[REF] qweb: simplify _compile method arguments

Strangely, the _compile method required a CompilationContext whenever it
was compiled as a sub template, but this parent context was actually not
really needed.  I guess that it was the case in the past, but this was
changed at some point.

This commit makes another significant change: the xml element is no
longer mandatory.  It is actually only required for slots (because the
template is not registered to qweb).

Finally, the interface for the whole method has been changed to use an
option object, which makes more sense with 3 optional paremeters.

src/component/directive.ts

@@ -446,7 +446,7 @@ QWeb.addDirective({
           slotNode.removeAttribute("t-set-slot");
           slotNode.parentElement!.removeChild(slotNode);
 
-          const slotFn = qweb._compile(`slot_${key}_template`, slotNode, ctx);
+          const slotFn = qweb._compile(`slot_${key}_template`, { elem: slotNode, hasParent: true });
           QWeb.slots[`${slotId}_${key}`] = slotFn;
         }
       }
@@ -455,7 +455,7 @@ QWeb.addDirective({
         for (let child of Object.values(clone.childNodes)) {
           t.appendChild(child);
         }
-        const slotFn = qweb._compile(`slot_default_template`, t, ctx);
+        const slotFn = qweb._compile(`slot_default_template`, { elem: t, hasParent: true });
         QWeb.slots[`${slotId}_default`] = slotFn;
       }
     }

src/qweb/base_directives.ts

@@ -235,7 +235,7 @@ QWeb.addDirective({
     if (!subId) {
       subId = QWeb.nextId++;
       qweb.subTemplates[subTemplate] = subId;
-      const subTemplateFn = qweb._compile(subTemplate, nodeTemplate.elem, ctx, true);
+      const subTemplateFn = qweb._compile(subTemplate, { hasParent: true, defineKey: true });
       QWeb.subTemplates[subId] = subTemplateFn;
     }
 

src/qweb/qweb.ts

@@ -312,7 +312,7 @@ export class QWeb extends EventBus {
     const template = {
       elem,
       fn: function (this: QWeb, context, extra) {
-        const compiledFunction = this._compile(name, elem);
+        const compiledFunction = this._compile(name);
         template.fn = compiledFunction;
         return compiledFunction.call(this, context, extra);
       },
@@ -417,30 +417,33 @@ export class QWeb extends EventBus {
 
   _compile(
     name: string,
-    elem: Element,
-    parentContext?: CompilationContext,
-    defineKey?: boolean
+    options: {
+      elem?: Element;
+      hasParent?: boolean;
+      defineKey?: boolean;
+    } = {}
   ): CompiledTemplate {
+    const elem = options.elem || this.templates[name].elem;
     const isDebug = elem.attributes.hasOwnProperty("t-debug");
     const ctx = new CompilationContext(name);
     if (elem.tagName !== "t") {
       ctx.shouldDefineResult = false;
     }
-    if (parentContext) {
-      ctx.variables = Object.create(parentContext.variables);
-      ctx.parentNode = parentContext.parentNode || ctx.generateID();
+    if (options.hasParent) {
+      ctx.variables = Object.create(null);
+      ctx.parentNode = ctx.generateID();
       ctx.allowMultipleRoots = true;
       ctx.hasParentWidget = true;
       ctx.shouldDefineResult = false;
       ctx.addLine(`let c${ctx.parentNode} = extra.parentNode;`);
-      if (defineKey) {
+      if (options.defineKey) {
         ctx.addLine(`let key0 = extra.key || "";`);
         ctx.hasKey0 = true;
       }
     }
     this._compileNode(elem, ctx);
 
-    if (!parentContext) {
+    if (!options.hasParent) {
       if (ctx.shouldDefineResult) {
         ctx.addLine(`return result;`);
       } else {