fix(select): unable to preselect array value in single selection mode

src/lib/select/select.spec.ts

@@ -99,6 +99,7 @@ describe('MatSelect', () => {
         SelectInsideFormGroup,
         NgModelCompareWithSelect,
         CustomErrorBehaviorSelect,
+        SingleSelectWithPreselectedArrayValues,
       ],
       providers: [
         {provide: OverlayContainer, useFactory: () => {
@@ -1006,6 +1007,19 @@ describe('MatSelect', () => {
         });
       });
     }));
+
+    it('should be able to preselect an array value in single-selection mode', fakeAsync(() => {
+      const fixture = TestBed.createComponent(SingleSelectWithPreselectedArrayValues);
+      fixture.detectChanges();
+      tick();
+      fixture.detectChanges();
+
+      const trigger = fixture.debugElement.query(By.css('.mat-select-trigger')).nativeElement;
+
+      expect(trigger.textContent).toContain('Pizza');
+      expect(fixture.componentInstance.options.toArray()[1].selected).toBe(true);
+    }));
+
   });
 
   describe('misc forms', () => {
@@ -3814,3 +3828,28 @@ class CustomErrorBehaviorSelect {
   ];
   errorStateMatcher: ErrorStateMatcher;
 }
+
+
+@Component({
+  template: `
+    <mat-form-field>
+      <mat-select placeholder="Food" [(ngModel)]="selectedFoods">
+        <mat-option *ngFor="let food of foods"
+                    [value]="food.value">{{ food.viewValue }}
+        </mat-option>
+      </mat-select>
+    </mat-form-field>
+  `
+})
+class SingleSelectWithPreselectedArrayValues {
+  foods: any[] = [
+    { value: ['steak-0', 'steak-1'], viewValue: 'Steak' },
+    { value: ['pizza-1', 'pizza-2'], viewValue: 'Pizza' },
+    { value: ['tacos-2', 'tacos-3'], viewValue: 'Tacos' },
+  ];
+
+  selectedFoods = this.foods[1].value;
+
+  @ViewChild(MatSelect) select: MatSelect;
+  @ViewChildren(MatOption) options: QueryList<MatOption>;
+}

src/lib/select/select.ts

@@ -711,18 +711,17 @@ export class MatSelect extends _MatSelectMixinBase implements AfterContentInit,
    * found with the designated value, the select trigger is cleared.
    */
   private _setSelectionByValue(value: any | any[], isUserInput = false): void {
-    const isArray = Array.isArray(value);
-
-    if (this.multiple && value && !isArray) {
-      throw getMatSelectNonArrayValueError();
-    }
-
-    this._clearSelection();
+    if (this.multiple && value) {
+      if (!Array.isArray(value)) {
+        throw getMatSelectNonArrayValueError();
+      }
 
-    if (isArray) {
+      this._clearSelection();
       value.forEach((currentValue: any) => this._selectValue(currentValue, isUserInput));
       this._sortValues();
     } else {
+      this._clearSelection();
+
       const correspondingOption = this._selectValue(value, isUserInput);
 
       // Shift focus to the active item. Note that we shouldn't do this in multiple
@@ -856,10 +855,10 @@ export class MatSelect extends _MatSelectMixinBase implements AfterContentInit,
   private _propagateChanges(fallbackValue?: any): void {
     let valueToEmit: any = null;
 
-    if (Array.isArray(this.selected)) {
-      valueToEmit = this.selected.map(option => option.value);
+    if (this.multiple) {
+      valueToEmit = (this.selected as MatOption[]).map(option => option.value);
     } else {
-      valueToEmit = this.selected ? this.selected.value : fallbackValue;
+      valueToEmit = this.selected ? (this.selected as MatOption).value : fallbackValue;
     }
 
     this._value = valueToEmit;