cgen: fix struct field init from shared struct field (#13343)

vlib/v/gen/c/cgen.v

@@ -2471,10 +2471,8 @@ fn (mut g Gen) expr_with_cast(expr ast.Expr, got_type_raw ast.Type, expected_typ
 	}
 	// Generic dereferencing logic
 	neither_void := ast.voidptr_type !in [got_type, expected_type]
-	to_shared := expected_type.has_flag(.shared_f) && !got_type_raw.has_flag(.shared_f)
-		&& !expected_type.has_flag(.optional)
-	// from_shared := got_type_raw.has_flag(.shared_f) && !expected_type.has_flag(.shared_f)
-	if to_shared {
+	if expected_type.has_flag(.shared_f) && !got_type_raw.has_flag(.shared_f)
+		&& !expected_type.has_flag(.optional) {
 		shared_styp := exp_styp[0..exp_styp.len - 1] // `shared` implies ptr, so eat one `*`
 		if got_type_raw.is_ptr() {
 			g.error('cannot convert reference to `shared`', expr.pos())
@@ -2492,6 +2490,13 @@ fn (mut g Gen) expr_with_cast(expr ast.Expr, got_type_raw ast.Type, expected_typ
 		g.is_shared = old_is_shared
 		g.writeln('}, sizeof($shared_styp))')
 		return
+	} else if got_type_raw.has_flag(.shared_f) && !expected_type.has_flag(.shared_f) {
+		if expected_type.is_ptr() {
+			g.write('&')
+		}
+		g.expr(expr)
+		g.write('->val')
+		return
 	}
 	if got_is_ptr && !expected_is_ptr && neither_void && exp_sym.kind != .placeholder
 		&& expr !is ast.InfixExpr {
@@ -6315,10 +6320,6 @@ fn (mut g Gen) size_of(node ast.SizeOf) {
 	g.write('sizeof(${util.no_dots(styp)})')
 }
 
-fn (g &Gen) is_importing_os() bool {
-	return 'os' in g.table.imports
-}
-
 fn (mut g Gen) go_expr(node ast.GoExpr) {
 	line := g.go_before_stmt(0)
 	mut handle := ''

vlib/v/tests/struct_shared_field_init_test.v

@@ -0,0 +1,22 @@
+struct AA {
+	b shared BB
+}
+
+struct BB {
+	a &int
+}
+
+struct CC {
+	a BB
+}
+
+fn test_struct_shared_field_init() {
+	a := 3
+	table := &AA{
+		b: BB{&a}
+	}
+	c := CC{
+		a: table.b
+	}
+	assert *c.a.a == 3
+}