fix(compiler): improve error message when a module imports itself (#14646)

Closes #14644

modules/@angular/compiler/src/metadata_resolver.ts

@@ -425,6 +425,7 @@ export class CompileMetadataResolver {
         }
 
         if (importedModuleType) {
+          if (this._checkSelfImport(moduleType, importedModuleType)) return;
           const importedModuleSummary = this.getNgModuleSummary(importedModuleType);
           if (!importedModuleSummary) {
             this._reportError(
@@ -567,6 +568,15 @@ export class CompileMetadataResolver {
     return compileMeta;
   }
 
+  private _checkSelfImport(moduleType: Type<any>, importedModuleType: Type<any>): boolean {
+    if (moduleType === importedModuleType) {
+      this._reportError(
+          syntaxError(`'${stringifyType(moduleType)}' module can't import itself`), moduleType);
+      return true;
+    }
+    return false;
+  }
+
   private _getTypeDescriptor(type: Type<any>): string {
     if (this._directiveResolver.isDirective(type)) {
       return 'directive';

modules/@angular/compiler/test/metadata_resolver_spec.ts

@@ -135,6 +135,15 @@ export function main() {
              .toThrowError(`Expected 'styles' to be an array of strings.`);
        }));
 
+    it('should throw with descriptive error message when a module imports itself',
+       inject([CompileMetadataResolver], (resolver: CompileMetadataResolver) => {
+         @NgModule({imports: [SomeModule]})
+         class SomeModule {
+         }
+         expect(() => resolver.loadNgModuleDirectiveAndPipeMetadata(SomeModule, true))
+             .toThrowError(`'SomeModule' module can't import itself`);
+       }));
+
     it('should throw with descriptive error message when provider token can not be resolved',
        inject([CompileMetadataResolver], (resolver: CompileMetadataResolver) => {
          @NgModule({declarations: [MyBrokenComp1]})
@@ -144,6 +153,7 @@ export function main() {
          expect(() => resolver.loadNgModuleDirectiveAndPipeMetadata(SomeModule, true))
              .toThrowError(`Can't resolve all parameters for MyBrokenComp1: (?).`);
        }));
+
     it('should throw with descriptive error message when a directive is passed to imports',
        inject([CompileMetadataResolver], (resolver: CompileMetadataResolver) => {
          @NgModule({imports: [ComponentWithoutModuleId]})