cgen: fix interface generic smartcast (#20609)

vlib/v/gen/c/cgen.v

@@ -4649,7 +4649,7 @@ fn (mut g Gen) ident(node ast.Ident) {
 				g.write('(*(')
 			}
 			if node.obj.smartcasts.len > 0 {
-				obj_sym := g.table.sym(node.obj.typ)
+				obj_sym := g.table.sym(g.unwrap_generic(node.obj.typ))
 				if !prevent_sum_type_unwrapping_once {
 					for _, typ in node.obj.smartcasts {
 						is_option_unwrap := is_option && typ == node.obj.typ.clear_flag(.option)
@@ -4665,7 +4665,9 @@ fn (mut g Gen) ident(node ast.Ident) {
 								g.write('*')
 							}
 						} else if (g.inside_interface_deref && g.table.is_interface_var(node.obj))
-							|| node.obj.ct_type_var == .smartcast {
+							|| node.obj.ct_type_var == .smartcast
+							|| (obj_sym.kind == .interface_
+							&& g.table.type_kind(node.obj.typ) == .any) {
 							g.write('*')
 						} else if is_option {
 							g.write('*(${g.base_type(node.obj.typ)}*)')

vlib/v/tests/generic_smartcast_test.v

@@ -0,0 +1,46 @@
+fn cast_interface[T, U](u U) T {
+	$if U is $interface {
+		if u is T {
+			return u
+		} else {
+			panic('expected t to be ${typeof[T]().name}, got ${typeof[U]().name}')
+		}
+	} $else {
+		$compile_error('not an interface')
+	}
+}
+
+interface Foo {
+	f()
+}
+
+struct Bar {}
+
+fn (bar Bar) f() {}
+
+struct Baz {}
+
+fn (baz Baz) f() {}
+
+fn (_ Bar) g() int {
+	return 0
+}
+
+fn (_ Baz) g() int {
+	return 1
+}
+
+fn f(foo Foo) int {
+	if foo is Bar {
+		return cast_interface[Bar, Foo](foo).g()
+	}
+	if foo is Baz {
+		return cast_interface[Baz, Foo](foo).g()
+	}
+	return -1
+}
+
+fn test_main() {
+	assert f(Bar{}) == 0
+	assert f(Baz{}) == 1
+}