refactor(compiler-cli): add NullishCoalescingNotNullableCheck (#43232)

Add a template check that returns diagnostics if the left side of a
nullish coalescing operation is not nullable.

Refs #42966

PR Close #43232

packages/compiler-cli/src/ngtsc/core/BUILD.bazel

@@ -40,6 +40,7 @@ ts_library(
         "//packages/compiler-cli/src/ngtsc/typecheck/extended",
         "//packages/compiler-cli/src/ngtsc/typecheck/extended/api",
         "//packages/compiler-cli/src/ngtsc/typecheck/extended/checks/invalid_banana_in_box",
+        "//packages/compiler-cli/src/ngtsc/typecheck/extended/checks/nullish_coalescing_not_nullable",
         "//packages/compiler-cli/src/ngtsc/util",
         "//packages/compiler-cli/src/ngtsc/xi18n",
         "@npm//typescript",

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

@@ -31,8 +31,9 @@ import {aliasTransformFactory, CompilationMode, declarationTransformFactory, Dec
 import {TemplateTypeCheckerImpl} from '../../typecheck';
 import {OptimizeFor, TemplateTypeChecker, TypeCheckingConfig} from '../../typecheck/api';
 import {ExtendedTemplateCheckerImpl} from '../../typecheck/extended';
-import {ExtendedTemplateChecker} from '../../typecheck/extended/api';
+import {ExtendedTemplateChecker, TemplateCheck} from '../../typecheck/extended/api';
 import {InvalidBananaInBoxCheck} from '../../typecheck/extended/checks/invalid_banana_in_box';
+import {NullishCoalescingNotNullableCheck} from '../../typecheck/extended/checks/nullish_coalescing_not_nullable';
 import {getSourceFileOrNull, isDtsPath, resolveModuleName, toUnredirectedSourceFile} from '../../util/src/typescript';
 import {Xi18nContext} from '../../xi18n';
 import {LazyRoute, NgCompilerAdapter, NgCompilerOptions} from '../api';
@@ -1109,7 +1110,10 @@ export class NgCompiler {
         reflector, this.adapter, this.incrementalCompilation, scopeRegistry, typeCheckScopeRegistry,
         this.delegatingPerfRecorder);
 
-    const templateChecks = [new InvalidBananaInBoxCheck()];
+    const templateChecks: TemplateCheck<ErrorCode>[] = [new InvalidBananaInBoxCheck()];
+    if (this.options.strictNullChecks) {
+      templateChecks.push(new NullishCoalescingNotNullableCheck());
+    }
     const extendedTemplateChecker =
         new ExtendedTemplateCheckerImpl(templateTypeChecker, checker, templateChecks);
 

packages/compiler-cli/src/ngtsc/diagnostics/src/error_code.ts

@@ -182,6 +182,16 @@ export enum ErrorCode {
    */
   INVALID_BANANA_IN_BOX = 8101,
 
+  /**
+   * The left side of a nullish coalescing operation is not nullable.
+   *
+   * ```
+   * {{ foo ?? bar }}
+   * ```
+   * When the type of foo doesn't include `null` or `undefined`.
+   */
+  NULLISH_COALESCING_NOT_NULLABLE = 8102,
+
   /**
    * The template type-checking engine would need to generate an inline type check block for a
    * component, but the current type-checking environment doesn't support it.

packages/compiler-cli/src/ngtsc/typecheck/extended/checks/nullish_coalescing_not_nullable/BUILD.bazel

@@ -0,0 +1,14 @@
+load("//tools:defaults.bzl", "ts_library")
+
+ts_library(
+    name = "nullish_coalescing_not_nullable",
+    srcs = ["index.ts"],
+    visibility = ["//packages/compiler-cli/src/ngtsc:__subpackages__"],
+    deps = [
+        "//packages/compiler",
+        "//packages/compiler-cli/src/ngtsc/diagnostics",
+        "//packages/compiler-cli/src/ngtsc/typecheck/api",
+        "//packages/compiler-cli/src/ngtsc/typecheck/extended/api",
+        "@npm//typescript",
+    ],
+)

packages/compiler-cli/src/ngtsc/typecheck/extended/checks/nullish_coalescing_not_nullable/index.ts

@@ -0,0 +1,51 @@
+/**
+ * @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 {AST, Binary, TmplAstNode} from '@angular/compiler';
+import * as ts from 'typescript';
+
+import {ErrorCode} from '../../../../diagnostics';
+import {NgTemplateDiagnostic, SymbolKind} from '../../../api';
+import {TemplateCheckWithVisitor, TemplateContext} from '../../api';
+
+/**
+ * Ensures the left side of a nullish coalescing operation is nullable.
+ * Returns diagnostics for the cases where the operator is useless.
+ * This check should only be use if `strictNullChecks` is enabled,
+ * otherwise it would produce inaccurate results.
+ */
+export class NullishCoalescingNotNullableCheck extends
+    TemplateCheckWithVisitor<ErrorCode.NULLISH_COALESCING_NOT_NULLABLE> {
+  override code = ErrorCode.NULLISH_COALESCING_NOT_NULLABLE as const;
+
+  override visitNode(ctx: TemplateContext, component: ts.ClassDeclaration, node: TmplAstNode|AST):
+      NgTemplateDiagnostic<ErrorCode.NULLISH_COALESCING_NOT_NULLABLE>[] {
+    if (!(node instanceof Binary) || node.operation !== '??') return [];
+
+    const symbolLeft = ctx.templateTypeChecker.getSymbolOfNode(node.left, component)!;
+    if (symbolLeft.kind !== SymbolKind.Expression) {
+      return [];
+    }
+    const typeLeft = symbolLeft.tsType;
+    // If the left operand's type is different from its non-nullable self, then it must
+    // contain a null or undefined so this nullish coalescing operator is useful. No diagnostic to
+    // report.
+    if (typeLeft.getNonNullableType() !== typeLeft) return [];
+
+    const symbol = ctx.templateTypeChecker.getSymbolOfNode(node, component)!;
+    if (symbol.kind !== SymbolKind.Expression) {
+      return [];
+    }
+    const span =
+        ctx.templateTypeChecker.getTemplateMappingAtShimLocation(symbol.shimLocation)!.span;
+    const diagnostic = ctx.templateTypeChecker.makeTemplateDiagnostic(
+        component, span, ts.DiagnosticCategory.Warning, ErrorCode.NULLISH_COALESCING_NOT_NULLABLE,
+        `The left side of this nullish coalescing operation does not include 'null' or 'undefined' in its type, therefore the '??' operator can be safely removed.`);
+    return [diagnostic];
+  }
+}