checker: disallow Bar{...foo} when Bar needs more fields, than what foo has (#16609)

vlib/v/checker/checker.v

@@ -4003,6 +4003,22 @@ fn (mut c Checker) error(message string, pos token.Pos) {
 	c.warn_or_error(msg, pos, false)
 }
 
+fn (c &Checker) check_struct_signature_init_fields(from ast.Struct, to ast.Struct, node ast.StructInit) bool {
+	if node.fields.len == 0 {
+		return from.fields.len == to.fields.len
+	}
+
+	mut count_not_in_from := 0
+	for field in node.fields {
+		filtered := from.fields.filter(it.name == field.name)
+		if filtered.len != 1 {
+			count_not_in_from++
+		}
+	}
+
+	return (from.fields.len + count_not_in_from) == to.fields.len
+}
+
 // check `to` has all fields of `from`
 fn (c &Checker) check_struct_signature(from ast.Struct, to ast.Struct) bool {
 	// Note: `to` can have extra fields

vlib/v/checker/struct.v

@@ -603,7 +603,8 @@ fn (mut c Checker) struct_init(mut node ast.StructInit) ast.Type {
 			from_info := from_sym.info as ast.Struct
 			to_info := to_sym.info as ast.Struct
 			// TODO this check is too strict
-			if !c.check_struct_signature(from_info, to_info) {
+			if !c.check_struct_signature(from_info, to_info)
+				|| !c.check_struct_signature_init_fields(from_info, to_info, node) {
 				c.error('struct `${from_sym.name}` is not compatible with struct `${to_sym.name}`',
 					node.update_expr.pos())
 			}

vlib/v/checker/tests/check_incompatible_struct.out

@@ -0,0 +1,14 @@
+vlib/v/checker/tests/check_incompatible_struct.vv:17:6: error: struct `Foo` is not compatible with struct `Bar`
+   15 |     }
+   16 |     bar3 := Bar{
+   17 |         ...foo
+      |            ~~~
+   18 |         b: 1
+   19 |     }
+vlib/v/checker/tests/check_incompatible_struct.vv:26:6: error: struct `Foo` is not compatible with struct `Bar`
+   24 |     }
+   25 |     bar := Bar{
+   26 |         ...foo
+      |            ~~~
+   27 |     }
+   28 |     print(bar)

vlib/v/checker/tests/check_incompatible_struct.vv

@@ -0,0 +1,32 @@
+struct Foo {
+	b int
+}
+
+struct Bar {
+	a int
+	b int
+}
+
+fn main() {
+	foo := Foo{2}
+	bar2 := Bar{
+		...foo
+		a: 1
+	}
+	bar3 := Bar{
+		...foo
+		b: 1
+	}
+	bar4 := Bar{
+		...foo
+		b: 1
+		a: 2
+	}
+	bar := Bar{
+		...foo
+	}
+	print(bar)
+	print(bar2)
+	print(bar3)
+	print(bar4)
+}