Verify mounted diagnostic paths and locations

docs/roadmap/implementation.md

@@ -284,7 +284,7 @@ ownership and lifecycle timing are preserved.
 
 ## Stage 6 — Reconcile diagnostic coordinate systems
 
-Status: planned. Scope: useful paths without fictional source locations.
+Status: complete. Scope: useful paths without fictional source locations.
 
 1. Audit early acquisition, import, fragment-version, and composition failures.
    Use one narrow path-prefix translation where diagnostics genuinely identify
@@ -300,6 +300,14 @@ Status: planned. Scope: useful paths without fictional source locations.
    nil results, not whole messages or callback choreography.
 
 Validate: destination/diagnostic/import/version/decode tests and everyday check.
+Validated with `go test ./internal/engine -run 'LayeredDestinationDiagnostics'`,
+`go test ./internal/engine -run 'Destination|Diagnostic|Import|Version|Decode'`,
+and `./scripts/check.sh` (go1.26.8 linux/amd64); all passed. The mounted
+conflict, decode, and imported-version assertions use effective application
+paths with authored source locations; invalid destinations have no physical
+location; malformed source-root directives retain their authored path and
+location. No path-prefix translation was needed because value diagnostics are
+already produced from placed trees and directive diagnostics from raw trees.
 Done: diagnostics consistently distinguish application paths from source syntax.
 
 ## Stage 7 — Complete shared placement accounting

internal/engine/destination_diagnostic_test.go

@@ -0,0 +1,142 @@
+package engine
+
+import (
+	"path/filepath"
+	"testing"
+	"testing/fstest"
+)
+
+func TestLayeredDestinationDiagnosticsUseEffectivePathsAndAuthoredLocations(t *testing.T) {
+	t.Parallel()
+
+	t.Run("strict conflict retains both source locations", func(t *testing.T) {
+		t.Parallel()
+		loaded, err := LoadLayered(
+			Input{Source: NoSource()},
+			testSecretBinding(),
+			LayeredOptions[map[string]any]{Underlays: []Underlay{{Inputs: []Input{
+				{Source: Bytes("left", []byte("section:\n  value: left\n")), Destination: Keys("application")},
+				{Source: Bytes("right", []byte("section:\n  value: right\n")), Destination: Keys("application")},
+			}}}},
+		)
+		if loaded != nil {
+			t.Fatalf("conflicting load returned a result: %#v", loaded)
+		}
+		diagnostic := firstDestinationDiagnostic(t, err)
+		if diagnostic.Code != CodeComposeConflict || diagnostic.Path.String() != "/application/section/value" {
+			t.Fatalf("conflict diagnostic = %#v", diagnostic)
+		}
+		if diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-1" || diagnostic.Primary.Line != 2 {
+			t.Fatalf("primary location = %#v", diagnostic.Primary)
+		}
+		if len(diagnostic.Related) != 1 || diagnostic.Related[0].SourceID != "source-2" || diagnostic.Related[0].Line != 2 {
+			t.Fatalf("related locations = %#v", diagnostic.Related)
+		}
+	})
+
+	t.Run("decode failure keeps effective path and content line", func(t *testing.T) {
+		t.Parallel()
+		type section struct {
+			Count int `yaml:"count"`
+		}
+		type config struct {
+			Application struct {
+				Section section `yaml:"section"`
+			} `yaml:"application"`
+		}
+		loaded, err := LoadLayered(
+			Input{Source: Bytes("main", []byte("section:\n  count: invalid\n")), Destination: Keys("application")},
+			testSecretBinding(),
+			LayeredOptions[config]{},
+		)
+		if loaded != nil {
+			t.Fatalf("invalid load returned a result: %#v", loaded)
+		}
+		diagnostic := firstDestinationDiagnostic(t, err)
+		if diagnostic.Code != CodeDecodeType || diagnostic.Path.String() != "/application/section/count" {
+			t.Fatalf("decode diagnostic = %#v", diagnostic)
+		}
+		if diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-1" || diagnostic.Primary.Line != 2 {
+			t.Fatalf("decode location = %#v", diagnostic.Primary)
+		}
+	})
+
+	t.Run("imported version failure uses effective path and import source", func(t *testing.T) {
+		t.Parallel()
+		directory := t.TempDir()
+		writeImportFile(t, directory, "main.yml", "configkit: {imports: [{path: fragment.yml}]}\n")
+		writeImportFile(t, directory, "fragment.yml", "version: forbidden\n")
+		rule := &VersionRule{Path: Keys("application", "version"), Check: func(Version, *Validation) error { return nil }}
+		loaded, err := LoadLayered(
+			Input{Source: NoSource()},
+			testSecretBinding(),
+			LayeredOptions[map[string]any]{
+				Definition: Definition[map[string]any]{Version: rule},
+				Underlays: []Underlay{{Inputs: []Input{{
+					Source: File(filepath.Join(directory, "main.yml")), Imports: true, Destination: Keys("application"),
+				}}}},
+			},
+		)
+		if loaded != nil {
+			t.Fatalf("invalid import returned a result: %#v", loaded)
+		}
+		diagnostic := firstDestinationDiagnostic(t, err)
+		if diagnostic.Code != CodeImportVersion || diagnostic.Path.String() != "/application/version" {
+			t.Fatalf("import version diagnostic = %#v", diagnostic)
+		}
+		if diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-2" || diagnostic.Primary.Line != 1 {
+			t.Fatalf("import version location = %#v", diagnostic.Primary)
+		}
+	})
+
+	t.Run("invalid destination has no fictional source location", func(t *testing.T) {
+		t.Parallel()
+		loaded, err := LoadLayered(
+			Input{Source: Bytes("main", []byte("value: present\n")), Destination: Keys("configkit")},
+			testSecretBinding(),
+			LayeredOptions[map[string]any]{},
+		)
+		if loaded != nil {
+			t.Fatalf("invalid destination returned a result: %#v", loaded)
+		}
+		diagnostic := firstDestinationDiagnostic(t, err)
+		if diagnostic.Code != CodeOptionsInvalid || diagnostic.Path.String() != "/configkit" || diagnostic.Primary != nil {
+			t.Fatalf("invalid destination diagnostic = %#v", diagnostic)
+		}
+	})
+
+	t.Run("malformed source directive keeps its local path and location", func(t *testing.T) {
+		t.Parallel()
+		filesystem := NewFilesystem("bundle", fstest.MapFS{
+			"main.yml": &fstest.MapFile{Data: []byte("configkit: {imports: [true]}\nvalue: present\n")},
+		})
+		loaded, err := LoadLayered(
+			Input{Source: filesystem.File("main.yml"), Imports: true, Destination: Keys("application")},
+			testSecretBinding(),
+			LayeredOptions[map[string]any]{},
+		)
+		if loaded != nil {
+			t.Fatalf("malformed directive returned a result: %#v", loaded)
+		}
+		diagnostic := firstDestinationDiagnostic(t, err)
+		if diagnostic.Code != CodeImportInvalid || diagnostic.Path.String() != "/configkit/imports/0" {
+			t.Fatalf("malformed directive diagnostic = %#v", diagnostic)
+		}
+		if diagnostic.Primary == nil || diagnostic.Primary.SourceID != "source-1" || diagnostic.Primary.Line != 1 {
+			t.Fatalf("malformed directive location = %#v", diagnostic.Primary)
+		}
+	})
+}
+
+func firstDestinationDiagnostic(t *testing.T, err error) Diagnostic {
+	t.Helper()
+	loadError, ok := err.(*LoadError)
+	if !ok {
+		t.Fatalf("load error = %#v", err)
+	}
+	diagnostics := loadError.Diagnostics()
+	if len(diagnostics) == 0 {
+		t.Fatal("load error has no diagnostics")
+	}
+	return diagnostics[0]
+}