees:
 
 .. code-block:: ampl
 
-  ampl: option gurobi_options 'global=-1'; solve;
+  ampl: option gurobi_options 'global=-1 acc:_expr=0'; solve;
 
 For individual constraints and objectives, the choice of global solving vs
 piecewise-linear approximation can be performed via the ``.global`` suffix:

doc/source/modeling-tools.rst

@@ -51,7 +51,7 @@ Next subsection explains how a user can switch between
 flat constraints and formulas, if available,
 or force reformulation.
 
-Reformulation options
+Acceptance options
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Sometimes it is handy to disable all automatic reformulations,

solvers/gurobi/gurobibackend.cc

@@ -2190,7 +2190,8 @@ void GurobiBackend::InitCustomOptions() {
     "with their constraint's or objective's suffix .funcnonlinear "
     "or, if not available, .global unset (or set to 0) are treated "
     "(ATTENTION: different meaning than Gurobi FuncNonLinear "
-                  "parameter and attribute):\n"
+                  "parameter and attribute; ATTENTION: also set acc:_expr=0 "
+                  "with pre:funcnonlinear=-1 to use 'general constraints'):\n"
                   "\n.. value-table::\n"
     "Suffix values mean the same.",
     storedOptions_.fFuncNonlinear_, values_funcnonlinear);