[framework] Fix diagram scalar conversion losing external constraints

[jwnimmer-tri]

When a subsystem in a diagram had an ExternalSystemConstraint attached to it, the constraint wasn't being propagated through scalar conversion.

(I noticed this while testing the forthcoming new Python logic for scalar conversion in #21968.)

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This change is 

[jwnimmer-tri]

+@sherm1 for feature review, please.

[sherm1]

Feature checkpoint. I'm confused about the intended execution sequence -- PTAL.
(Out of time for this evening so no rush to respond)

Reviewed 6 of 8 files at r1, all commit messages.
Reviewable status: 1 unresolved discussion, LGTM missing from assignee sherm1(platform), needs at least two assigned reviewers

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systems/framework/system.h line 1437 at r2 (raw file):

    auto result = system_scalar_converter_.Convert<U, T>(*this);
    if (result) {
      result->HandlePostConstructionScalarConversion(*this, result.get());

BTW Stepping through this in the debugger, the Convert above ultimately calls Diagram::ConvertScalarType which now calls HandlePostConstructionScalarConversion().

So it looks like that gets called twice. Is that your intention, or am I misunderstanding?

[jwnimmer-tri]

Reviewed 8 of 8 files at r1, all commit messages.
Reviewable status: 1 unresolved discussion, LGTM missing from assignee sherm1(platform), needs at least two assigned reviewers

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systems/framework/system.h line 1437 at r2 (raw file):

Previously, sherm1 (Michael Sherman) wrote…

BTW Stepping through this in the debugger, the Convert above ultimately calls Diagram::ConvertScalarType which now calls HandlePostConstructionScalarConversion().

So it looks like that gets called twice. Is that your intention, or am I misunderstanding?

My expectation is that HandlePostConstructionScalarConversion is called exactly once for everything being converted -- both leaf systems and diagrams.

I can also trace it through tomorrow to see if the code is actually doing that =P.

[jwnimmer-tri]

Reviewable status: 1 unresolved discussion, LGTM missing from assignee sherm1(platform), needs at least two assigned reviewers

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systems/framework/system.h line 1437 at r2 (raw file):

Previously, jwnimmer-tri (Jeremy Nimmer) wrote…

My expectation is that HandlePostConstructionScalarConversion is called exactly once for everything being converted -- both leaf systems and diagrams.

I can also trace it through tomorrow to see if the code is actually doing that =P.

Yes, if I add this logger:

--- a/systems/framework/system.h
+++ b/systems/framework/system.h
@@ -21,6 +21,7 @@
 #include "drake/common/nice_type_name.h"
 #include "drake/common/pointer_cast.h"
 #include "drake/common/random.h"
+#include "drake/common/text_logging.h"
 #include "drake/systems/framework/context.h"
 #include "drake/systems/framework/event_collection.h"
 #include "drake/systems/framework/input_port.h"
@@ -1953,6 +1954,7 @@ class System : public SystemBase {
   static void HandlePostConstructionScalarConversion(const System<U>& from,
                                                      System<T>* to) {
     DRAKE_DEMAND(to != nullptr);
+    drake::log()->warn("AddExternalConstraints from {} {}", from.GetSystemPathname(), from.GetMemoryObjectName());
     to->AddExternalConstraints(from.external_constraints_);
   }
 

Then I see this output:

Two scalar conversions (the test case does ToAutoDiffXd and then ToSymbolic), and for each conversion the function is called both subsystems and then on the diagram.

[ RUN      ] DiagramConstraintTest.SystemConstraintsTest
[2025-01-19 15:56:27.286] [console] [warning] AddExternalConstraints from ::_::drake/systems/(anonymous)/ConstraintTestSystem@000064afcd8abe80 drake/systems/(anonymous)/ConstraintTestSystem@000064afcd8abe80
[2025-01-19 15:56:27.286] [console] [warning] AddExternalConstraints from ::_::drake/systems/(anonymous)/ConstraintTestSystem@000064afcd8c1600 drake/systems/(anonymous)/ConstraintTestSystem@000064afcd8c1600
[2025-01-19 15:56:27.286] [console] [warning] AddExternalConstraints from ::_ drake/systems/Diagram@000064afcd8c11d0
[2025-01-19 15:56:27.286] [console] [warning] AddExternalConstraints from ::_::drake/systems/(anonymous)/ConstraintTestSystem@000064afcd8abe80 drake/systems/(anonymous)/ConstraintTestSystem@000064afcd8abe80
[2025-01-19 15:56:27.286] [console] [warning] AddExternalConstraints from ::_::drake/systems/(anonymous)/ConstraintTestSystem@000064afcd8c1600 drake/systems/(anonymous)/ConstraintTestSystem@000064afcd8c1600
[2025-01-19 15:56:27.286] [console] [warning] AddExternalConstraints from ::_ drake/systems/Diagram@000064afcd8c11d0
[       OK ] DiagramConstraintTest.SystemConstraintsTest (0 ms)