[IMP] slots: make defaultContent of slot available to slot as zero

In some cases, it is useful for a slot to be able to keep the default
content of the slot it is supposed to replace and simply add extra
content instead of simply replacing it entirely. This commit makes the
default content of a slot available in the context as `zero` similar to
how t-call can access the content that was inside the t-call.

src/runtime/template_helpers.ts

@@ -29,6 +29,13 @@ function callSlot(
   const slots = ctx.props.slots || {};
   const { __render, __ctx, __scope } = slots[name] || {};
   const slotScope = ObjectCreate(__ctx || {});
+  if (defaultContent) {
+    Object.defineProperty(slotScope, helpers.zero, {
+      get() {
+        return defaultContent(ctx, parent, key);
+      },
+    });
+  }
   if (__scope) {
     slotScope[__scope] = extra;
   }

tests/components/__snapshots__/slots.test.ts.snap

@@ -349,6 +349,42 @@ exports[`slots content is the default slot 2`] = `
 }"
 `;
 
+exports[`slots default content is available as 0 1`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { zero, markRaw } = helpers;
+  const comp1 = app.createComponent(\`Dialog\`, true, true, false, true);
+  
+  function slot1(ctx, node, key = \\"\\") {
+    const b2 = text(\`Before - \`);
+    const b3 = ctx[zero];
+    const b4 = text(\` - After\`);
+    return multi([b2, b3, b4]);
+  }
+  
+  return function template(ctx, node, key = \\"\\") {
+    return comp1({slots: markRaw({'default': {__render: slot1.bind(this), __ctx: ctx}})}, key + \`__1\`, node, this, null);
+  }
+}"
+`;
+
+exports[`slots default content is available as 0 2`] = `
+"function anonymous(app, bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, comment } = bdom;
+  let { callSlot } = helpers;
+  
+  function defaultContent1(ctx, node, key = \\"\\") {
+    return text(\`default content\`);
+  }
+  
+  return function template(ctx, node, key = \\"\\") {
+    return callSlot(ctx, node, key, 'default', false, {}, defaultContent1.bind(this));
+  }
+}"
+`;
+
 exports[`slots default content is not rendered if named slot is provided 1`] = `
 "function anonymous(app, bdom, helpers
 ) {

tests/components/slots.test.ts

@@ -1933,4 +1933,17 @@ describe("slots", () => {
       "<p>1 hello <div>child</div></p><p>2 hello <div>child</div></p>"
     );
   });
+
+  test("default content is available as 0", async () => {
+    class Dialog extends Component {
+      static template = xml`<t t-slot="default">default content</t>`;
+    }
+    class Parent extends Component {
+      static template = xml`<Dialog>Before - <t t-out="0"/> - After</Dialog>`;
+      static components = { Dialog };
+    }
+
+    await mount(Parent, fixture);
+    expect(fixture.innerHTML).toBe("Before - default content - After");
+  });
 });