Fix optional type resolution

extensions/src/main/java/dev/cel/extensions/CelOptionalLibrary.java

@@ -28,12 +28,14 @@
 import dev.cel.common.CelFunctionDecl;
 import dev.cel.common.CelIssue;
 import dev.cel.common.CelOverloadDecl;
+import dev.cel.common.CelVarDecl;
 import dev.cel.common.ast.CelExpr;
 import dev.cel.common.types.ListType;
 import dev.cel.common.types.MapType;
 import dev.cel.common.types.OptionalType;
 import dev.cel.common.types.SimpleType;
 import dev.cel.common.types.TypeParamType;
+import dev.cel.common.types.TypeType;
 import dev.cel.compiler.CelCompilerLibrary;
 import dev.cel.parser.CelMacro;
 import dev.cel.parser.CelMacroExprFactory;
@@ -93,6 +95,11 @@ public void setCheckerOptions(CelCheckerBuilder checkerBuilder) {
     OptionalType optionalTypeV = OptionalType.create(paramTypeV);
     ListType listTypeV = ListType.create(paramTypeV);
     MapType mapTypeKv = MapType.create(paramTypeK, paramTypeV);
+
+    // Type declaration for optional_type -> type(optional_type(V))
+    checkerBuilder.addVarDeclarations(
+        CelVarDecl.newVarDeclaration(OptionalType.NAME, TypeType.create(optionalTypeV)));
+
     checkerBuilder.addFunctionDeclarations(
         CelFunctionDecl.newFunctionDeclaration(
             Function.OPTIONAL_OF.getFunction(),

extensions/src/test/java/dev/cel/extensions/BUILD.bazel

@@ -27,6 +27,7 @@ java_library(
         "//parser:macro",
         "//runtime",
         "//runtime:interpreter_util",
+        "@cel_spec//proto/cel/expr:expr_java_proto",
         "@maven//:com_google_guava_guava",
         "@maven//:com_google_protobuf_protobuf_java",
         "@maven//:com_google_protobuf_protobuf_java_util",

extensions/src/test/java/dev/cel/extensions/CelOptionalLibraryTest.java

@@ -17,6 +17,7 @@
 import static com.google.common.truth.Truth.assertThat;
 import static org.junit.Assert.assertThrows;
 
+import dev.cel.expr.Value;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.primitives.UnsignedLong;
@@ -1409,4 +1410,23 @@ public void optionalFlatMapMacro_withNonIdent_throws() {
         .hasMessageThat()
         .contains("optFlatMap() variable name must be a simple identifier");
   }
+
+  @Test
+  public void optionalType_typeResolution() throws Exception {
+    Cel cel = newCelBuilder().build();
+
+    CelAbstractSyntaxTree ast = cel.compile("optional_type").getAst();
+
+    assertThat(cel.createProgram(ast).eval())
+        .isEqualTo(Value.newBuilder().setTypeValue("optional_type").build());
+  }
+
+  @Test
+  public void optionalType_typeComparison() throws Exception {
+    Cel cel = newCelBuilder().build();
+
+    CelAbstractSyntaxTree ast = cel.compile("type(optional.none()) == optional_type").getAst();
+
+    assertThat(cel.createProgram(ast).eval()).isEqualTo(true);
+  }
 }

runtime/src/main/java/dev/cel/runtime/StandardTypeResolver.java

@@ -34,6 +34,7 @@
 import dev.cel.common.types.TypeType;
 import java.util.Collection;
 import java.util.Map;
+import java.util.Optional;
 import org.jspecify.nullness.Nullable;
 
 /**
@@ -76,6 +77,8 @@ private static ImmutableMap<Value, Class<?>> commonTypes(boolean unsignedLongs)
         // Aggregate types.
         .put(createType("list"), Collection.class)
         .put(createType("map"), Map.class)
+        // Optional type
+        .put(createType("optional_type"), Optional.class)
         .buildOrThrow();
   }