fix(compilers/openapi): read $id past an empty pointer segment

compilers/openapi/internal/schema/schema.go

@@ -1443,20 +1443,35 @@ func componentSchemaAt(c lowering.Ctx, pointer string) *oas3.Schema {
 func declaresResourceIDAbove(c lowering.Ctx, pointer string) bool {
 	view := nodeview.New()
 	cur := nodeview.DocumentRoot(nodeview.Deref(c.Doc.GetRootNode()))
-	for seg := range strings.SplitSeq(pointer, "/") {
+	for _, token := range pointerTokens(pointer) {
 		if cur == nil {
 			return false
 		}
 		if view.ChildByToken(cur, "$id") != nil {
 			return true
 		}
-		if seg != "" { // every pointer starts with the empty segment
-			cur = nodeview.Deref(view.ChildByToken(cur, ids.UnescapeSegment(seg)))
-		}
+		cur = nodeview.Deref(view.ChildByToken(cur, ids.UnescapeSegment(token)))
 	}
 	return cur != nil && view.ChildByToken(cur, "$id") != nil
 }
 
+// pointerTokens returns the RFC 6901 reference tokens of pointer, still escaped.
+// Splitting on '/' yields one leading empty segment that is the split's artifact
+// rather than a token, and only that one is: a later empty segment names the key
+// "", which is how a component schema named "" is addressed
+// (/components/schemas/). Dropping every empty segment stopped the walk above
+// such a position and read the $id of its parent instead of its own.
+//
+// A string with no leading '/' has no tokens at all: the empty pointer names the
+// whole document, and a relative pointer names no position in it.
+func pointerTokens(pointer string) []string {
+	rest, found := strings.CutPrefix(pointer, "/")
+	if !found {
+		return nil
+	}
+	return strings.Split(rest, "/")
+}
+
 // dynamicFragment returns the plain fragment name a $dynamicRef addresses. Only
 // the same-document `#name` spelling resolves here: a URI part names another
 // resource (Milestone 1 interns only same-file targets) and a `#/…` pointer

compilers/openapi/internal/schema/schema_internal_test.go

@@ -212,6 +212,42 @@ func TestDeclaresResourceIDAbove_WithoutARawTree(t *testing.T) {
 		"a document with no raw tree declares no resource anywhere")
 }
 
+// TestDeclaresResourceIDAbove_EmptySegmentIsATokenNotAnArtifact pins which empty
+// segments the path walk may drop. Splitting a pointer on '/' produces one
+// leading empty segment that no reference token stands behind; every later one
+// is the token naming the key "", which is how a schema literally named "" is
+// addressed. Dropping those stopped the walk above the position and read the
+// $id of the components/schemas map instead of the schema's own.
+func TestDeclaresResourceIDAbove_EmptySegmentIsATokenNotAnArtifact(t *testing.T) {
+	t.Parallel()
+	l, diags := loweredFor(t, `openapi: 3.1.0
+info: {title: T, version: "1"}
+paths: {}
+components:
+  schemas:
+    "":
+      $id: https://example.com/empty
+      properties: {x: {type: string}}
+    Sibling: {type: string}
+`)
+	requireNoErrorDiags(t, diags)
+
+	for name, tc := range map[string]struct {
+		pointer string
+		want    bool
+	}{
+		"the position is named by a trailing empty token": {"/components/schemas/", true},
+		"an interior empty token still descends":          {"/components/schemas//properties/x", true},
+		"a sibling sits above no $id at all":              {"/components/schemas/Sibling", false},
+		"the empty pointer names the document root":       {"", false},
+	} {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+			assert.Equal(t, tc.want, declaresResourceIDAbove(l.ctx, tc.pointer))
+		})
+	}
+}
+
 // TestDynamicAnchors_WalksEveryNodeShape drives the raw-tree walk over the
 // shapes a YAML document can present, rather than only the mappings a schema
 // happens to be written as. The walk reads the raw tree because oas3.Schema has

compilers/openapi/internal/schema/schema_test.go

@@ -3021,6 +3021,17 @@ func TestDynamicRef_IrreducibleIsKeptAndSaysWhy(t *testing.T) {
 			schemas: anchor + "    A: {$id: 'https://example.com/a', $dynamicRef: '#m'}\n",
 			wantWhy: `an $id at or above "/components/schemas/A"`,
 		},
+		{
+			// The pointer of a schema named "" ends in an empty reference token,
+			// which the path walk once dropped as if it were the artifact of
+			// splitting a pointer on '/' — reading the components/schemas map for
+			// the $id instead of the schema itself, and expanding across a
+			// boundary it should have stopped at.
+			name:    `a schema named "" is still in a resource of its own`,
+			schemas: anchor + "    \"\": {$id: 'https://example.com/empty', $dynamicRef: '#m'}\n",
+			wantWhy: `an $id at or above "/components/schemas/"`,
+			at:      "t/anon/components/schemas/",
+		},
 		{
 			name: "an enclosing schema starts the resource",
 			schemas: anchor +