<style>
.grid {
    display: grid;
    grid-template-columns: 1fr 1fr;
    grid-template-rows: min-content auto;
}
.right {
    background-color: lightpink;
    grid-area: 1 / 2 / auto / -1;
}
.left {
    grid-area: 1 / 1 / -1 / auto;
    background-color: lightslategrey;
}
.inner {
    background-color: lightskyblue;
}
</style>
<div class="grid">
<div class="right">a</div>
<div class="left">
    <div class="inner">b</div>
</div>
</div>