fix(material/input): preserve native placeholder on non-legacy appearances

The `legacy` form field appearance has a feature where it promotes the input placeholder
to the form field label which introduces a problem where screen readers will read out
the placeholder twice. Some time ago we added logic to clear the placeholder, but
it seems to be a bit too aggressive since it also clears the placeholder for other
appearances.

These changes scope the workaround only to the case when a placeholder would be
promoted to a label.

Fixes #20903.

Author: crisbeto

src/material/input/input.spec.ts

@@ -958,7 +958,6 @@ describe('MatInput without forms', () => {
     expect(container.classList).toContain('mat-form-field-hide-placeholder');
     expect(container.classList).not.toContain('mat-form-field-should-float');
     expect(label.textContent.trim()).toBe('Label');
-    expect(input.hasAttribute('placeholder')).toBe(false);
 
     input.value = 'Value';
     fixture.detectChanges();
@@ -1005,6 +1004,16 @@ describe('MatInput without forms', () => {
     expect(container.classList).not.toContain('mat-form-field-should-float');
   });
 
+  it('should preserve the native placeholder on a non-legacy appearance', fakeAsync(() => {
+    const fixture = createComponent(MatInputWithLabelAndPlaceholder);
+    fixture.componentInstance.floatLabel = 'auto';
+    fixture.componentInstance.appearance = 'standard';
+    fixture.detectChanges();
+
+    expect(fixture.nativeElement.querySelector('input').hasAttribute('placeholder')).toBe(true);
+  }));
+
+
   it('should not add the native select class if the control is not a native select', () => {
     const fixture = createComponent(MatInputWithId);
     fixture.detectChanges();
@@ -2109,14 +2118,15 @@ class MatInputWithLabel {}
 
 @Component({
   template: `
-    <mat-form-field [floatLabel]="floatLabel">
+    <mat-form-field [floatLabel]="floatLabel" [appearance]="appearance">
       <mat-label>Label</mat-label>
       <input matInput placeholder="Placeholder">
     </mat-form-field>
   `
 })
 class MatInputWithLabelAndPlaceholder {
   floatLabel: FloatLabelType;
+  appearance: MatFormFieldAppearance = 'legacy';
 }
 
 @Component({

src/material/input/input.ts

@@ -369,7 +369,9 @@ export class MatInput extends _MatInputMixinBase implements MatFormFieldControl<
     // screen readers will read it out twice: once from the label and once from the attribute.
     // TODO: can be removed once we get rid of the `legacy` style for the form field, because it's
     // the only one that supports promoting the placeholder to a label.
-    const placeholder = this._formField?._hideControlPlaceholder?.() ? null : this.placeholder;
+    const formField = this._formField;
+    const placeholder = formField && formField.appearance === 'legacy' && !formField._hasLabel() ?
+      null : this.placeholder;
     if (placeholder !== this._previousPlaceholder) {
       const element = this._elementRef.nativeElement;
       this._previousPlaceholder = placeholder;