add describedbyids and use to preserve existing ids

Author: adolgachev

src/material/form-field/form-field-control.ts

@@ -75,6 +75,8 @@ export abstract class MatFormFieldControl<T> {
    */
   readonly disableAutomaticLabeling?: boolean;
 
+  readonly describedByIds?: string[];
+
   /** Sets the list of element IDs that currently describe this control. */
   abstract setDescribedByIds(ids: string[]): void;
 

src/material/form-field/form-field.ts

@@ -308,6 +308,9 @@ export class MatFormField
   // Unique id for the hint label.
   readonly _hintLabelId = this._idGenerator.getId('mat-mdc-hint-');
 
+  // Ids obtained from the fields
+  private _describedByIds: Array<String> = [];
+
   /** Gets the current form field control */
   get _control(): MatFormFieldControl<any> {
     return this._explicitFormFieldControl || this._formFieldControl;
@@ -705,7 +708,17 @@ export class MatFormField
         ids.push(...this._errorChildren.map(error => error.id));
       }
 
-      this._control.setDescribedByIds(ids);
+      let currentIds = this._control.describedByIds ?? [];
+      let combinedIds: Array<string> = [];
+      currentIds.forEach(id => {
+        if (!this._describedByIds.includes(id)) {
+          combinedIds.push(id);
+        }
+      });
+      combinedIds.concat(...ids);
+
+      this._control.setDescribedByIds(combinedIds);
+      this._describedByIds = ids;
     }
   }