cgen: fix option ptr initialization (#18893)

vlib/v/gen/c/cgen.v

@@ -1917,6 +1917,9 @@ fn (mut g Gen) expr_with_tmp_var(expr ast.Expr, expr_typ ast.Type, ret_typ ast.T
 				// option ptr assignment simplification
 				if is_ptr_to_ptr_assign {
 					g.write('${tmp_var} = ')
+				} else if expr is ast.PrefixExpr && expr.right is ast.StructInit
+					&& (expr.right as ast.StructInit).init_fields.len == 0 {
+					g.write('_option_none(&(${styp}[]) { ')
 				} else {
 					g.write('_option_ok(&(${styp}[]) { ')
 				}

vlib/v/gen/c/struct.v

@@ -59,7 +59,12 @@ fn (mut g Gen) struct_init(node ast.StructInit) {
 		shared_styp = g.typ(shared_typ)
 		g.writeln('(${shared_styp}*)__dup${shared_styp}(&(${shared_styp}){.mtx = {0}, .val =(${styp}){')
 	} else if is_amp || g.inside_cast_in_heap > 0 {
+		if node.typ.has_flag(.option) {
+			basetyp := g.base_type(node.typ)
+			g.write('(${basetyp}*)memdup(&(${basetyp}){')
+		} else {
 			g.write('(${styp}*)memdup(&(${styp}){')
+		}
 	} else if node.typ.is_ptr() {
 		basetyp := g.typ(node.typ.set_nr_muls(0))
 		if is_multiline {
@@ -305,8 +310,13 @@ fn (mut g Gen) struct_init(node ast.StructInit) {
 	if g.is_shared && !g.inside_opt_data && !g.is_arraymap_set {
 		g.write('}, sizeof(${shared_styp}))')
 	} else if is_amp || g.inside_cast_in_heap > 0 {
+		if node.typ.has_flag(.option) {
+			basetyp := g.base_type(node.typ)
+			g.write(', sizeof(${basetyp}))')
+		} else {
 			g.write(', sizeof(${styp}))')
 		}
+	}
 }
 
 fn (mut g Gen) zero_struct_field(field ast.StructField) bool {

vlib/v/tests/option_init_ptr_test.v

@@ -0,0 +1,26 @@
+struct Abc {
+	a int
+}
+
+fn test_option_init() {
+	a := ?Abc{
+		a: 1
+	}
+	dump(a)
+	b := &?Abc{
+		a: 1
+	}
+	dump(b)
+
+	assert *b? == a?
+}
+
+fn test_option_empty() {
+	a := ?Abc{}
+	dump(a)
+	b := &?Abc{}
+	dump(b)
+
+	assert a == none
+	assert b == none
+}