Identify conflicting sequence registration positions

internal/engine/sequence_registration_test.go

@@ -178,6 +178,34 @@ func TestCompileSequenceIdentityPolicyRejectsConflictingOverlapsWithoutInput(t *
 	}
 }
 
+func TestSequenceOverlapDiagnosticsIdentifyOriginalRegistrationPositions(t *testing.T) {
+	t.Parallel()
+	_, diagnostics := compileSequenceIdentityPolicy([]sequenceIdentityRegistration{
+		{selector: sequenceIdentitySelector{literalSequenceSelector("invalid")}},
+		{selector: sequenceIdentitySelector{literalSequenceSelector("unrelated")}, identityFields: []string{"id"}},
+		{selector: sequenceIdentitySelector{literalSequenceSelector("collections"), anyMappingSequenceSelector(), literalSequenceSelector("items")}, identityFields: []string{"id"}},
+		{selector: sequenceIdentitySelector{literalSequenceSelector("collections"), literalSequenceSelector("foo"), literalSequenceSelector("items")}, identityFields: []string{"name"}},
+		{selector: sequenceIdentitySelector{literalSequenceSelector("collections"), literalSequenceSelector("bar"), literalSequenceSelector("items")}, identityFields: []string{"code"}},
+	})
+	if len(diagnostics) != 3 {
+		t.Fatalf("diagnostics = %#v, want the invalid registration and two conflicts", diagnostics)
+	}
+	for _, pair := range []string{"3 and 4", "3 and 5"} {
+		found := false
+		for _, diagnostic := range diagnostics {
+			if strings.Contains(diagnostic.Message, pair) {
+				found = true
+				if diagnostic.Code != CodeOptionsInvalid || diagnostic.Path.String() != "" || len(diagnostic.Related) != 0 {
+					t.Fatalf("definition overlap invented input coordinates: %#v", diagnostic)
+				}
+			}
+		}
+		if !found {
+			t.Errorf("conflicting registration positions %s not identified: %#v", pair, diagnostics)
+		}
+	}
+}
+
 func TestCompileSequenceIdentityPolicyAllowsDisjointSelectorsAndRoot(t *testing.T) {
 	policy, diagnostics := compileSequenceIdentityPolicy([]sequenceIdentityRegistration{
 		{selector: sequenceIdentitySelector{}, identityFields: []string{"id"}},