2 files changed, 34 insertions, 19 deletions

diff --git a/cli/tsc/dts/lib.deno.unstable.d.ts b/cli/tsc/dts/lib.deno.unstable.d.ts
index ce609736b..9e91c8800 100644
--- a/cli/tsc/dts/lib.deno.unstable.d.ts
+++ b/cli/tsc/dts/lib.deno.unstable.d.ts
@@ -443,7 +443,7 @@ declare namespace Deno {
     /** The definition of the function. */
     definition: Fn;
 
-    constructor(pointer: PointerValue, definition: Fn);
+    constructor(pointer: PointerValue, definition: Const<Fn>);
 
     /** Call the foreign function. */
     call: FromForeignFunction<Fn>;
@@ -494,7 +494,7 @@ declare namespace Deno {
     Definition extends UnsafeCallbackDefinition = UnsafeCallbackDefinition,
   > {
     constructor(
-      definition: Definition,
+      definition: Const<Definition>,
       callback: UnsafeCallbackFunction<
         Definition["parameters"],
         Definition["result"]
@@ -562,6 +562,17 @@ declare namespace Deno {
     close(): void;
   }
 
+  /**
+   *  This magic code used to implement better type hints for {@linkcode Deno.dlopen}
+   */
+  type Cast<A, B> = A extends B ? A : B;
+  type Const<T> = Cast<
+    T,
+    | (T extends string | number | bigint | boolean ? T : never)
+    | { [K in keyof T]: Const<T[K]> }
+    | []
+  >;
+
   /** **UNSTABLE**: New API, yet to be vetted.
    *
    * Opens an external dynamic library and registers symbols, making foreign
@@ -611,7 +622,7 @@ declare namespace Deno {
    */
   export function dlopen<S extends ForeignLibraryInterface>(
     filename: string | URL,
-    symbols: S,
+    symbols: Const<S>,
   ): DynamicLibrary<S>;
 
   /** **UNSTABLE**: New API, yet to be vetted.
diff --git a/test_ffi/tests/ffi_types.ts b/test_ffi/tests/ffi_types.ts
index e2054cfe1..a49224cfd 100644
--- a/test_ffi/tests/ffi_types.ts
+++ b/test_ffi/tests/ffi_types.ts
@@ -5,7 +5,7 @@
 const remote = Deno.dlopen(
   "dummy_lib.so",
   {
-    method1: { parameters: ["usize", "usize"], result: "void", callback: true },
+    method1: { parameters: ["usize", "bool"], result: "void", callback: true },
     method2: { parameters: [], result: "void" },
     method3: { parameters: ["usize"], result: "void" },
     method4: { parameters: ["isize"], result: "void" },
@@ -61,16 +61,16 @@ const remote = Deno.dlopen(
     static13: { type: "f32" },
     static14: { type: "f64" },
     static15: { type: "bool" },
-  } as const,
+  },
 );
 
 Deno.dlopen(
   "dummy_lib_2.so",
-  // @ts-expect-error: Returning a function pointer
   // is declared using "pointer" or "function" + UnsafeFnPointer
   {
     wrong_method1: {
       parameters: [],
+      // @ts-expect-error not assignable to type 'NativeResultType'
       result: {
         function: {
           parameters: [],
@@ -78,14 +78,18 @@ Deno.dlopen(
         },
       },
     },
-  } as const,
+  },
 );
 
 // @ts-expect-error: Invalid argument
 remote.symbols.method1(0);
+// @ts-expect-error: Invalid argument
+remote.symbols.method1(0, 0);
+// @ts-expect-error: Invalid argument
+remote.symbols.method1(true, true);
 // @ts-expect-error: Invalid return type
-<number> remote.symbols.method1(0, 0);
-<void> remote.symbols.method1(0n, 0n);
+<number> remote.symbols.method1(0, true);
+<void> remote.symbols.method1(0n, true);
 
 // @ts-expect-error: Expected 0 arguments, but got 1.
 remote.symbols.method2(null);
@@ -170,7 +174,7 @@ const fnptr = new Deno.UnsafeFnPointer(
   {
     parameters: ["u32", "pointer"],
     result: "void",
-  } as const,
+  },
 );
 // @ts-expect-error: Invalid argument
 fnptr.call(null, null);
@@ -180,7 +184,7 @@ const unsafe_callback_wrong1 = new Deno.UnsafeCallback(
   {
     parameters: ["i8"],
     result: "void",
-  } as const,
+  },
   // @ts-expect-error: i8 is not a pointer
   (_: bigint) => {},
 );
@@ -188,7 +192,7 @@ const unsafe_callback_wrong2 = new Deno.UnsafeCallback(
   {
     parameters: ["pointer"],
     result: "u64",
-  } as const,
+  },
   // @ts-expect-error: must return a number or bigint
   (_: Deno.UnsafePointer) => {},
 );
@@ -196,7 +200,7 @@ const unsafe_callback_wrong3 = new Deno.UnsafeCallback(
   {
     parameters: [],
     result: "void",
-  } as const,
+  },
   // @ts-expect-error: no parameters
   (_: Deno.UnsafePointer) => {},
 );
@@ -204,7 +208,7 @@ const unsafe_callback_wrong4 = new Deno.UnsafeCallback(
   {
     parameters: ["u64"],
     result: "void",
-  } as const,
+  },
   // @ts-expect-error: Callback's 64bit parameters are either number or bigint
   (_: number) => {},
 );
@@ -212,21 +216,21 @@ const unsafe_callback_right1 = new Deno.UnsafeCallback(
   {
     parameters: ["u8", "u32", "pointer"],
     result: "void",
-  } as const,
+  },
   (_1: number, _2: number, _3: Deno.PointerValue) => {},
 );
 const unsafe_callback_right2 = new Deno.UnsafeCallback(
   {
     parameters: [],
     result: "u8",
-  } as const,
+  },
   () => 3,
 );
 const unsafe_callback_right3 = new Deno.UnsafeCallback(
   {
     parameters: [],
     result: "function",
-  } as const,
+  },
   // Callbacks can return other callbacks' pointers, if really wanted.
   () => unsafe_callback_right2.pointer,
 );
@@ -234,14 +238,14 @@ const unsafe_callback_right4 = new Deno.UnsafeCallback(
   {
     parameters: ["u8", "u32", "pointer"],
     result: "u8",
-  } as const,
+  },
   (_1: number, _2: number, _3: Deno.PointerValue) => 3,
 );
 const unsafe_callback_right5 = new Deno.UnsafeCallback(
   {
     parameters: ["u8", "i32", "pointer"],
     result: "void",
-  } as const,
+  },
   (_1: number, _2: number, _3: Deno.PointerValue) => {},
 );