fix(compiler-cli): symbol feature detection for the compiler

packages/compiler-cli/src/ngtsc/core/src/compiler.ts

@@ -6,6 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
+import {R3Identifiers} from '@angular/compiler';
 import ts from 'typescript';
 
 import {ComponentDecoratorHandler, DirectiveDecoratorHandler, InjectableDecoratorHandler, NgModuleDecoratorHandler, NoopReferencesRegistry, PipeDecoratorHandler, ReferencesRegistry} from '../../annotations';
@@ -37,6 +38,7 @@ import {getSourceFileOrNull, isDtsPath, toUnredirectedSourceFile} from '../../ut
 import {Xi18nContext} from '../../xi18n';
 import {DiagnosticCategoryLabel, NgCompilerAdapter, NgCompilerOptions} from '../api';
 
+import {coreHasSymbol} from './core_version';
 import {coreVersionSupportsFeature} from './feature_detection';
 
 const SHOULD_USE_TEMPLATE_PIPELINE = true;
@@ -796,10 +798,15 @@ export class NgCompiler {
 
     const useInlineTypeConstructors = this.programDriver.supportsInlineOperations;
 
-    // Only Angular versions greater than 17.2 have the necessary symbols to type check signals in
-    // two-way bindings. We also allow version 0.0.0 in case somebody is using Angular at head.
-    const allowSignalsInTwoWayBindings = this.angularCoreVersion === null ||
-        coreVersionSupportsFeature(this.angularCoreVersion, '>= 17.2.0-0');
+    // Check whether the loaded version of `@angular/core` in the `ts.Program` supports unwrapping
+    // writable signals for type-checking. If this check fails to find a suitable .d.ts file, fall
+    // back to version detection. Only Angular versions greater than 17.2 have the necessary symbols
+    // to type check signals in two-way bindings. We also allow version 0.0.0 in case somebody is
+    // using Angular at head.
+    let allowSignalsInTwoWayBindings =
+        coreHasSymbol(this.inputProgram, R3Identifiers.unwrapWritableSignal) ??
+        (this.angularCoreVersion === null ||
+         coreVersionSupportsFeature(this.angularCoreVersion, '>= 17.2.0-0'));
 
     // First select a type-checking configuration, based on whether full template type-checking is
     // requested.

packages/compiler-cli/src/ngtsc/core/src/core_version.ts

@@ -0,0 +1,32 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {ExternalReference} from '@angular/compiler';
+import ts from 'typescript';
+
+export function coreHasSymbol(program: ts.Program, symbol: ExternalReference): boolean|null {
+  const checker = program.getTypeChecker();
+  for (const sf of program.getSourceFiles().filter(isMaybeCore)) {
+    const sym = checker.getSymbolAtLocation(sf);
+    if (sym === undefined || sym.exports === undefined) {
+      continue;
+    }
+    if (!sym.exports.has('ɵɵtemplate' as ts.__String)) {
+      // This is not @angular/core.
+      continue;
+    }
+    return sym.exports.has(symbol.name as ts.__String);
+  }
+  // No @angular/core file found, so we have no information.
+  return null;
+}
+
+export function isMaybeCore(sf: ts.SourceFile): boolean {
+  return sf.isDeclarationFile && sf.fileName.includes('@angular/core') &&
+      sf.fileName.endsWith('index.d.ts');
+}