checker: ensure ?Struct is not parsed as ??Struct when param is ?T (fix #25559) (#25564)

Author: dy-tea

vlib/v/checker/check_types.v

@@ -1030,7 +1030,16 @@ fn (mut c Checker) infer_fn_generic_types(func &ast.Fn, mut node ast.CallExpr) {
 			arg := node.args[arg_i]
 			param_sym := c.table.sym(param.typ)
 
-			if param.typ.has_flag(.generic) && param_sym.name == gt_name {
+			if (param.typ.has_flag(.option) && arg.typ.has_flag(.option))
+				|| (param.typ.has_flag(.result) && arg.typ.has_flag(.result)) {
+				param_inner := param.typ.clear_option_and_result()
+				if param_inner.has_flag(.generic) && c.table.sym(param_inner).name == gt_name {
+					typ = arg.typ.clear_option_and_result()
+					if param_inner.nr_muls() > 0 && typ.nr_muls() > 0 {
+						typ = typ.set_nr_muls(0)
+					}
+				}
+			} else if param.typ.has_flag(.generic) && param_sym.name == gt_name {
 				typ = ast.mktyp(arg.typ)
 				if typ == ast.nil_type {
 					typ = ast.voidptr_type

vlib/v/tests/options/option_unwrap_unwrap_or_generic_fn_test.v

@@ -0,0 +1,37 @@
+fn unwrap[T](t ?T) T {
+	return t or { panic('none') }
+}
+
+fn unwrap_or[T](t ?T, t_or T) T {
+	return t or { t_or }
+}
+
+struct S1 {
+	value int
+}
+
+struct S2 {}
+
+fn test_unwrap() {
+	s1 := ?S1{
+		value: 42
+	}
+	assert unwrap(s1).value == 42
+
+	// see #25566
+	s2 := ?S2(S2{})
+	assert unwrap(s2) == S2{}
+}
+
+fn test_unwrap_or() {
+	s1 := ?S1{
+		value: 42
+	}
+	assert unwrap_or(s1, S1{ value: 12 }).value == 42
+
+	s1n := ?S1(none)
+	assert unwrap_or(s1n, S1{ value: 12 }).value == 12
+
+	s2 := ?S2{}
+	assert unwrap_or(s2, S2{}) == S2{}
+}