[FIX] qweb: t-set directive

This commit reintroduces some tests for the t-set directive and make
them pass. For that, it was necessary to adapt the qweb compiler in
order to get the following behaviors:

A t-set can affect parent contexts (up to the first parent tagged as
boundary) when the key changed is found in one of the parent contexts.
Some context are marked as boundaries in such a way that

   - rendering contexts (e.g. components) cannot be modified via a t-set.
   - a t-set in a t-call body or in a called template can never change a
     context above the t-call context.

Code prettification has been done.
Snapshots have been modified.

tests/qweb/t_set.test.ts

@@ -1,4 +1,4 @@
-import { renderToString, snapshotEverything } from "../helpers";
+import { renderToString, snapshotEverything, TestContext } from "../helpers";
 
 snapshotEverything();
 
@@ -12,6 +12,13 @@ describe("t-set", () => {
     expect(renderToString(template)).toBe("<div>ok</div>");
   });
 
+  test("t-set does not modify render context existing key values", () => {
+    const template = `<div><t t-set="value" t-value="35"/><t t-esc="value"/></div>`;
+    const ctx = { value: 17 };
+    expect(renderToString(template, ctx)).toBe("<div>35</div>");
+    expect(ctx.value).toBe(17);
+  });
+
   test("set from attribute literal (no outside div)", () => {
     const template = `<t><t t-set="value" t-value="'ok'"/><t t-esc="value"/></t>`;
     expect(renderToString(template)).toBe("ok");
@@ -179,4 +186,92 @@ describe("t-set", () => {
 
     expect(renderToString(template)).toBe("<div>Truthy</div>");
   });
+
+  test("t-set outside modified in t-foreach", async () => {
+    const template = `
+      <div>
+        <t t-set="iter" t-value="0"/>
+        <t t-foreach="['a','b']" t-as="val" t-key="val">
+          <p>InLoop: <t t-esc="iter"/></p>
+          <t t-set="iter" t-value="iter + 1"/>
+        </t>
+        <p>EndLoop: <t t-esc="iter"/></p>
+      </div>
+    `;
+    expect(renderToString(template)).toBe(
+      "<div><p>InLoop: 0</p><p>InLoop: 1</p><p>EndLoop: 2</p></div>"
+    );
+  });
+
+  test("t-set outside modified in t-foreach increment-after operator", async () => {
+    const template = `
+      <div>
+        <t t-set="iter" t-value="0"/>
+        <t t-foreach="['a','b']" t-as="val" t-key="val">
+          <p>InLoop: <t t-esc="iter"/></p>
+          <t t-set="iter" t-value="iter++"/>
+        </t>
+        <p>EndLoop: <t t-esc="iter"/></p>
+      </div>
+    `;
+    expect(renderToString(template)).toBe(
+      "<div><p>InLoop: 0</p><p>InLoop: 0</p><p>EndLoop: 0</p></div>"
+    );
+  });
+
+  test("t-set outside modified in t-foreach increment-before operator", async () => {
+    const template = `
+      <div>
+        <t t-set="iter" t-value="0"/>
+        <t t-foreach="['a','b']" t-as="val" t-key="val">
+          <p>InLoop: <t t-esc="iter"/></p>
+          <t t-set="iter" t-value="++iter"/>
+        </t>
+        <p>EndLoop: <t t-esc="iter"/></p>
+      </div>
+    `;
+    expect(renderToString(template)).toBe(
+      "<div><p>InLoop: 0</p><p>InLoop: 1</p><p>EndLoop: 0</p></div>"
+    );
+  });
+
+  test("t-set can't alter from within callee", async () => {
+    const context = new TestContext();
+    const sub = `<div><t t-esc="iter"/><t t-set="iter" t-value="'called'"/><t t-esc="iter"/></div>`;
+    const main = `
+      <div>
+        <t t-set="iter" t-value="'source'"/>
+        <p><t t-esc="iter"/></p>
+        <t t-call="sub"/>
+        <p><t t-esc="iter"/></p>
+      </div>
+    `;
+    context.addTemplate("sub", sub);
+    context.addTemplate("main", main);
+
+    expect(context.renderToString("main")).toBe(
+      "<div><p>source</p><div>sourcecalled</div><p>source</p></div>"
+    );
+  });
+
+  test("t-set can't alter in t-call body", async () => {
+    const context = new TestContext();
+    const sub = `<div><t t-esc="iter"/><t t-set="iter" t-value="'called'"/><t t-esc="iter"/></div>`;
+    const main = `
+      <div>
+        <t t-set="iter" t-value="'source'"/>
+        <p><t t-esc="iter"/></p>
+        <t t-call="sub">
+          <t t-set="iter" t-value="'inCall'"/>
+        </t>
+        <p><t t-esc="iter"/></p>
+      </div>
+    `;
+    context.addTemplate("sub", sub);
+    context.addTemplate("main", main);
+
+    expect(context.renderToString("main")).toBe(
+      "<div><p>source</p><div>inCallcalled</div><p>source</p></div>"
+    );
+  });
 });