cgen: fix generated code for assignments of if ternary non-opt values to an option struct member (#19485)

vlib/v/gen/c/cgen.v

@@ -1813,7 +1813,11 @@ fn (mut g Gen) stmts_with_tmp_var(stmts []ast.Stmt, tmp_var string) bool {
 							g.expr(stmt.expr)
 							g.writeln(';')
 						} else {
-							ret_typ := g.fn_decl.return_type.clear_flag(.option)
+							ret_typ := if g.inside_assign {
+								stmt.typ
+							} else {
+								g.fn_decl.return_type.clear_flag(.option)
+							}
 							styp = g.base_type(ret_typ)
 							g.write('_option_ok(&(${styp}[]) { ')
 							g.expr_with_cast(stmt.expr, stmt.typ, ret_typ)

vlib/v/tests/if_assign_test.v

@@ -0,0 +1,15 @@
+struct Foo {
+mut:
+	option ?string
+}
+
+fn test_main() {
+	mut foo := Foo{}
+	some_val := 1
+	foo.option = if some_val > 0 {
+		'awesome'
+	} else {
+		none
+	}
+	assert foo.option? == 'awesome'
+}