feat(ngcc): support `__read` helper as used by TypeScript 4.2 (#41201)

This commit complements the support for the `__spreadArray` helper that
was added in microsoft/TypeScript#41523. The prior helpers `__spread`
and `__spreadArrays` used the `__read` helper internally, but the helper
is now emitted as an argument to `__spreadArray` so ngcc now needs to
support evaluating it statically. The real implementation of `__read`
reads an iterable into an array, but for ngcc's static evaluation
support it is sufficient to only deal with arrays as is. Additionally,
the optional `n` parameter is not supported as that is only emitted for
array destructuring syntax, which ngcc does not have to support.

PR Close #41201

packages/compiler-cli/ngcc/src/utils.ts

@@ -163,6 +163,8 @@ export function getTsHelperFnFromIdentifier(id: ts.Identifier): KnownDeclaration
       return KnownDeclaration.TsHelperSpreadArrays;
     case '__spreadArray':
       return KnownDeclaration.TsHelperSpreadArray;
+    case '__read':
+      return KnownDeclaration.TsHelperRead;
     default:
       return null;
   }

packages/compiler-cli/ngcc/test/host/commonjs_host_spec.ts

@@ -2000,11 +2000,13 @@ exports.MissingClass2 = MissingClass2;
               function __spread(...args) { /* ... */ }
               function __spreadArrays(...args) { /* ... */ }
               function __spreadArray(to, from) { /* ... */ }
+              function __read(o) { /* ... */ }
 
               var a = __assign({foo: 'bar'}, {baz: 'qux'});
               var b = __spread(['foo', 'bar'], ['baz', 'qux']);
               var c = __spreadArrays(['foo', 'bar'], ['baz', 'qux']);
               var d = __spreadArray(['foo', 'bar'], ['baz', 'qux']);
+              var e = __read(['foo', 'bar']);
             `,
           };
           loadTestFiles([file]);
@@ -2021,6 +2023,7 @@ exports.MissingClass2 = MissingClass2;
           testForHelper('b', '__spread', KnownDeclaration.TsHelperSpread);
           testForHelper('c', '__spreadArrays', KnownDeclaration.TsHelperSpreadArrays);
           testForHelper('d', '__spreadArray', KnownDeclaration.TsHelperSpreadArray);
+          testForHelper('e', '__read', KnownDeclaration.TsHelperRead);
         });
 
         it('should recognize suffixed TypeScript helpers (as function declarations)', () => {
@@ -2031,11 +2034,13 @@ exports.MissingClass2 = MissingClass2;
               function __spread$2(...args) { /* ... */ }
               function __spreadArrays$3(...args) { /* ... */ }
               function __spreadArray$3(to, from) { /* ... */ }
+              function __read$3(o) { /* ... */ }
 
               var a = __assign$1({foo: 'bar'}, {baz: 'qux'});
               var b = __spread$2(['foo', 'bar'], ['baz', 'qux']);
               var c = __spreadArrays$3(['foo', 'bar'], ['baz', 'qux']);
               var d = __spreadArray$3(['foo', 'bar'], ['baz', 'qux']);
+              var e = __read$3(['foo', 'bar']);
             `,
           };
           loadTestFiles([file]);
@@ -2052,6 +2057,7 @@ exports.MissingClass2 = MissingClass2;
           testForHelper('b', '__spread$2', KnownDeclaration.TsHelperSpread);
           testForHelper('c', '__spreadArrays$3', KnownDeclaration.TsHelperSpreadArrays);
           testForHelper('d', '__spreadArray$3', KnownDeclaration.TsHelperSpreadArray);
+          testForHelper('e', '__read$3', KnownDeclaration.TsHelperRead);
         });
 
         it('should recognize TypeScript helpers (as variable declarations)', () => {
@@ -2062,11 +2068,13 @@ exports.MissingClass2 = MissingClass2;
               var __spread = (this && this.__spread) || function (...args) { /* ... */ }
               var __spreadArrays = (this && this.__spreadArrays) || function (...args) { /* ... */ }
               var __spreadArray = (this && this.__spreadArray) || function (to, from) { /* ... */ }
+              var __read = (this && this.__read) || function (o) { /* ... */ }
 
               var a = __assign({foo: 'bar'}, {baz: 'qux'});
               var b = __spread(['foo', 'bar'], ['baz', 'qux']);
               var c = __spreadArrays(['foo', 'bar'], ['baz', 'qux']);
               var d = __spreadArray(['foo', 'bar'], ['baz', 'qux']);
+              var e = __read(['foo', 'bar']);
             `,
           };
           loadTestFiles([file]);
@@ -2083,6 +2091,7 @@ exports.MissingClass2 = MissingClass2;
           testForHelper('b', '__spread', KnownDeclaration.TsHelperSpread);
           testForHelper('c', '__spreadArrays', KnownDeclaration.TsHelperSpreadArrays);
           testForHelper('d', '__spreadArray', KnownDeclaration.TsHelperSpreadArray);
+          testForHelper('e', '__read', KnownDeclaration.TsHelperRead);
         });
 
         it('should recognize suffixed TypeScript helpers (as variable declarations)', () => {
@@ -2093,11 +2102,13 @@ exports.MissingClass2 = MissingClass2;
               var __spread$2 = (this && this.__spread$2) || function (...args) { /* ... */ }
               var __spreadArrays$3 = (this && this.__spreadArrays$3) || function (...args) { /* ... */ }
               var __spreadArray$3 = (this && this.__spreadArray$3) || function (to, from) { /* ... */ }
+              var __read$3 = (this && this.__read$3) || function (o) { /* ... */ }
 
               var a = __assign$1({foo: 'bar'}, {baz: 'qux'});
               var b = __spread$2(['foo', 'bar'], ['baz', 'qux']);
               var c = __spreadArrays$3(['foo', 'bar'], ['baz', 'qux']);
               var d = __spreadArray$3(['foo', 'bar'], ['baz', 'qux']);
+              var e = __read$3(['foo', 'bar']);
             `,
           };
           loadTestFiles([file]);
@@ -2114,6 +2125,7 @@ exports.MissingClass2 = MissingClass2;
           testForHelper('b', '__spread$2', KnownDeclaration.TsHelperSpread);
           testForHelper('c', '__spreadArrays$3', KnownDeclaration.TsHelperSpreadArrays);
           testForHelper('d', '__spreadArray$3', KnownDeclaration.TsHelperSpreadArray);
+          testForHelper('e', '__read$3', KnownDeclaration.TsHelperRead);
         });
 
         it('should recognize imported TypeScript helpers', () => {
@@ -2127,6 +2139,7 @@ exports.MissingClass2 = MissingClass2;
                 var b = tslib_1.__spread(['foo', 'bar'], ['baz', 'qux']);
                 var c = tslib_1.__spreadArrays(['foo', 'bar'], ['baz', 'qux']);
                 var d = tslib_1.__spreadArray(['foo', 'bar'], ['baz', 'qux']);
+                var e = tslib_1.__read(['foo', 'bar']);
               `,
             },
             {
@@ -2136,6 +2149,7 @@ exports.MissingClass2 = MissingClass2;
                 export declare function __spread(...args: any[][]): any[];
                 export declare function __spreadArrays(...args: any[][]): any[];
                 export declare function __spreadArray(to: any[], from: any[]): any[];
+                export declare function __read(o: any, n?: number): any[];
               `,
             },
           ];
@@ -2155,6 +2169,7 @@ exports.MissingClass2 = MissingClass2;
           testForHelper('b', '__spread', KnownDeclaration.TsHelperSpread, 'tslib');
           testForHelper('c', '__spreadArrays', KnownDeclaration.TsHelperSpreadArrays, 'tslib');
           testForHelper('d', '__spreadArray', KnownDeclaration.TsHelperSpreadArray, 'tslib');
+          testForHelper('e', '__read', KnownDeclaration.TsHelperRead, 'tslib');
         });
 
         it('should recognize undeclared, unimported TypeScript helpers (by name)', () => {
@@ -2165,6 +2180,7 @@ exports.MissingClass2 = MissingClass2;
               var b = __spread(['foo', 'bar'], ['baz', 'qux']);
               var c = __spreadArrays(['foo', 'bar'], ['baz', 'qux']);
               var d = __spreadArray(['foo', 'bar'], ['baz', 'qux']);
+              var e = __read(['foo', 'bar']);
             `,
           };
           loadTestFiles([file]);
@@ -2191,6 +2207,7 @@ exports.MissingClass2 = MissingClass2;
           testForHelper('b', '__spread', KnownDeclaration.TsHelperSpread);
           testForHelper('c', '__spreadArrays', KnownDeclaration.TsHelperSpreadArrays);
           testForHelper('d', '__spreadArray', KnownDeclaration.TsHelperSpreadArray);
+          testForHelper('e', '__read', KnownDeclaration.TsHelperRead);
         });
 
         it('should recognize suffixed, undeclared, unimported TypeScript helpers (by name)', () => {
@@ -2201,6 +2218,7 @@ exports.MissingClass2 = MissingClass2;
               var b = __spread$2(['foo', 'bar'], ['baz', 'qux']);
               var c = __spreadArrays$3(['foo', 'bar'], ['baz', 'qux']);
               var d = __spreadArray$3(['foo', 'bar'], ['baz', 'qux']);
+              var e = __read$3(['foo', 'bar']);
             `,
           };
           loadTestFiles([file]);
@@ -2227,6 +2245,7 @@ exports.MissingClass2 = MissingClass2;
           testForHelper('b', '__spread$2', KnownDeclaration.TsHelperSpread);
           testForHelper('c', '__spreadArrays$3', KnownDeclaration.TsHelperSpreadArrays);
           testForHelper('d', '__spreadArray$3', KnownDeclaration.TsHelperSpreadArray);
+          testForHelper('e', '__read$3', KnownDeclaration.TsHelperRead);
         });
 
         it('should recognize enum declarations with string values', () => {
@@ -2461,6 +2480,7 @@ exports.MissingClass2 = MissingClass2;
               export declare function __spread(...args: any[][]): any[];
               export declare function __spreadArrays(...args: any[][]): any[];
               export declare function __spreadArray(to: any[], from: any[]): any[];
+              export declare function __read(o: any, n?: number): any[];
               export declare function __unknownHelper(...args: any[]): any;
             `,
           };
@@ -2477,6 +2497,7 @@ exports.MissingClass2 = MissingClass2;
                 ['__spread', KnownDeclaration.TsHelperSpread],
                 ['__spreadArrays', KnownDeclaration.TsHelperSpreadArrays],
                 ['__spreadArray', KnownDeclaration.TsHelperSpreadArray],
+                ['__read', KnownDeclaration.TsHelperRead],
                 ['__unknownHelper', null],
               ]);
         });

packages/compiler-cli/ngcc/test/host/esm5_host_spec.ts

@@ -2060,11 +2060,13 @@ runInEachFileSystem(() => {
             function __spread(...args) { /* ... */ }
             function __spreadArrays(...args) { /* ... */ }
             function __spreadArray(to, from) { /* ... */ }
+            function __read(o) { /* ... */ }
 
             var a = __assign({foo: 'bar'}, {baz: 'qux'});
             var b = __spread(['foo', 'bar'], ['baz', 'qux']);
             var c = __spreadArrays(['foo', 'bar'], ['baz', 'qux']);
             var d = __spreadArray(['foo', 'bar'], ['baz', 'qux']);
+            var e = __read(['foo', 'bar']);
           `,
         };
         loadTestFiles([file]);
@@ -2080,6 +2082,7 @@ runInEachFileSystem(() => {
         testForHelper('b', '__spread', KnownDeclaration.TsHelperSpread);
         testForHelper('c', '__spreadArrays', KnownDeclaration.TsHelperSpreadArrays);
         testForHelper('d', '__spreadArray', KnownDeclaration.TsHelperSpreadArray);
+        testForHelper('e', '__read', KnownDeclaration.TsHelperRead);
       });
 
       it('should recognize suffixed TypeScript helpers (as function declarations)', () => {
@@ -2090,11 +2093,13 @@ runInEachFileSystem(() => {
             function __spread$2(...args) { /* ... */ }
             function __spreadArrays$3(...args) { /* ... */ }
             function __spreadArray$3(to, from) { /* ... */ }
+            function __read$3(o) { /* ... */ }
 
             var a = __assign$1({foo: 'bar'}, {baz: 'qux'});
             var b = __spread$2(['foo', 'bar'], ['baz', 'qux']);
             var c = __spreadArrays$3(['foo', 'bar'], ['baz', 'qux']);
             var d = __spreadArray$3(['foo', 'bar'], ['baz', 'qux']);
+            var e = __read$3(['foo', 'bar']);
           `,
         };
         loadTestFiles([file]);
@@ -2110,6 +2115,7 @@ runInEachFileSystem(() => {
         testForHelper('b', '__spread$2', KnownDeclaration.TsHelperSpread);
         testForHelper('c', '__spreadArrays$3', KnownDeclaration.TsHelperSpreadArrays);
         testForHelper('d', '__spreadArray$3', KnownDeclaration.TsHelperSpreadArray);
+        testForHelper('e', '__read$3', KnownDeclaration.TsHelperRead);
       });
 
       it('should recognize TypeScript helpers (as variable declarations)', () => {
@@ -2120,11 +2126,13 @@ runInEachFileSystem(() => {
             var __spread = (this && this.__spread) || function (...args) { /* ... */ }
             var __spreadArrays = (this && this.__spreadArrays) || function (...args) { /* ... */ }
             var __spreadArray = (this && this.__spreadArray) || function (to, from) { /* ... */ }
+            var __read = (this && this._read) || function (o) { /* ... */ }
 
             var a = __assign({foo: 'bar'}, {baz: 'qux'});
             var b = __spread(['foo', 'bar'], ['baz', 'qux']);
             var c = __spreadArrays(['foo', 'bar'], ['baz', 'qux']);
             var d = __spreadArray(['foo', 'bar'], ['baz', 'qux']);
+            var e = __read(['foo', 'bar']);
        `,
         };
         loadTestFiles([file]);
@@ -2140,6 +2148,7 @@ runInEachFileSystem(() => {
         testForHelper('b', '__spread', KnownDeclaration.TsHelperSpread);
         testForHelper('c', '__spreadArrays', KnownDeclaration.TsHelperSpreadArrays);
         testForHelper('d', '__spreadArray', KnownDeclaration.TsHelperSpreadArray);
+        testForHelper('e', '__read', KnownDeclaration.TsHelperRead);
       });
 
       it('should recognize suffixed TypeScript helpers (as variable declarations)', () => {
@@ -2150,11 +2159,13 @@ runInEachFileSystem(() => {
             var __spread$2 = (this && this.__spread$2) || function (...args) { /* ... */ }
             var __spreadArrays$3 = (this && this.__spreadArrays$3) || function (...args) { /* ... */ }
             var __spreadArray$3 = (this && this.__spreadArray$3) || function (to, from) { /* ... */ }
+            var __read$3 = (this && this.__read$3) || function (o) { /* ... */ }
 
             var a = __assign$1({foo: 'bar'}, {baz: 'qux'});
             var b = __spread$2(['foo', 'bar'], ['baz', 'qux']);
             var c = __spreadArrays$3(['foo', 'bar'], ['baz', 'qux']);
             var d = __spreadArray$3(['foo', 'bar'], ['baz', 'qux']);
+            var e = __read$3(['foo', 'bar']);
           `,
         };
         loadTestFiles([file]);
@@ -2170,6 +2181,7 @@ runInEachFileSystem(() => {
         testForHelper('b', '__spread$2', KnownDeclaration.TsHelperSpread);
         testForHelper('c', '__spreadArrays$3', KnownDeclaration.TsHelperSpreadArrays);
         testForHelper('d', '__spreadArray$3', KnownDeclaration.TsHelperSpreadArray);
+        testForHelper('e', '__read$3', KnownDeclaration.TsHelperRead);
       });
 
       it('should recognize imported TypeScript helpers (named imports)', () => {
@@ -2177,12 +2189,13 @@ runInEachFileSystem(() => {
           {
             name: _('/test.js'),
             contents: `
-              import {__assign, __spread, __spreadArrays, __spreadArray} from 'tslib';
+              import {__assign, __spread, __spreadArrays, __spreadArray, __read} from 'tslib';
 
               var a = __assign({foo: 'bar'}, {baz: 'qux'});
               var b = __spread(['foo', 'bar'], ['baz', 'qux']);
               var c = __spreadArrays(['foo', 'bar'], ['baz', 'qux']);
               var d = __spreadArray(['foo', 'bar'], ['baz', 'qux']);
+              var e = __read(['foo', 'bar']);
             `,
           },
           {
@@ -2192,6 +2205,7 @@ runInEachFileSystem(() => {
               export declare function __spread(...args: any[][]): any[];
               export declare function __spreadArrays(...args: any[][]): any[];
               export declare function __spreadArray(to: any[], from: any[]): any[];
+              export declare function __read(o: any, n?: number): any[];
             `,
           },
         ];
@@ -2210,6 +2224,7 @@ runInEachFileSystem(() => {
         testForHelper('b', '__spread', KnownDeclaration.TsHelperSpread, 'tslib');
         testForHelper('c', '__spreadArrays', KnownDeclaration.TsHelperSpreadArrays, 'tslib');
         testForHelper('d', '__spreadArray', KnownDeclaration.TsHelperSpreadArray, 'tslib');
+        testForHelper('e', '__read', KnownDeclaration.TsHelperRead, 'tslib');
       });
 
       it('should recognize imported TypeScript helpers (star import)', () => {
@@ -2223,6 +2238,7 @@ runInEachFileSystem(() => {
               var b = tslib_1.__spread(['foo', 'bar'], ['baz', 'qux']);
               var c = tslib_1.__spreadArrays(['foo', 'bar'], ['baz', 'qux']);
               var d = tslib_1.__spreadArray(['foo', 'bar'], ['baz', 'qux']);
+              var e = tslib_1.__read(['foo', 'bar']);
             `,
           },
           {
@@ -2232,6 +2248,7 @@ runInEachFileSystem(() => {
               export declare function __spread(...args: any[][]): any[];
               export declare function __spreadArrays(...args: any[][]): any[];
               export declare function __spreadArray(to: any[], from: any[]): any[];
+              export declare function __read(o: any, n?: number): any[];
             `,
           },
         ];
@@ -2250,6 +2267,7 @@ runInEachFileSystem(() => {
         testForHelper('b', '__spread', KnownDeclaration.TsHelperSpread, 'tslib');
         testForHelper('c', '__spreadArrays', KnownDeclaration.TsHelperSpreadArrays, 'tslib');
         testForHelper('d', '__spreadArray', KnownDeclaration.TsHelperSpreadArray, 'tslib');
+        testForHelper('e', '__read', KnownDeclaration.TsHelperRead, 'tslib');
       });
 
       it('should recognize undeclared, unimported TypeScript helpers (by name)', () => {
@@ -2260,6 +2278,7 @@ runInEachFileSystem(() => {
             var b = __spread(['foo', 'bar'], ['baz', 'qux']);
             var c = __spreadArrays(['foo', 'bar'], ['baz', 'qux']);
             var d = __spreadArray(['foo', 'bar'], ['baz', 'qux']);
+            var e = __read(['foo', 'bar']);
           `,
         };
         loadTestFiles([file]);
@@ -2283,6 +2302,7 @@ runInEachFileSystem(() => {
         testForHelper('b', '__spread', KnownDeclaration.TsHelperSpread);
         testForHelper('c', '__spreadArrays', KnownDeclaration.TsHelperSpreadArrays);
         testForHelper('d', '__spreadArray', KnownDeclaration.TsHelperSpreadArray);
+        testForHelper('e', '__read', KnownDeclaration.TsHelperRead);
       });
 
       it('should recognize suffixed, undeclared, unimported TypeScript helpers (by name)', () => {
@@ -2293,6 +2313,7 @@ runInEachFileSystem(() => {
             var b = __spread$2(['foo', 'bar'], ['baz', 'qux']);
             var c = __spreadArrays$3(['foo', 'bar'], ['baz', 'qux']);
             var d = __spreadArray$3(['foo', 'bar'], ['baz', 'qux']);
+            var e = __read$3(['foo', 'bar']);
           `,
         };
         loadTestFiles([file]);
@@ -2316,6 +2337,7 @@ runInEachFileSystem(() => {
         testForHelper('b', '__spread$2', KnownDeclaration.TsHelperSpread);
         testForHelper('c', '__spreadArrays$3', KnownDeclaration.TsHelperSpreadArrays);
         testForHelper('d', '__spreadArray$3', KnownDeclaration.TsHelperSpreadArray);
+        testForHelper('e', '__read', KnownDeclaration.TsHelperRead);
       });
 
       it('should recognize enum declarations with string values', () => {
@@ -2479,6 +2501,7 @@ runInEachFileSystem(() => {
             export declare function __spread(...args: any[][]): any[];
             export declare function __spreadArrays(...args: any[][]): any[];
             export declare function __spreadArray(to: any[], from: any[]): any[];
+            export declare function __read(o: any, n?: number): any[];
             export declare function __unknownHelper(...args: any[]): any;
           `,
         };
@@ -2494,6 +2517,7 @@ runInEachFileSystem(() => {
               ['__spread', KnownDeclaration.TsHelperSpread],
               ['__spreadArrays', KnownDeclaration.TsHelperSpreadArrays],
               ['__spreadArray', KnownDeclaration.TsHelperSpreadArray],
+              ['__read', KnownDeclaration.TsHelperRead],
               ['__unknownHelper', null],
             ]);
       });

packages/compiler-cli/ngcc/test/host/umd_host_spec.ts

@@ -2332,11 +2332,13 @@ runInEachFileSystem(() => {
               function __spread(...args) { /* ... */ }
               function __spreadArrays(...args) { /* ... */ }
               function __spreadArray(to, from) { /* ... */ }
+              function __read(o) { /* ... */ }
 
               var a = __assign({foo: 'bar'}, {baz: 'qux'});
               var b = __spread(['foo', 'bar'], ['baz', 'qux']);
               var c = __spreadArrays(['foo', 'bar'], ['baz', 'qux']);
               var d = __spreadArray(['foo', 'bar'], ['baz', 'qux']);
+              var e = __read(['foo', 'bar']);
             })));
           `,
           };
@@ -2352,6 +2354,7 @@ runInEachFileSystem(() => {
           testForHelper('b', '__spread', KnownDeclaration.TsHelperSpread);
           testForHelper('c', '__spreadArrays', KnownDeclaration.TsHelperSpreadArrays);
           testForHelper('d', '__spreadArray', KnownDeclaration.TsHelperSpreadArray);
+          testForHelper('e', '__read', KnownDeclaration.TsHelperRead);
         });
 
         it('should recognize suffixed TypeScript helpers (as function declarations)', () => {
@@ -2367,11 +2370,13 @@ runInEachFileSystem(() => {
               function __spread$2(...args) { /* ... */ }
               function __spreadArrays$3(...args) { /* ... */ }
               function __spreadArray$3(to, from) { /* ... */ }
+              function __read$3(o) { /* ... */ }
 
               var a = __assign$1({foo: 'bar'}, {baz: 'qux'});
               var b = __spread$2(['foo', 'bar'], ['baz', 'qux']);
               var c = __spreadArrays$3(['foo', 'bar'], ['baz', 'qux']);
               var d = __spreadArray$3(['foo', 'bar'], ['baz', 'qux']);
+              var e = __read$3(['foo', 'bar']);
             })));
           `,
           };
@@ -2387,6 +2392,7 @@ runInEachFileSystem(() => {
           testForHelper('b', '__spread$2', KnownDeclaration.TsHelperSpread);
           testForHelper('c', '__spreadArrays$3', KnownDeclaration.TsHelperSpreadArrays);
           testForHelper('d', '__spreadArray$3', KnownDeclaration.TsHelperSpreadArray);
+          testForHelper('e', '__read$3', KnownDeclaration.TsHelperRead);
         });
 
         it('should recognize TypeScript helpers (as variable declarations)', () => {
@@ -2402,11 +2408,13 @@ runInEachFileSystem(() => {
               var __spread = (this && this.__spread) || function (...args) { /* ... */ }
               var __spreadArrays = (this && this.__spreadArrays) || function (...args) { /* ... */ }
               var __spreadArray = (this && this.__spreadArray) || function (to, from) { /* ... */ }
+              var __read = (this && this.__read) || function (o) { /* ... */ }
 
               var a = __assign({foo: 'bar'}, {baz: 'qux'});
               var b = __spread(['foo', 'bar'], ['baz', 'qux']);
               var c = __spreadArrays(['foo', 'bar'], ['baz', 'qux']);
               var d = __spreadArray(['foo', 'bar'], ['baz', 'qux']);
+              var e = __read(['foo', 'bar']);
             })));
           `,
           };
@@ -2422,6 +2430,7 @@ runInEachFileSystem(() => {
           testForHelper('b', '__spread', KnownDeclaration.TsHelperSpread);
           testForHelper('c', '__spreadArrays', KnownDeclaration.TsHelperSpreadArrays);
           testForHelper('d', '__spreadArray', KnownDeclaration.TsHelperSpreadArray);
+          testForHelper('e', '__read', KnownDeclaration.TsHelperRead);
         });
 
         it('should recognize suffixed TypeScript helpers (as variable declarations)', () => {
@@ -2437,11 +2446,13 @@ runInEachFileSystem(() => {
               var __spread$2 = (this && this.__spread$2) || function (...args) { /* ... */ }
               var __spreadArrays$3 = (this && this.__spreadArrays$3) || function (...args) { /* ... */ }
               var __spreadArray$3 = (this && this.__spreadArray$3) || function (to, from) { /* ... */ }
+              var __read$3 = (this && this.__read$3) || function (o) { /* ... */ }
 
               var a = __assign$1({foo: 'bar'}, {baz: 'qux'});
               var b = __spread$2(['foo', 'bar'], ['baz', 'qux']);
               var c = __spreadArrays$3(['foo', 'bar'], ['baz', 'qux']);
               var d = __spreadArray$3(['foo', 'bar'], ['baz', 'qux']);
+              var e = __read$3(['foo', 'bar']);
             })));
           `,
           };
@@ -2457,6 +2468,7 @@ runInEachFileSystem(() => {
           testForHelper('b', '__spread$2', KnownDeclaration.TsHelperSpread);
           testForHelper('c', '__spreadArrays$3', KnownDeclaration.TsHelperSpreadArrays);
           testForHelper('d', '__spreadArray$3', KnownDeclaration.TsHelperSpreadArray);
+          testForHelper('e', '__read$3', KnownDeclaration.TsHelperRead);
         });
 
         it('should recognize imported TypeScript helpers', () => {
@@ -2473,6 +2485,7 @@ runInEachFileSystem(() => {
                 var b = tslib_1.__spread(['foo', 'bar'], ['baz', 'qux']);
                 var c = tslib_1.__spreadArrays(['foo', 'bar'], ['baz', 'qux']);
                 var d = tslib_1.__spreadArray(['foo', 'bar'], ['baz', 'qux']);
+                var e = tslib_1.__read(['foo', 'bar']);
               })));
             `,
             },
@@ -2483,6 +2496,7 @@ runInEachFileSystem(() => {
               export declare function __spread(...args: any[][]): any[];
               export declare function __spreadArrays(...args: any[][]): any[];
               export declare function __spreadArray(to: any[], from: any[]): any[];
+              export declare function __read(o: any, n?: number): any[];
             `,
             },
           ];
@@ -2502,6 +2516,7 @@ runInEachFileSystem(() => {
           testForHelper('b', '__spread', KnownDeclaration.TsHelperSpread, 'tslib');
           testForHelper('c', '__spreadArrays', KnownDeclaration.TsHelperSpreadArrays, 'tslib');
           testForHelper('d', '__spreadArray', KnownDeclaration.TsHelperSpreadArray, 'tslib');
+          testForHelper('e', '__read', KnownDeclaration.TsHelperRead, 'tslib');
         });
 
         it('should recognize undeclared, unimported TypeScript helpers (by name)', () => {
@@ -2517,6 +2532,7 @@ runInEachFileSystem(() => {
               var b = __spread(['foo', 'bar'], ['baz', 'qux']);
               var c = __spreadArrays(['foo', 'bar'], ['baz', 'qux']);
               var d = __spreadArray(['foo', 'bar'], ['baz', 'qux']);
+              var e = __read(['foo', 'bar']);
             })));
           `,
           };
@@ -2544,6 +2560,7 @@ runInEachFileSystem(() => {
           testForHelper('b', '__spread', KnownDeclaration.TsHelperSpread);
           testForHelper('c', '__spreadArrays', KnownDeclaration.TsHelperSpreadArrays);
           testForHelper('d', '__spreadArray', KnownDeclaration.TsHelperSpreadArray);
+          testForHelper('e', '__read', KnownDeclaration.TsHelperRead);
         });
 
         it('should recognize suffixed, undeclared, unimported TypeScript helpers (by name)', () => {
@@ -2559,6 +2576,7 @@ runInEachFileSystem(() => {
               var b = __spread$2(['foo', 'bar'], ['baz', 'qux']);
               var c = __spreadArrays$3(['foo', 'bar'], ['baz', 'qux']);
               var d = __spreadArray$3(['foo', 'bar'], ['baz', 'qux']);
+              var e = __read$3(['foo', 'bar']);
             })));
           `,
           };
@@ -2586,6 +2604,7 @@ runInEachFileSystem(() => {
           testForHelper('b', '__spread$2', KnownDeclaration.TsHelperSpread);
           testForHelper('c', '__spreadArrays$3', KnownDeclaration.TsHelperSpreadArrays);
           testForHelper('d', '__spreadArray$3', KnownDeclaration.TsHelperSpreadArray);
+          testForHelper('e', '__read$3', KnownDeclaration.TsHelperRead);
         });
 
         it('should recognize enum declarations with string values', () => {
@@ -2861,6 +2880,7 @@ runInEachFileSystem(() => {
             export declare function __spread(...args: any[][]): any[];
             export declare function __spreadArrays(...args: any[][]): any[];
             export declare function __spreadArray(to: any[], from: any[]): any[];
+            export declare function __read(o: any, n?: number): any[];
             export declare function __unknownHelper(...args: any[]): any;
           `,
           };
@@ -2876,6 +2896,7 @@ runInEachFileSystem(() => {
                 ['__spread', KnownDeclaration.TsHelperSpread],
                 ['__spreadArrays', KnownDeclaration.TsHelperSpreadArrays],
                 ['__spreadArray', KnownDeclaration.TsHelperSpreadArray],
+                ['__read', KnownDeclaration.TsHelperRead],
                 ['__unknownHelper', null],
               ]);
         });

packages/compiler-cli/src/ngtsc/partial_evaluator/src/known_declaration.ts

@@ -10,7 +10,7 @@ import {KnownDeclaration} from '../../reflection/src/host';
 
 import {ObjectAssignBuiltinFn} from './builtin';
 import {ResolvedValue} from './result';
-import {AssignHelperFn, SpreadArrayHelperFn, SpreadHelperFn} from './ts_helpers';
+import {AssignHelperFn, ReadHelperFn, SpreadArrayHelperFn, SpreadHelperFn} from './ts_helpers';
 
 /** Resolved value for the JavaScript global `Object` declaration. */
 export const jsGlobalObjectValue = new Map([['assign', new ObjectAssignBuiltinFn()]]);
@@ -24,6 +24,9 @@ const spreadTsHelperFn = new SpreadHelperFn();
 /** Resolved value for the `__spreadArray()` TypeScript helper declarations. */
 const spreadArrayTsHelperFn = new SpreadArrayHelperFn();
 
+/** Resolved value for the `__read()` TypeScript helper declarations. */
+const readTsHelperFn = new ReadHelperFn();
+
 /**
  * Resolves the specified known declaration to a resolved value. For example,
  * the known JavaScript global `Object` will resolve to a `Map` that provides the
@@ -40,6 +43,8 @@ export function resolveKnownDeclaration(decl: KnownDeclaration): ResolvedValue {
       return spreadTsHelperFn;
     case KnownDeclaration.TsHelperSpreadArray:
       return spreadArrayTsHelperFn;
+    case KnownDeclaration.TsHelperRead:
+      return readTsHelperFn;
     default:
       throw new Error(`Cannot resolve known declaration. Received: ${KnownDeclaration[decl]}.`);
   }

packages/compiler-cli/src/ngtsc/partial_evaluator/src/ts_helpers.ts

@@ -59,3 +59,24 @@ export class SpreadArrayHelperFn extends KnownFn {
     return to.concat(from);
   }
 }
+
+// Used for `__read` TypeScript helper function.
+export class ReadHelperFn extends KnownFn {
+  evaluate(node: ts.Node, args: ResolvedValueArray): ResolvedValue {
+    if (args.length !== 1) {
+      // The `__read` helper accepts a second argument `n` but that case is not supported.
+      return DynamicValue.fromUnknown(node);
+    }
+
+    const [value] = args;
+    if (value instanceof DynamicValue) {
+      return DynamicValue.fromDynamicInput(node, value);
+    }
+
+    if (!Array.isArray(value)) {
+      return DynamicValue.fromInvalidExpressionType(node, value);
+    }
+
+    return value;
+  }
+}

packages/compiler-cli/src/ngtsc/partial_evaluator/test/evaluator_spec.ts

@@ -647,6 +647,7 @@ runInEachFileSystem(() => {
               export declare function __spread(...args: any[][]): any[];
               export declare function __spreadArrays(...args: any[][]): any[];
               export declare function __spreadArray(to: any[], from: any[]): any[];
+              export declare function __read(o: any, n?: number): any[];
             `,
           },
         ]);
@@ -758,6 +759,28 @@ runInEachFileSystem(() => {
 
         expect(arr).toEqual([4, 5, 6]);
       });
+
+      it('should evaluate `__read()` (named import)', () => {
+        const arr: number[] = evaluateExpression(
+            `
+              import {__read} from 'tslib';
+              const a = [5, 6];
+            `,
+            '__read(a)');
+
+        expect(arr).toEqual([5, 6]);
+      });
+
+      it('should evaluate `__read()` (star import)', () => {
+        const arr: number[] = evaluateExpression(
+            `
+              import * as tslib from 'tslib';
+              const a = [5, 6];
+            `,
+            'tslib.__read(a)');
+
+        expect(arr).toEqual([5, 6]);
+      });
     });
 
     describe('(with emitted TypeScript helpers as functions)', () => {
@@ -768,6 +791,7 @@ runInEachFileSystem(() => {
           function __spread(...args) { /* ... */ }
           function __spreadArrays(...args) { /* ... */ }
           function __spreadArray(to, from) { /* ... */ }
+          function __read(o) { /* ... */ }
         `;
         const {checker, expression} = makeExpression(helpers + code, expr);
 
@@ -823,6 +847,16 @@ runInEachFileSystem(() => {
 
         expect(arr).toEqual([4, 5, 6]);
       });
+
+      it('should evaluate `__read()`', () => {
+        const arr: number[] = evaluateExpression(
+            `
+              const a = [5, 6];
+            `,
+            '__read(a)');
+
+        expect(arr).toEqual([5, 6]);
+      });
     });
 
     describe('(with emitted TypeScript helpers as variables)', () => {
@@ -833,6 +867,7 @@ runInEachFileSystem(() => {
           var __spread = (this && this.__spread) || function (...args) { /* ... */ }
           var __spreadArrays = (this && this.__spreadArrays) || function (...args) { /* ... */ }
           var __spreadArray = (this && this.__spreadArray) || function (to, from) { /* ... */ }
+          var __read = (this && this.__read) || function (o) { /* ... */ }
         `;
         const {checker, expression} = makeExpression(helpers + code, expr);
 
@@ -888,6 +923,16 @@ runInEachFileSystem(() => {
 
         expect(arr).toEqual([4, 5, 6]);
       });
+
+      it('should evaluate `__read()`', () => {
+        const arr: number[] = evaluateExpression(
+            `
+              const a = [5, 6];
+            `,
+            '__read(a)');
+
+        expect(arr).toEqual([5, 6]);
+      });
     });
 
     describe('(visited file tracking)', () => {
@@ -1031,6 +1076,8 @@ runInEachFileSystem(() => {
         return KnownDeclaration.TsHelperSpreadArrays;
       case '__spreadArray':
         return KnownDeclaration.TsHelperSpreadArray;
+      case '__read':
+        return KnownDeclaration.TsHelperRead;
       default:
         return null;
     }

packages/compiler-cli/src/ngtsc/reflection/src/host.ts

@@ -479,6 +479,11 @@ export enum KnownDeclaration {
    * Indicates the `__spreadArray` TypeScript helper function.
    */
   TsHelperSpreadArray,
+
+  /**
+   * Indicates the `__read` TypeScript helper function.
+   */
+  TsHelperRead,
 }
 
 /**