checker: fix struct init with update expr (fix #9472) (#15460)

vlib/v/checker/struct.v

@@ -563,11 +563,6 @@ pub fn (mut c Checker) struct_init(mut node ast.StructInit) ast.Type {
 					node.update_expr.pos())
 			}
 		}
-		if !node.update_expr.is_lvalue() {
-			// cgen will repeat `update_expr` for each field
-			// so enforce an lvalue for efficiency
-			c.error('expression is not an lvalue', node.update_expr.pos())
-		}
 	}
 	return node.typ
 }

vlib/v/checker/tests/struct_init_update_type_err.out

@@ -19,13 +19,6 @@ vlib/v/checker/tests/struct_init_update_type_err.vv:20:6: error: struct `Foo2` i
       |            ~~
    21 |     }
    22 |     _ = Foo{
-vlib/v/checker/tests/struct_init_update_type_err.vv:23:6: error: expression is not an lvalue
-   21 |     }
-   22 |     _ = Foo{
-   23 |         ...Foo{}
-      |            ~~~~~
-   24 |     }
-   25 | }
 vlib/v/checker/tests/struct_init_update_type_err.vv:32:6: error: struct `Empty` is not compatible with struct `Foo`
    30 |     e := Empty{}
    31 |     _ = Foo{

vlib/v/gen/c/struct.v

@@ -8,6 +8,20 @@ import v.ast
 const skip_struct_init = ['struct stat', 'struct addrinfo']
 
 fn (mut g Gen) struct_init(node ast.StructInit) {
+	mut is_update_tmp_var := false
+	mut tmp_update_var := ''
+	if node.has_update_expr && !node.update_expr.is_lvalue() {
+		tmp_update_var = g.new_tmp_var()
+		is_update_tmp_var = true
+		s := g.go_before_stmt(0)
+		styp := g.typ(node.update_expr_type)
+		g.empty_line = true
+		g.write('$styp $tmp_update_var = ')
+		g.expr(node.update_expr)
+		g.writeln(';')
+		g.empty_line = false
+		g.write(s)
+	}
 	styp := g.typ(node.typ)
 	mut shared_styp := '' // only needed for shared x := St{...
 	if styp in c.skip_struct_init {
@@ -167,8 +181,8 @@ fn (mut g Gen) struct_init(node ast.StructInit) {
 				// unions thould have exactly one explicit initializer
 				continue
 			}
+			field_name := c_name(field.name)
 			if field.typ.has_flag(.optional) {
-				field_name := c_name(field.name)
 				g.write('.$field_name = {EMPTY_STRUCT_INITIALIZATION},')
 				initialized = true
 				continue
@@ -177,7 +191,12 @@ fn (mut g Gen) struct_init(node ast.StructInit) {
 				continue
 			}
 			if node.has_update_expr {
-				g.expr(node.update_expr)
+				g.write('.$field_name = ')
+				if is_update_tmp_var {
+					g.write(tmp_update_var)
+				} else {
+					g.expr(node.update_expr)
+				}
 				if node.update_expr_type.is_ptr() {
 					g.write('->')
 				} else {

vlib/v/tests/struct_init_with_update_test.v

@@ -0,0 +1,32 @@
+module main
+
+struct Author {
+	username string
+	name     string
+	pass     string
+	height   int
+	age      int
+}
+
+fn cool_author() Author {
+	return Author{
+		username: 'Terisback'
+		name: 'Bob'
+		pass: '123456'
+		height: 175
+		age: 18
+	}
+}
+
+fn test_struct_init_with_update_expr() {
+	mut o := Author{
+		...cool_author()
+		age: 21
+	}
+	println(o)
+	assert o.username == 'Terisback'
+	assert o.name == 'Bob'
+	assert o.pass == '123456'
+	assert o.height == 175
+	assert o.age == 21
+}