Preserve failing search candidate diagnostics

internal/engine/discovery_load_test.go

@@ -157,6 +157,56 @@ func TestSearchSelectedFileFailuresDoNotFallBack(t *testing.T) {
 	})
 }
 
+func TestSearchProbeFailureRetainsFileEvidence(t *testing.T) {
+	t.Parallel()
+
+	root := t.TempDir()
+	main := writeImportFile(t, root, "main.yml", "value: main\n")
+	underlay := writeImportFile(t, root, "underlay.yml", "settings: {first: underlay}\n")
+	bad := filepath.Join(main, "child.yml")
+	later := writeImportFile(t, root, "later.yml", "value: later\n")
+	missing := filepath.Join(root, "missing.yml")
+	for _, entry := range []struct {
+		name string
+		id   string
+		load func(Source) (*Loaded[discoveryLoadConfig], error)
+	}{
+		{"ordinary", "source-1", func(source Source) (*Loaded[discoveryLoadConfig], error) {
+			return Load(source, testSecretBinding(), Options[discoveryLoadConfig]{destination: Keys("settings")})
+		}},
+		{"underlay after acquired sources", "source-3", func(source Source) (*Loaded[discoveryLoadConfig], error) {
+			return LoadLayered(Input{Source: File(main)}, testSecretBinding(), LayeredOptions[discoveryLoadConfig]{
+				Underlays: []Underlay{{Inputs: []Input{
+					{Source: File(underlay)}, {Source: source, Destination: Keys("service", "settings")},
+				}}},
+			})
+		}},
+	} {
+		t.Run(entry.name, func(t *testing.T) {
+			_, err := entry.load(File(bad))
+			var explicit *LoadError
+			if !errors.As(err, &explicit) || len(explicit.Diagnostics()) != 1 {
+				t.Fatalf("explicit failed file = %v", err)
+			}
+			if explicit.Diagnostics()[0].Code == CodeSourceMissing {
+				t.Skip("host classifies a non-directory ancestor as absence; controlled probe tests cover access failures")
+			}
+			result, err := entry.load(SearchConfig("", Paths(missing, bad, later)))
+			var searched *LoadError
+			if result != nil || !errors.As(err, &searched) {
+				t.Fatalf("failed search = %#v, %v", result, err)
+			}
+			if !reflect.DeepEqual(searched.Sources(), explicit.Sources()) || !reflect.DeepEqual(searched.Diagnostics(), explicit.Diagnostics()) {
+				t.Fatalf("search failure differs from explicit file:\nsources %#v / %#v\ndiagnostics %#v / %#v", searched.Sources(), explicit.Sources(), searched.Diagnostics(), explicit.Diagnostics())
+			}
+			primary := searched.Diagnostics()[0].Primary
+			if primary == nil || primary.SourceID != entry.id {
+				t.Fatalf("failed candidate location = %#v, want %s", primary, entry.id)
+			}
+		})
+	}
+}
+
 func TestSearchValidationPrecedesCandidateProbing(t *testing.T) {
 	t.Parallel()