fix(compiler): project control flow root elements into correct slot

With the directive-based control flow users were able to conditionally project content using the `*` syntax. E.g. `<div *ngIf="expr" projectMe></div>` will be projected into `<ng-content select="[projectMe]"/>`, because the attributes and tag name from the `div` are copied to the template via the template creation instruction. With `@if` and `@for` that is not the case, because the conditional is placed *around* elements, rather than *on* them. The result is that content projection won't work in the same way if a user converts from `*ngIf` to `@if`.

These changes aim to cover the most common case by doing the same copying when a control flow node has *one and only one* root element or template node.

This approach comes with some caveats:
1. As soon as any other node is added to the root, the copying behavior won't work anymore. A diagnostic will be added to flag cases like this and to explain how to work around it.
2. If `preserveWhitespaces` is enabled, it's very likely that indentation will break this workaround, because it'll include an additional text node as the first child. We can work around it here, but in a discussion it was decided not to, because the user explicitly opted into preserving the whitespace and we would have to drop it from the generated code. The diagnostic mentioned point #1 will flag such cases to users.

Fixes #52277.

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/GOLDEN_PARTIAL.js

@@ -1652,3 +1652,77 @@ export declare class MyApp {
     static ɵcmp: i0.ɵɵComponentDeclaration<MyApp, "ng-component", never, {}, {}, never, never, true, never>;
 }
 
+/****************************************************************************************************
+ * PARTIAL FILE: if_element_root_node.js
+ ****************************************************************************************************/
+import { Component } from '@angular/core';
+import * as i0 from "@angular/core";
+export class MyApp {
+    constructor() {
+        this.expr = true;
+    }
+}
+MyApp.ɵfac = i0.ɵɵngDeclareFactory({ minVersion: "12.0.0", version: "0.0.0-PLACEHOLDER", ngImport: i0, type: MyApp, deps: [], target: i0.ɵɵFactoryTarget.Component });
+MyApp.ɵcmp = i0.ɵɵngDeclareComponent({ minVersion: "17.0.0", version: "0.0.0-PLACEHOLDER", type: MyApp, selector: "ng-component", ngImport: i0, template: `
+    @if (expr) {
+      <div foo="1" bar="2">{{expr}}</div>
+    }
+  `, isInline: true });
+i0.ɵɵngDeclareClassMetadata({ minVersion: "12.0.0", version: "0.0.0-PLACEHOLDER", ngImport: i0, type: MyApp, decorators: [{
+            type: Component,
+            args: [{
+                    template: `
+    @if (expr) {
+      <div foo="1" bar="2">{{expr}}</div>
+    }
+  `,
+                }]
+        }] });
+
+/****************************************************************************************************
+ * PARTIAL FILE: if_element_root_node.d.ts
+ ****************************************************************************************************/
+import * as i0 from "@angular/core";
+export declare class MyApp {
+    expr: boolean;
+    static ɵfac: i0.ɵɵFactoryDeclaration<MyApp, never>;
+    static ɵcmp: i0.ɵɵComponentDeclaration<MyApp, "ng-component", never, {}, {}, never, never, false, never>;
+}
+
+/****************************************************************************************************
+ * PARTIAL FILE: for_element_root_node.js
+ ****************************************************************************************************/
+import { Component } from '@angular/core';
+import * as i0 from "@angular/core";
+export class MyApp {
+    constructor() {
+        this.items = [1, 2, 3];
+    }
+}
+MyApp.ɵfac = i0.ɵɵngDeclareFactory({ minVersion: "12.0.0", version: "0.0.0-PLACEHOLDER", ngImport: i0, type: MyApp, deps: [], target: i0.ɵɵFactoryTarget.Component });
+MyApp.ɵcmp = i0.ɵɵngDeclareComponent({ minVersion: "17.0.0", version: "0.0.0-PLACEHOLDER", type: MyApp, selector: "ng-component", ngImport: i0, template: `
+    @for (item of items; track item) {
+      <div foo="1" bar="2">{{item}}</div>
+    }
+  `, isInline: true });
+i0.ɵɵngDeclareClassMetadata({ minVersion: "12.0.0", version: "0.0.0-PLACEHOLDER", ngImport: i0, type: MyApp, decorators: [{
+            type: Component,
+            args: [{
+                    template: `
+    @for (item of items; track item) {
+      <div foo="1" bar="2">{{item}}</div>
+    }
+  `,
+                }]
+        }] });
+
+/****************************************************************************************************
+ * PARTIAL FILE: for_element_root_node.d.ts
+ ****************************************************************************************************/
+import * as i0 from "@angular/core";
+export declare class MyApp {
+    items: number[];
+    static ɵfac: i0.ɵɵFactoryDeclaration<MyApp, never>;
+    static ɵcmp: i0.ɵɵComponentDeclaration<MyApp, "ng-component", never, {}, {}, never, never, false, never>;
+}
+

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/TEST_CASES.json

@@ -513,6 +513,40 @@
           "failureMessage": "Incorrect template"
         }
       ]
+    },
+    {
+      "description": "should generate an if block with an element root node",
+      "inputFiles": [
+        "if_element_root_node.ts"
+      ],
+      "expectations": [
+        {
+          "files": [
+            {
+              "expected": "if_element_root_node_template.js",
+              "generated": "if_element_root_node.js"
+            }
+          ],
+          "failureMessage": "Incorrect template"
+        }
+      ]
+    },
+    {
+      "description": "should generate a for block with an element root node",
+      "inputFiles": [
+        "for_element_root_node.ts"
+      ],
+      "expectations": [
+        {
+          "files": [
+            {
+              "expected": "for_element_root_node_template.js",
+              "generated": "for_element_root_node.js"
+            }
+          ],
+          "failureMessage": "Incorrect template"
+        }
+      ]
     }
   ]
-}
+}

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/basic_for_template.js

@@ -12,7 +12,7 @@ function MyApp_Template(rf, ctx) {
   if (rf & 1) {
     $r3$.ɵɵelementStart(0, "div");
     $r3$.ɵɵtext(1);
-    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 1, 1, $r3$.ɵɵrepeaterTrackByIdentity);
+    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 1, 1, null, null, $r3$.ɵɵrepeaterTrackByIdentity);
     $r3$.ɵɵelementEnd();
   }
   if (rf & 2) {

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/for_aliased_template_variables_template.js

@@ -13,7 +13,7 @@ function MyApp_Template(rf, ctx) {
   if (rf & 1) {
     $r3$.ɵɵelementStart(0, "div");
     $r3$.ɵɵtext(1);
-    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 1, 6, $r3$.ɵɵrepeaterTrackByIdentity);
+    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 1, 6, null, null, $r3$.ɵɵrepeaterTrackByIdentity);
     $r3$.ɵɵelementEnd();
   }
   if (rf & 2) {

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/for_data_slots_template.js

@@ -1,7 +1,7 @@
 function MyApp_Template(rf, ctx) {
   if (rf & 1) {
     $r3$.ɵɵtemplate(0, MyApp_ng_template_0_Template, 0, 0, "ng-template");
-    $r3$.ɵɵrepeaterCreate(1, MyApp_For_2_Template, 1, 1, $r3$.ɵɵrepeaterTrackByIdentity, false, MyApp_ForEmpty_3_Template, 1, 0);
+    $r3$.ɵɵrepeaterCreate(1, MyApp_For_2_Template, 1, 1, null, null, $r3$.ɵɵrepeaterTrackByIdentity, false, MyApp_ForEmpty_3_Template, 1, 0);
     $r3$.ɵɵtemplate(4, MyApp_ng_template_4_Template, 0, 0, "ng-template");
   }
   if (rf & 2) {

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/for_element_root_node.ts

@@ -0,0 +1,12 @@
+import {Component} from '@angular/core';
+
+@Component({
+  template: `
+    @for (item of items; track item) {
+      <div foo="1" bar="2">{{item}}</div>
+    }
+  `,
+})
+export class MyApp {
+  items = [1, 2, 3];
+}

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/for_element_root_node_template.js

@@ -0,0 +1,3 @@
+consts: [["foo", "1", "bar", "2"]]
+…
+$r3$.ɵɵrepeaterCreate(0, MyApp_For_1_Template, 2, 1, "div", 0, i0.ɵɵrepeaterTrackByIdentity);

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/for_impure_track_reuse_template.js

@@ -4,8 +4,8 @@ function $_forTrack0$($index, $item) {
 …
 function MyApp_Template(rf, ctx) {
   if (rf & 1) {
-    $r3$.ɵɵrepeaterCreate(0, MyApp_For_1_Template, 1, 1, $_forTrack0$, true);
-    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 1, 1, $_forTrack0$, true);
+    $r3$.ɵɵrepeaterCreate(0, MyApp_For_1_Template, 1, 1, null, null, $_forTrack0$, true);
+    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 1, 1, null, null, $_forTrack0$, true);
   }
   if (rf & 2) {
     $r3$.ɵɵrepeater(0, ctx.items);

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/for_pure_track_reuse_template.js

@@ -2,8 +2,8 @@ const $_forTrack0$ = ($index, $item) => $item.name[0].toUpperCase();
 …
 function MyApp_Template(rf, ctx) {
   if (rf & 1) {
-    $r3$.ɵɵrepeaterCreate(0, MyApp_For_1_Template, 1, 1, $_forTrack0$);
-    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 1, 1, $_forTrack0$);
+    $r3$.ɵɵrepeaterCreate(0, MyApp_For_1_Template, 1, 1, null, null, $_forTrack0$);
+    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 1, 1, null, null, $_forTrack0$);
   }
   if (rf & 2) {
     $r3$.ɵɵrepeater(0, ctx.items);

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/for_template_track_method_nested_template.js

@@ -1 +1 @@
-$r3$.ɵɵrepeaterCreate(0, MyApp_ng_template_2_For_1_Template, 0, 0, $r3$.ɵɵcomponentInstance().trackFn);
+$r3$.ɵɵrepeaterCreate(0, MyApp_ng_template_2_For_1_Template, 0, 0, null, null, $r3$.ɵɵcomponentInstance().trackFn);

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/for_template_track_method_root_template.js

@@ -1 +1 @@
-$r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 0, 0, ctx.trackFn);
+$r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 0, 0, null, null, ctx.trackFn);

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/for_template_variables_listener_template.js

@@ -17,7 +17,7 @@ function MyApp_Template(rf, ctx) {
   if (rf & 1) {
     $r3$.ɵɵelementStart(0, "div");
     $r3$.ɵɵtext(1);
-    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 1, 0, $r3$.ɵɵrepeaterTrackByIdentity);
+    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 1, 0, "div", null, $r3$.ɵɵrepeaterTrackByIdentity);
     $r3$.ɵɵelementEnd();
   }
   if (rf & 2) {

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/for_template_variables_scope_template.js

@@ -12,7 +12,7 @@ function MyApp_For_2_Template(rf, ctx) {
 function MyApp_Template(rf, ctx) {
   if (rf & 1) {
     $r3$.ɵɵtext(0);
-    $r3$.ɵɵrepeaterCreate(1, MyApp_For_2_Template, 1, 4, $r3$.ɵɵrepeaterTrackByIdentity);
+    $r3$.ɵɵrepeaterCreate(1, MyApp_For_2_Template, 1, 4, null, null, $r3$.ɵɵrepeaterTrackByIdentity);
     $r3$.ɵɵtext(3);
   }
   if (rf & 2) {

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/for_template_variables_template.js

@@ -13,7 +13,7 @@ function MyApp_Template(rf, ctx) {
   if (rf & 1) {
     $r3$.ɵɵelementStart(0, "div");
     $r3$.ɵɵtext(1);
-    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 1, 6, $r3$.ɵɵrepeaterTrackByIdentity);
+    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 1, 6, null, null, $r3$.ɵɵrepeaterTrackByIdentity);
     $r3$.ɵɵelementEnd();
   }
   if (rf & 2) {

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/for_track_by_field_template.js

@@ -13,7 +13,7 @@ function MyApp_Template(rf, ctx) {
   if (rf & 1) {
     $r3$.ɵɵelementStart(0, "div");
     $r3$.ɵɵtext(1);
-    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 1, 1, $_forTrack0$);
+    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 1, 1, null, null, $_forTrack0$);
     $r3$.ɵɵelementEnd();
   }
   if (rf & 2) {

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/for_track_by_index_template.js

@@ -12,7 +12,7 @@ function MyApp_Template(rf, ctx) {
   if (rf & 1) {
     $r3$.ɵɵelementStart(0, "div");
     $r3$.ɵɵtext(1);
-    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 1, 1, $r3$.ɵɵrepeaterTrackByIndex);
+    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 1, 1, null, null, $r3$.ɵɵrepeaterTrackByIndex);
     $r3$.ɵɵelementEnd();
   }
   if (rf & 2) {

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/for_track_literals_template.js

@@ -7,7 +7,7 @@ function $_forTrack0$($index, $item) {
 …
 function MyApp_Template(rf, ctx) {
   if (rf & 1) {
-    $r3$.ɵɵrepeaterCreate(0, MyApp_For_1_Template, 1, 1, $_forTrack0$, true);
+    $r3$.ɵɵrepeaterCreate(0, MyApp_For_1_Template, 1, 1, null, null, $_forTrack0$, true);
   }
   if (rf & 2) {
     $r3$.ɵɵrepeater(0, ctx.items);

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/for_with_empty_template.js

@@ -18,7 +18,7 @@ function MyApp_Template(rf, ctx) {
   if (rf & 1) {
     $r3$.ɵɵelementStart(0, "div");
     $r3$.ɵɵtext(1);
-    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 1, 1, $r3$.ɵɵrepeaterTrackByIdentity, false, MyApp_ForEmpty_4_Template, 1, 0);
+    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 1, 1, null, null, $r3$.ɵɵrepeaterTrackByIdentity, false, MyApp_ForEmpty_4_Template, 1, 0);
     $r3$.ɵɵelementEnd();
   }
   if (rf & 2) {

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/for_with_pipe_template.js

@@ -2,7 +2,7 @@ function MyApp_Template(rf, ctx) {
   if (rf & 1) {
     $r3$.ɵɵelementStart(0, "div");
     $r3$.ɵɵtext(1);
-    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 1, 1, $r3$.ɵɵrepeaterTrackByIdentity);
+    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 1, 1, null, null, $r3$.ɵɵrepeaterTrackByIdentity);
     $r3$.ɵɵpipe(4, "test");
     $r3$.ɵɵelementEnd();
   }

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/if_element_root_node.ts

@@ -0,0 +1,12 @@
+import {Component} from '@angular/core';
+
+@Component({
+  template: `
+    @if (expr) {
+      <div foo="1" bar="2">{{expr}}</div>
+    }
+  `,
+})
+export class MyApp {
+  expr = true;
+}

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/if_element_root_node_template.js

@@ -0,0 +1,3 @@
+consts: [["foo", "1", "bar", "2"]]
+…
+$r3$.ɵɵtemplate(0, MyApp_Conditional_0_Template, 2, 1, "div", 0);

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/if_nested_alias_listeners_template.js

@@ -26,7 +26,7 @@ function MyApp_Conditional_0_Conditional_1_Template(rf, ctx) {
       return $r3$.ɵɵresetView($ctx_r10$.log($ctx_r10$.value(), $root_r1$, $inner_r3$));
     });
     $r3$.ɵɵelementEnd();
-    $r3$.ɵɵtemplate(1, MyApp_Conditional_0_Conditional_1_Conditional_1_Template, 1, 0);
+    $r3$.ɵɵtemplate(1, MyApp_Conditional_0_Conditional_1_Conditional_1_Template, 1, 0, "button");
   }
   if (rf & 2) {
     const $ctx_r2$ = $r3$.ɵɵnextContext(2);

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/if_nested_alias_listeners_template.pipeline.js

@@ -24,7 +24,7 @@ function MyApp_Conditional_0_Conditional_1_Conditional_1_Template(rf, ctx) {
 		return $r3$.ɵɵresetView($ctx_r10$.log($ctx_r10$.value(), $root_r1$, $inner_r3$));
 	  });
 	  $r3$.ɵɵelementEnd();
-	  $r3$.ɵɵtemplate(1, MyApp_Conditional_0_Conditional_1_Conditional_1_Template, 1, 0);
+	  $r3$.ɵɵtemplate(1, MyApp_Conditional_0_Conditional_1_Conditional_1_Template, 1, 0, "button");
 	}
 	if (rf & 2) {
 	  let $MyApp_Conditional_0_Conditional_1_contFlowTmp$;

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/nested_for_template.js

@@ -12,7 +12,7 @@ function MyApp_For_3_For_2_Template(rf, ctx) {
 function MyApp_For_3_Template(rf, ctx) {
   if (rf & 1) {
     $r3$.ɵɵtext(0);
-    $r3$.ɵɵrepeaterCreate(1, MyApp_For_3_For_2_Template, 1, 2, $r3$.ɵɵrepeaterTrackByIndex);
+    $r3$.ɵɵrepeaterCreate(1, MyApp_For_3_For_2_Template, 1, 2, null, null, $r3$.ɵɵrepeaterTrackByIndex);
   }
   if (rf & 2) {
     const $item_r1$ = ctx.$implicit;
@@ -25,7 +25,7 @@ function MyApp_Template(rf, ctx) {
   if (rf & 1) {
     $r3$.ɵɵelementStart(0, "div");
     $r3$.ɵɵtext(1);
-    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 3, 1, $r3$.ɵɵrepeaterTrackByIdentity);
+    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 3, 1, null, null, $r3$.ɵɵrepeaterTrackByIdentity);
     $r3$.ɵɵelementEnd();
   }
   if (rf & 2) {

packages/compiler-cli/test/compliance/test_cases/r3_view_compiler_control_flow/nested_for_template_variables_template.js

@@ -12,7 +12,7 @@ function MyApp_For_3_For_2_Template(rf, ctx) {
 function MyApp_For_3_Template(rf, ctx) {
   if (rf & 1) {
     $r3$.ɵɵtext(0);
-    $r3$.ɵɵrepeaterCreate(1, MyApp_For_3_For_2_Template, 1, 2, $r3$.ɵɵrepeaterTrackByIdentity);
+    $r3$.ɵɵrepeaterCreate(1, MyApp_For_3_For_2_Template, 1, 2, null, null, $r3$.ɵɵrepeaterTrackByIdentity);
   }
   if (rf & 2) {
     const $item_r1$ = ctx.$implicit;
@@ -25,7 +25,7 @@ function MyApp_Template(rf, ctx) {
   if (rf & 1) {
     $r3$.ɵɵelementStart(0, "div");
     $r3$.ɵɵtext(1);
-    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 3, 1, $r3$.ɵɵrepeaterTrackByIdentity);
+    $r3$.ɵɵrepeaterCreate(2, MyApp_For_3_Template, 3, 1, null, null, $r3$.ɵɵrepeaterTrackByIdentity);
     $r3$.ɵɵelementEnd();
   }
   if (rf & 2) {