checker: allow for f() or { T{} } in a generic method, for fn f() ?T {, being called with T, being a container like []int etc, not just a primitive type like int (#22672)

vlib/v/checker/checker.v

@@ -1419,6 +1419,9 @@ fn (mut c Checker) check_or_last_stmt(mut stmt ast.Stmt, ret_type ast.Type, expr
 					}
 					type_name := c.table.type_to_str(last_stmt_typ)
 					expected_type_name := c.table.type_to_str(ret_type.clear_option_and_result())
+					if ret_type.has_flag(.generic) {
+						return
+					}
 					c.error('wrong return type `${type_name}` in the `or {}` block, expected `${expected_type_name}`',
 						stmt.expr.pos())
 				}

vlib/v/tests/generics/generic_default_expression_in_or_block_test.v

@@ -0,0 +1,91 @@
+struct St[T] {
+mut:
+	a []T
+}
+
+fn (s St[T]) peek() ?T {
+	if s.a.len > 0 {
+		return s.a[0]
+	} else {
+		return none
+	}
+}
+
+fn (mut s St[T]) peek_or_default() T {
+	return s.peek() or { T{} }
+}
+
+fn (mut s St[T]) push(e T) {
+	x := s.peek() or { T{} } // this is deliberate
+	$if x is $array {
+		dump(x)
+		if s.a.len > 0 {
+			assert x.len > 0
+		} else {
+			assert x.len == 0
+		}
+	}
+	$if x is $int {
+		dump(x)
+		if s.a.len > 0 {
+			assert x == 2
+		} else {
+			assert x == 0
+		}
+	}
+	$if x is $map {
+		dump(x)
+		if s.a.len > 0 {
+			assert x == {
+				'abc': u8(4)
+				'def': 7
+			}
+		}
+	}
+	s.a << e
+}
+
+fn test_ints() {
+	mut s := St[int]{}
+	r := s.peek_or_default()
+	s.push(2)
+	s.push(3)
+	s.push(99)
+	assert s.a == [2, 3, 99]
+	dump(s)
+}
+
+fn test_array_of_ints() {
+	mut s := St[[]int]{}
+	r := s.peek_or_default()
+	s.push([2, 3, 5])
+	s.push([]int{})
+	s.push([10, 20, 99])
+	s.push([55, 88])
+	assert s.a == [[2, 3, 5], []int{}, [10, 20, 99], [55, 88]]
+	dump(s)
+}
+
+fn test_maps_of_u8s() {
+	mut s := St[map[string]u8]{}
+	r := s.peek_or_default()
+	s.push({
+		'abc': u8(4)
+		'def': 7
+	})
+	s.push({
+		'xyz': u8(13)
+	})
+	s.push({
+		'zzz': u8(99)
+	})
+	assert s.a == [{
+		'abc': u8(4)
+		'def': 7
+	}, {
+		'xyz': u8(13)
+	}, {
+		'zzz': u8(99)
+	}]
+	dump(s)
+}