fix(language-service): infer $implicit value for ngIf template contexts (#35941)

Today, the language service infers the type of variables bound to the
"ngIf" template context member of an NgIf directive, but does not do the
same for the the "$implicit" context member. This commit adds support
for that.

Fixes https://github.com/angular/vscode-ng-language-service/issues/676

PR Close #35941

packages/language-service/src/expression_diagnostics.ts

@@ -133,6 +133,7 @@ function getVariableTypeFromDirectiveContext(
       }
     }
   }
+
   return query.getBuiltinType(BuiltinType.Any);
 }
 
@@ -149,7 +150,7 @@ function refinedVariableType(
     value: string, mergedTable: SymbolTable, query: SymbolQuery,
     templateElement: EmbeddedTemplateAst): Symbol {
   if (value === '$implicit') {
-    // Special case the ngFor directive
+    // Special case: ngFor directive
     const ngForDirective = templateElement.directives.find(d => {
       const name = identifierName(d.directive.type);
       return name == 'NgFor' || name == 'NgForOf';
@@ -169,13 +170,20 @@ function refinedVariableType(
     }
   }
 
-  // Special case the ngIf directive ( *ngIf="data$ | async as variable" )
-  if (value === 'ngIf') {
+  if (value === 'ngIf' || value === '$implicit') {
     const ngIfDirective =
         templateElement.directives.find(d => identifierName(d.directive.type) === 'NgIf');
     if (ngIfDirective) {
+      // Special case: ngIf directive. The NgIf structural directive owns a template context with
+      // "$implicit" and "ngIf" members. These properties are typed as generics. Until the language
+      // service uses an Ivy and TypecheckBlock backend, we cannot bind these values to a concrete
+      // type without manual inference. To get the concrete type, look up the type of the "ngIf"
+      // import on the NgIf directive bound to the template.
+      //
+      // See @angular/common/ng_if.ts for more information.
       const ngIfBinding = ngIfDirective.inputs.find(i => i.directiveName === 'ngIf');
       if (ngIfBinding) {
+        // Check if there is a known type bound to the ngIf input.
         const bindingType = new AstType(mergedTable, query, {}).getType(ngIfBinding.value);
         if (bindingType) {
           return bindingType;

packages/language-service/test/diagnostics_spec.ts

@@ -144,6 +144,44 @@ describe('diagnostics', () => {
     });
   });
 
+  describe('diagnostics for ngIf exported values', () => {
+    it('should infer the type of an implicit value in an NgIf context', () => {
+      mockHost.override(TEST_TEMPLATE, `
+        <div *ngIf="title; let titleProxy;">
+            'titleProxy' is a string
+          {{~{start-err}titleProxy.notAProperty~{end-err}}}
+        </div>
+      `);
+      const diags = ngLS.getSemanticDiagnostics(TEST_TEMPLATE);
+      expect(diags.length).toBe(1);
+      const {messageText, start, length} = diags[0];
+      expect(messageText)
+          .toBe(
+              `Identifier 'notAProperty' is not defined. 'string' does not contain such a member`);
+      const span = mockHost.getLocationMarkerFor(TEST_TEMPLATE, 'err');
+      expect(start).toBe(span.start);
+      expect(length).toBe(span.length);
+    });
+
+    it('should infer the type of an ngIf value in an NgIf context', () => {
+      mockHost.override(TEST_TEMPLATE, `
+        <div *ngIf="title as titleProxy">
+            'titleProxy' is a string
+          {{~{start-err}titleProxy.notAProperty~{end-err}}}
+        </div>
+      `);
+      const diags = ngLS.getSemanticDiagnostics(TEST_TEMPLATE);
+      expect(diags.length).toBe(1);
+      const {messageText, start, length} = diags[0];
+      expect(messageText)
+          .toBe(
+              `Identifier 'notAProperty' is not defined. 'string' does not contain such a member`);
+      const span = mockHost.getLocationMarkerFor(TEST_TEMPLATE, 'err');
+      expect(start).toBe(span.start);
+      expect(length).toBe(span.length);
+    });
+  });
+
   describe('diagnostics for invalid indexed type property access', () => {
     it('should work with numeric index signatures (arrays)', () => {
       mockHost.override(TEST_TEMPLATE, `