checker: restrict multiple union fields initialised at once with a ni…

…cer checker error, instead of producing an enigmatic error at cgen time (#22196)

vlib/v/checker/struct.v

@@ -461,6 +461,10 @@ fn (mut c Checker) struct_init(mut node ast.StructInit, is_field_zero_struct_ini
 				c.cur_struct_concrete_types = old_cur_struct_concrete_types
 			}
 		}
+		if struct_sym.info.is_union && node.init_fields.len > 1 {
+			c.error('union `${struct_sym.name}` can have only one field initialised',
+				node.pos)
+		}
 	} else if struct_sym.info is ast.Alias {
 		parent_sym := c.table.sym(struct_sym.info.parent_type)
 		// e.g. ´x := MyMapAlias{}´, should be a cast to alias type ´x := MyMapAlias(map[...]...)´
@@ -973,6 +977,24 @@ fn (mut c Checker) check_uninitialized_struct_fields_and_embeds(node ast.StructI
 	}
 
 	for embed in info.embeds {
+		embed_sym := c.table.sym(embed)
+		if embed_sym.info is ast.Struct {
+			if embed_sym.info.is_union {
+				mut embed_union_fields := c.table.struct_fields(embed_sym)
+				mut found := false
+				for init_field in inited_fields {
+					for union_field in embed_union_fields {
+						if init_field == union_field.name && found {
+							c.error('embed union `${embed_sym.name}` can have only one field initialised',
+								node.pos)
+						}
+						if init_field == union_field.name {
+							found = true
+						}
+					}
+				}
+			}
+		}
 		mut zero_struct_init := ast.StructInit{
 			pos: node.pos
 			typ: embed

vlib/v/checker/tests/union_init_having_more_fields_err.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/union_init_having_more_fields_err.vv:23:7: error: embed union `Head` can have only one field initialised
+   21 | 
+   22 | fn union_embed__having_more_fields_used() {
+   23 |     _ := Message{
+      |          ~~~~~~~~
+   24 |         from: 1
+   25 |         src:  1
+vlib/v/checker/tests/union_init_having_more_fields_err.vv:30:7: error: union `Head` can have only one field initialised
+   28 | 
+   29 | fn union_having_more_fields_used() {
+   30 |     _ := Head{
+      |          ~~~~~
+   31 |         from: 1
+   32 |         src:  1

vlib/v/checker/tests/union_init_having_more_fields_err.vv

@@ -0,0 +1,34 @@
+pub struct Lines {
+pub:
+	from u8
+	to   u8
+	dst  u8
+	src  u8
+}
+
+pub union Head {
+	Lines
+pub:
+	serial [3]u8
+}
+
+pub struct Message {
+	Head
+pub:
+	cmd     u8
+	payload []u8
+}
+
+fn union_embed__having_more_fields_used() {
+	_ := Message{
+		from: 1
+		src:  1
+	}
+}
+
+fn union_having_more_fields_used() {
+	_ := Head{
+		from: 1
+		src:  1
+	}
+}