cgen: don't voidptr cast option/result functions (fix #17204) (#17207)

vlib/v/gen/c/cgen.v

@@ -2580,7 +2580,8 @@ fn (mut g Gen) expr_with_cast(expr ast.Expr, got_type_raw ast.Type, expected_typ
 			return
 		}
 	}
-	if exp_sym.kind == .function {
+	if exp_sym.kind == .function && !expected_type.has_flag(.option)
+		&& !expected_type.has_flag(.result) {
 		g.write('(voidptr)')
 	}
 	// no cast

vlib/v/tests/if_expr_nested_with_option_result_test.v

@@ -37,11 +37,30 @@ pub fn bar2(i int) !int {
 	}
 }
 
+pub fn bar3(ok bool) ?fn () string {
+	return if ok {
+		fn () string {
+			return 'yes'
+		}
+	} else {
+		none
+	}
+}
+
+pub fn bar4(ok bool) !fn () string {
+	return if ok {
+		fn () string {
+			return 'yes'
+		}
+	} else {
+		error('no:error')
+	}
+}
+
 fn test_if_expr_nested_with_option_result() {
 	ret11 := bar1(0) or { 0 }
 	println(ret11)
 	assert ret11 == 2
-
 	ret12 := bar1(1) or { 0 }
 	println(ret12)
 	assert ret12 == 3
@@ -49,8 +68,37 @@ fn test_if_expr_nested_with_option_result() {
 	ret21 := bar2(0) or { 0 }
 	println(ret21)
 	assert ret21 == 2
-
 	ret22 := bar2(1) or { 0 }
 	println(ret22)
 	assert ret22 == 3
+
+	ret31 := bar3(true) or {
+		fn () string {
+			return 'no:default'
+		}
+	}
+	println(ret31())
+	assert ret31() == 'yes'
+	ret32 := bar3(false) or {
+		fn () string {
+			return 'no:default'
+		}
+	}
+	println(ret32())
+	assert ret32() == 'no:default'
+
+	ret41 := bar4(true) or {
+		fn [err] () string {
+			return err.msg()
+		}
+	}
+	println(ret41())
+	assert ret41() == 'yes'
+	ret42 := bar4(false) or {
+		fn [err] () string {
+			return err.msg()
+		}
+	}
+	println(ret42())
+	assert ret42() == 'no:error'
 }