fix(compiler-cli): Produce diagnostic rather than crash when using in…

packages/compiler-cli/src/ngtsc/metadata/src/host_directives_resolver.ts

@@ -43,12 +43,9 @@ export class HostDirectivesResolver {
     for (const current of directives) {
       const hostMeta = flattenInheritedDirectiveMetadata(this.metaReader, current.directive);
 
-      // This case has been checked for already, but we keep the assertion here so that the
-      // user gets a better error message than "Cannot read property foo of null" in case
-      // something slipped through.
+      // This case has been checked for already and produces a diagnostic
       if (hostMeta === null) {
-        throw new Error(
-            `Could not resolve host directive metadata of ${current.directive.debugName}`);
+        continue;
       }
 
       if (hostMeta.hostDirectives) {

packages/compiler-cli/src/ngtsc/metadata/src/inheritance.ts

@@ -21,10 +21,10 @@ import {ClassPropertyMapping, ClassPropertyName} from './property_mapping';
  * followed.
  */
 export function flattenInheritedDirectiveMetadata(
-    reader: MetadataReader, dir: Reference<ClassDeclaration>): DirectiveMeta {
+    reader: MetadataReader, dir: Reference<ClassDeclaration>): DirectiveMeta|null {
   const topMeta = reader.getDirectiveMetadata(dir);
   if (topMeta === null) {
-    throw new Error(`Metadata not found for directive: ${dir.debugName}`);
+    return null;
   }
   if (topMeta.baseClass === null) {
     return topMeta;

packages/compiler-cli/src/ngtsc/scope/src/typecheck.ts

@@ -99,6 +99,9 @@ export class TypeCheckScopeRegistry {
     for (const meta of dependencies) {
       if (meta.kind === MetaKind.Directive && meta.selector !== null) {
         const extMeta = this.getTypeCheckDirectiveMetadata(meta.ref);
+        if (extMeta === null) {
+          continue;
+        }
         matcher.addSelectables(
             CssSelector.parse(meta.selector),
             [...this.hostDirectivesResolver.resolve(extMeta), extMeta]);
@@ -125,13 +128,16 @@ export class TypeCheckScopeRegistry {
     return typeCheckScope;
   }
 
-  getTypeCheckDirectiveMetadata(ref: Reference<ClassDeclaration>): DirectiveMeta {
+  getTypeCheckDirectiveMetadata(ref: Reference<ClassDeclaration>): DirectiveMeta|null {
     const clazz = ref.node;
     if (this.flattenedDirectiveMetaCache.has(clazz)) {
       return this.flattenedDirectiveMetaCache.get(clazz)!;
     }
 
     const meta = flattenInheritedDirectiveMetadata(this.metaReader, ref);
+    if (meta === null) {
+      return null;
+    }
     this.flattenedDirectiveMetaCache.set(clazz, meta);
     return meta;
   }

packages/compiler-cli/test/ngtsc/host_directives_spec.ts

@@ -451,6 +451,24 @@ runInEachFileSystem(() => {
         expect(messages).toEqual(['Host directive HostDir must be standalone']);
       });
 
+      it('should produce a diagnostic if a host directive is not a directive', () => {
+        env.write('test.ts', `
+          import {Directive, Pipe, Component, NgModule} from '@angular/core';
+
+          @Pipe({name: 'hostDir'})
+          export class HostDir {}
+
+          @Directive({
+            hostDirectives: [HostDir],
+          })
+          export class Dir {}
+        `);
+
+        const messages = env.driveDiagnostics().map(extractMessage);
+        expect(messages).toEqual(
+            ['HostDir must be a standalone directive to be used as a host directive']);
+      });
+
       it('should produce a diagnostic if a host directive is a component', () => {
         env.write('test.ts', `
           import {Directive, Component, NgModule} from '@angular/core';