Preserve composition conflict claimant order

internal/engine/diagnostic_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"errors"
 	"io"
+	"slices"
 	"strings"
 	"testing"
 )
@@ -110,6 +111,44 @@ func TestCanonicalDiagnosticsSortEscapedPaths(t *testing.T) {
 	}
 }
 
+func TestCanonicalDiagnosticsPreservesCompositionClaimantOrder(t *testing.T) {
+	composition := Diagnostic{
+		Code:     CodeComposeConflict,
+		Severity: SeverityError,
+		Path:     Keys("value"),
+		Primary:  &Location{SourceID: "source-4"},
+		Related: []Location{
+			{SourceID: "source-5"},
+			{SourceID: "source-3"},
+		},
+	}
+	other := Diagnostic{
+		Code:     "decode.type",
+		Severity: SeverityError,
+		Path:     Keys("other"),
+		Related: []Location{
+			{SourceID: "source-5"},
+			{SourceID: "source-3"},
+		},
+	}
+
+	canonical := canonicalDiagnostics([]Diagnostic{composition, other})
+	var compositionResult, genericResult Diagnostic
+	for _, diagnostic := range canonical {
+		if diagnostic.Code == CodeComposeConflict {
+			compositionResult = diagnostic
+		} else {
+			genericResult = diagnostic
+		}
+	}
+	if got := []string{compositionResult.Related[0].SourceID, compositionResult.Related[1].SourceID}; !slices.Equal(got, []string{"source-5", "source-3"}) {
+		t.Fatalf("composition related locations = %v, want claimant order", got)
+	}
+	if got := []string{genericResult.Related[0].SourceID, genericResult.Related[1].SourceID}; !slices.Equal(got, []string{"source-3", "source-5"}) {
+		t.Fatalf("generic related locations = %v, want sorted order", got)
+	}
+}
+
 func TestWriteDiagnosticsEscapesControls(t *testing.T) {
 	t.Parallel()