add compile error for async call of function pointer

test/compile_errors.zig

@@ -2,6 +2,18 @@ const tests = @import("tests.zig");
 const builtin = @import("builtin");
 
 pub fn addCases(cases: *tests.CompileErrorContext) void {
+    cases.add(
+        "runtime-known function called with async keyword",
+        \\export fn entry() void {
+        \\    var ptr = afunc;
+        \\    _ = async ptr();
+        \\}
+        \\
+        \\async fn afunc() void { }
+    ,
+        "tmp.zig:3:15: error: function is not comptime-known; @asyncCall required",
+    );
+
     cases.add(
         "function with ccc indirectly calling async function",
         \\export fn entry() void {