refactor(compiler): add details while throw error during expression convert (#32760)

Fixes #32759

PR Close #32760

packages/compiler/src/compiler_util/expression_converter.ts

@@ -551,7 +551,10 @@ class _AstToIrVisitor implements cdAst.AstVisitor {
           this.usesImplicitReceiver = prevUsesImplicitReceiver;
         } else {
           // Otherwise it's an error.
-          throw new Error('Cannot assign to a reference or variable!');
+          const receiver = ast.name;
+          const value = (ast.value instanceof cdAst.PropertyRead) ? ast.value.name : undefined;
+          throw new Error(
+              `Cannot assign value "${value}" to template variable "${receiver}". Template variables are read-only.`);
         }
       }
     }

packages/core/test/linker/change_detection_integration_spec.ts

@@ -662,9 +662,9 @@ const TEST_COMPILER_PROVIDERS: Provider[] = [
            }));
 
         it('should throw when trying to assign to a local', fakeAsync(() => {
-             expect(() => {
-               _bindSimpleProp('(event)="$event=1"');
-             }).toThrowError(new RegExp('Cannot assign to a reference or variable!'));
+             expect(() => { _bindSimpleProp('(event)="$event=1"'); })
+                 .toThrowError(new RegExp(
+                     'Cannot assign value (.*) to template variable (.*). Template variables are read-only.'));
            }));
 
         it('should support short-circuiting', fakeAsync(() => {