fix(material/datepicker): resolve repeater warnings in calendar

Fixes that the calendar was triggering some newly-introduced warnings from the framework.

Fixes #29008.

Author: crisbeto

src/material/datepicker/calendar-body.html

@@ -14,7 +14,7 @@
 }
 
 <!-- Create the first row separately so we can include a special spacer cell. -->
-@for (row of rows; track row; let rowIndex = $index) {
+@for (row of rows; track _trackRow(row); let rowIndex = $index) {
   <tr role="row">
     <!--
       This cell is purely decorative, but we can't put `aria-hidden` or `role="presentation"` on it,
@@ -36,7 +36,7 @@
       cell is interactable, as well as the selection state via `aria-pressed`. See #23476 for
       background.
     -->
-    @for (item of row; track item; let colIndex = $index) {
+    @for (item of row; track item.id; let colIndex = $index) {
       <td
         role="gridcell"
         class="mat-calendar-body-cell-container"

src/material/datepicker/calendar-body.ts

@@ -35,11 +35,15 @@ export type MatCalendarCellClassFunction<D> = (
   view: 'month' | 'year' | 'multi-year',
 ) => MatCalendarCellCssClasses;
 
+let uniqueIdCounter = 0;
+
 /**
  * An internal class that represents the data corresponding to a single calendar cell.
  * @docs-private
  */
 export class MatCalendarCell<D = any> {
+  readonly id = uniqueIdCounter++;
+
   constructor(
     public value: number,
     public displayValue: string,
@@ -192,6 +196,13 @@ export class MatCalendarBody<D = any> implements OnChanges, OnDestroy, AfterView
 
   private _injector = inject(Injector);
 
+  /**
+   * Tracking function for rows based on their identity. Ideally we would use some sort of
+   * key on the row, but that would require a breaking change for the `rows` input. We don't
+   * use the built-in identity tracking, because it logs warnings.
+   */
+  _trackRow = (row: MatCalendarCell[]) => row;
+
   constructor(
     private _elementRef: ElementRef<HTMLElement>,
     private _ngZone: NgZone,

src/material/datepicker/month-view.html

@@ -1,7 +1,7 @@
 <table class="mat-calendar-table" role="grid">
   <thead class="mat-calendar-table-header">
     <tr>
-      @for (day of _weekdays; track day) {
+      @for (day of _weekdays; track day.id) {
         <th scope="col">
           <span class="cdk-visually-hidden">{{day.long}}</span>
           <span aria-hidden="true">{{day.narrow}}</span>

src/material/datepicker/month-view.ts

@@ -55,6 +55,8 @@ import {
 
 const DAYS_PER_WEEK = 7;
 
+let uniqueIdCounter = 0;
+
 /**
  * An internal component used to display a single month in the datepicker.
  * @docs-private
@@ -206,7 +208,7 @@ export class MatMonthView<D> implements AfterContentInit, OnChanges, OnDestroy {
   _todayDate: number | null;
 
   /** The names of the weekdays. */
-  _weekdays: {long: string; narrow: string}[];
+  _weekdays: {long: string; narrow: string; id: number}[];
 
   constructor(
     readonly _changeDetectorRef: ChangeDetectorRef,
@@ -508,7 +510,7 @@ export class MatMonthView<D> implements AfterContentInit, OnChanges, OnDestroy {
 
     // Rotate the labels for days of the week based on the configured first day of the week.
     let weekdays = longWeekdays.map((long, i) => {
-      return {long, narrow: narrowWeekdays[i]};
+      return {long, narrow: narrowWeekdays[i], id: uniqueIdCounter++};
     });
     this._weekdays = weekdays.slice(firstDayOfWeek).concat(weekdays.slice(0, firstDayOfWeek));
   }

tools/public_api_guard/material/datepicker.md

@@ -248,6 +248,7 @@ export class MatCalendarBody<D = any> implements OnChanges, OnDestroy, AfterView
     _startDateLabelId: string;
     startValue: number;
     todayValue: number;
+    _trackRow: (row: MatCalendarCell[]) => MatCalendarCell<any>[];
     // (undocumented)
     static ɵcmp: i0.ɵɵComponentDeclaration<MatCalendarBody<any>, "[mat-calendar-body]", ["matCalendarBody"], { "label": { "alias": "label"; "required": false; }; "rows": { "alias": "rows"; "required": false; }; "todayValue": { "alias": "todayValue"; "required": false; }; "startValue": { "alias": "startValue"; "required": false; }; "endValue": { "alias": "endValue"; "required": false; }; "labelMinRequiredCells": { "alias": "labelMinRequiredCells"; "required": false; }; "numCols": { "alias": "numCols"; "required": false; }; "activeCell": { "alias": "activeCell"; "required": false; }; "isRange": { "alias": "isRange"; "required": false; }; "cellAspectRatio": { "alias": "cellAspectRatio"; "required": false; }; "comparisonStart": { "alias": "comparisonStart"; "required": false; }; "comparisonEnd": { "alias": "comparisonEnd"; "required": false; }; "previewStart": { "alias": "previewStart"; "required": false; }; "previewEnd": { "alias": "previewEnd"; "required": false; }; "startDateAccessibleName": { "alias": "startDateAccessibleName"; "required": false; }; "endDateAccessibleName": { "alias": "endDateAccessibleName"; "required": false; }; }, { "selectedValueChange": "selectedValueChange"; "previewChange": "previewChange"; "activeDateChange": "activeDateChange"; "dragStarted": "dragStarted"; "dragEnded": "dragEnded"; }, never, never, true, never>;
     // (undocumented)
@@ -268,6 +269,8 @@ export class MatCalendarCell<D = any> {
     // (undocumented)
     enabled: boolean;
     // (undocumented)
+    readonly id: number;
+    // (undocumented)
     rawValue?: D | undefined;
     // (undocumented)
     value: number;
@@ -748,6 +751,7 @@ export class MatMonthView<D> implements AfterContentInit, OnChanges, OnDestroy {
     _weekdays: {
         long: string;
         narrow: string;
+        id: number;
     }[];
     _weeks: MatCalendarCell[][];
     // (undocumented)