fix: honor branch anchor destination on split-to-merge flows

mdl-examples/bug-tests/310-anchor-split-to-merge-destination.mdl

@@ -0,0 +1,63 @@
+-- ============================================================================
+-- Bug #310: Branch anchor `to:` side dropped on direct split-to-merge flows
+-- ============================================================================
+--
+-- Symptom (before fix):
+--   When an IF had a branch anchor whose `to:` side wasn't the default Left
+--   AND that branch body was empty (so the split connected straight to the
+--   merge), the writer applied only the branch FROM side and let the
+--   default destination (Left) overwrite the user-provided `to:` side.
+--   Re-describing turned
+--     @anchor(to: left, false: (from: bottom, to: top))
+--   into
+--     @anchor(to: left, false: (from: bottom, to: left))
+--   silently changing the visual layout in Studio Pro on roundtrip.
+--
+-- Root cause:
+--   Several split-to-merge shortcuts called `applyUserAnchors` with the
+--   branch anchor only as the origin override and `nil` as the destination
+--   override.
+--
+-- After fix:
+--   `applyUserAnchors` now receives the branch anchor for both origin and
+--   destination overrides on split-to-merge shortcuts, matching the
+--   behavior used when a branch has a first statement.
+--
+-- Usage:
+--   mxcli exec mdl-examples/bug-tests/310-anchor-split-to-merge-destination.mdl -p app.mpr
+--   mxcli -p app.mpr -c "describe microflow BugTest310.MF_AnchorFalseBranch"
+--   The output must preserve `false: (from: bottom, to: top)` on the
+--   IF anchor and must be a fixpoint under describe → exec → describe.
+-- ============================================================================
+
+create module BugTest310;
+
+-- IF-without-ELSE with a non-default false-branch destination.
+create microflow BugTest310.MF_AnchorFalseBranch (
+  $value: integer
+)
+begin
+  @anchor(to: left, false: (from: bottom, to: top))
+  if $value > 0 then
+    log info node 'BugTest310' 'inside';
+  end if;
+
+  log info node 'BugTest310' 'after';
+end;
+/
+
+-- IF with empty THEN body — true branch goes straight to merge with a
+-- non-default destination side.
+create microflow BugTest310.MF_AnchorEmptyThen (
+  $value: integer
+)
+begin
+  @anchor(to: left, true: (from: right, to: top))
+  if $value > 0 then
+  else
+    log info node 'BugTest310' 'else only';
+  end if;
+
+  log info node 'BugTest310' 'after';
+end;
+/

mdl/executor/cmd_microflows_anchor_if_test.go

@@ -109,6 +109,60 @@ func TestBuilder_AnchorInsideElseBranch(t *testing.T) {
 	}
 }
 
+// TestBuilder_AnchorFalseBranchTo_IfWithoutElse pins a regression: when the
+// describer emits
+//
+//	@anchor(to: left, true: (from: right, to: left), false: (from: bottom, to: top))
+//
+// on an IF-without-ELSE split, the writer used to apply only the FROM side of
+// the false-branch anchor to the split→merge flow, letting the default (Left)
+// overwrite the intended `to: top`. Re-describing produced `false: (from: bottom, to: left)`.
+func TestBuilder_AnchorFalseBranchTo_IfWithoutElse(t *testing.T) {
+	body := []ast.MicroflowStatement{
+		&ast.IfStmt{
+			Condition: &ast.LiteralExpr{Kind: ast.LiteralBoolean, Value: true},
+			ThenBody: []ast.MicroflowStatement{
+				&ast.LogStmt{Level: ast.LogInfo, Message: &ast.LiteralExpr{Kind: ast.LiteralString, Value: "inside"}},
+			},
+			Annotations: &ast.ActivityAnnotations{
+				FalseBranchAnchor: &ast.FlowAnchors{From: ast.AnchorSideBottom, To: ast.AnchorSideTop},
+			},
+		},
+	}
+
+	oc := buildWithAnchors(body)
+
+	// The split → merge false flow must land on merge's Top side, not the default Left.
+	if !hasFlow(oc.Flows, AnchorBottom, AnchorTop) {
+		t.Errorf("expected false branch split→merge flow Bottom→Top, got %+v", oc.Flows)
+	}
+}
+
+// TestBuilder_AnchorTrueBranchTo_EmptyThenIfWithElse is the ELSE-branch mirror
+// of the cluster B4 regression: an empty THEN body connecting straight to the
+// merge must honor the trueBranchAnchor.To.
+func TestBuilder_AnchorTrueBranchTo_EmptyThenIfWithElse(t *testing.T) {
+	body := []ast.MicroflowStatement{
+		&ast.IfStmt{
+			Condition: &ast.LiteralExpr{Kind: ast.LiteralBoolean, Value: true},
+			ThenBody:  nil, // empty THEN body
+			ElseBody: []ast.MicroflowStatement{
+				&ast.LogStmt{Level: ast.LogInfo, Message: &ast.LiteralExpr{Kind: ast.LiteralString, Value: "else"}},
+			},
+			Annotations: &ast.ActivityAnnotations{
+				TrueBranchAnchor: &ast.FlowAnchors{From: ast.AnchorSideRight, To: ast.AnchorSideTop},
+			},
+		},
+	}
+
+	oc := buildWithAnchors(body)
+
+	// Empty THEN → merge: must honor the user's Right→Top anchor, not the default Right→Left.
+	if !hasFlow(oc.Flows, AnchorRight, AnchorTop) {
+		t.Errorf("expected true branch split→merge flow Right→Top, got %+v", oc.Flows)
+	}
+}
+
 func TestBuilder_AnchorToTopOnReturnPreservedInsideElse(t *testing.T) {
 	// Minimal case: single-statement ELSE whose only statement is a RETURN
 	// carrying @anchor(to: top). The flow from the split to that return's

mdl/executor/cmd_microflows_builder_control.go

@@ -155,9 +155,11 @@ func (fb *flowBuilder) addIfStatement(s *ast.IfStmt) model.ID {
 				applyUserAnchors(flow, prevThenAnchor, nil)
 				fb.flows = append(fb.flows, flow)
 			} else {
-				// Empty THEN body - connect split directly to merge with true case
+				// Empty THEN body - connect split directly to merge with true case.
+				// Pass trueBranchAnchor as destination too so the @anchor(true: (..., to: Y))
+				// from the describer round-trips into the merge side of the flow.
 				flow := newHorizontalFlowWithCase(splitID, mergeID, "true")
-				applyUserAnchors(flow, trueBranchAnchor, nil)
+				applyUserAnchors(flow, trueBranchAnchor, trueBranchAnchor)
 				fb.flows = append(fb.flows, flow)
 			}
 		}
@@ -214,9 +216,11 @@ func (fb *flowBuilder) addIfStatement(s *ast.IfStmt) model.ID {
 		// This keeps the "do nothing" path straight and the "do something" path below
 
 		if needMerge {
-			// FALSE path: connect split directly to merge horizontally
+			// FALSE path: connect split directly to merge horizontally.
+			// Pass falseBranchAnchor as destination too so @anchor(false: (..., to: Y))
+			// round-trips through the merge side of the split-to-merge flow.
 			flow := newHorizontalFlowWithCase(splitID, mergeID, "false")
-			applyUserAnchors(flow, falseBranchAnchor, nil)
+			applyUserAnchors(flow, falseBranchAnchor, falseBranchAnchor)
 			fb.flows = append(fb.flows, flow)
 		}
 		// When !needMerge (thenReturns): FALSE flow is deferred — the parent will
@@ -267,9 +271,11 @@ func (fb *flowBuilder) addIfStatement(s *ast.IfStmt) model.ID {
 				applyUserAnchors(flow, prevThenAnchor, nil)
 				fb.flows = append(fb.flows, flow)
 			} else {
-				// Empty THEN body - connect split directly to merge going down and back up
+				// Empty THEN body - connect split directly to merge going down and back up.
+				// Pass trueBranchAnchor as destination too so @anchor(true: (..., to: Y))
+				// round-trips into the merge side of the flow.
 				flow := newDownwardFlowWithCase(splitID, mergeID, "true")
-				applyUserAnchors(flow, trueBranchAnchor, nil)
+				applyUserAnchors(flow, trueBranchAnchor, trueBranchAnchor)
 				fb.flows = append(fb.flows, flow)
 			}
 		}