fix: Use runtime check in Array#flat (#1353)

Author: MaxGraey

Diff for: std/assembly/array.ts

@@ -4,7 +4,7 @@ import { BLOCK_MAXSIZE } from "./rt/common";
 import { COMPARATOR, SORT } from "./util/sort";
 import { joinBooleanArray, joinIntegerArray, joinFloatArray, joinStringArray, joinReferenceArray } from "./util/string";
 import { idof, isArray as builtin_isArray } from "./builtins";
-import { E_INDEXOUTOFRANGE, E_INVALIDLENGTH, E_EMPTYARRAY, E_HOLEYARRAY } from "./util/error";
+import { E_INDEXOUTOFRANGE, E_INVALIDLENGTH, E_ILLEGALGENTYPE, E_EMPTYARRAY, E_HOLEYARRAY } from "./util/error";
 
 /** Ensures that the given array has _at least_ the specified backing size. */
 function ensureSize(array: usize, minSize: usize, alignLog2: u32): void {
@@ -367,7 +367,12 @@ export class Array<T> {
       base + sizeof<T>(),
       <usize>lastIndex << alignof<T>()
     );
-    store<T>(base + (<usize>lastIndex << alignof<T>()), changetype<T>(0));
+    if (isReference<T>()) {
+      store<usize>(base + (<usize>lastIndex << alignof<T>()), 0);
+    } else {
+      // @ts-ignore
+      store<T>(base + (<usize>lastIndex << alignof<T>()), <T>0);
+    }
     this.length_ = lastIndex;
     return element; // no need to retain -> is moved
   }
@@ -496,7 +501,7 @@ export class Array<T> {
 
   flat(): T {
     if (!isArray<T>()) {
-      ERROR("Cannot call flat() on Array<T> where T is not an Array.");
+      throw new TypeError(E_ILLEGALGENTYPE);
     }
     // Get the length and data start values
     var length = this.length_;

Diff for: std/assembly/util/error.ts

@@ -9,6 +9,10 @@ export const E_INDEXOUTOFRANGE: string = "Index out of range";
 @lazy @inline
 export const E_INVALIDLENGTH: string = "Invalid length";
 
+// @ts-ignore: decorator
+@lazy @inline
+export const E_ILLEGALGENTYPE: string = "Illegal generic type";
+
 // @ts-ignore: decorator
 @lazy @inline
 export const E_EMPTYARRAY: string = "Array is empty";