feat(ApiIncrementalChecker): improve generation of diagnostics

when the options `checkSyntacticErrors: false` and `useTypescriptIncrementalApi: true` both were active, no semantic errors were emitted as soon as the first syntactic error was encountered
this patches the `typescript` import to override that behavior
see discussion in TypeStrong#257

src/patchTypescript.ts

@@ -0,0 +1,42 @@
+// tslint:disable-next-line:no-implicit-dependencies
+import * as ts from 'typescript'; // Imported for types alone
+
+export interface TypeScriptPatchConfig {
+  /**
+   * Ususally, the compilerHost created with typescript.createWatchCompilerHost will bail out of diagnostics collection if there has been any syntactic error.
+   * (see /~https://github.com/Microsoft/TypeScript/blob/89386ddda7dafc63cb35560e05412487f47cc267/src/compiler/watch.ts#L141 )
+   * If this plugin is running with `checkSyntacticErrors: false`, this might lead to situations where no syntactic errors are reported within webpack
+   * (because the file causing a syntactic error might not get processed by ts-loader), but there are semantic errors that would be missed due to this behavior.
+   * This ensures that the compilerHost always assumes that there were no syntactic errors to be found and continues to check for semantic errors.
+   */
+  skipGetSyntacticDiagnostics: boolean;
+}
+
+export function patchTypescript(
+  typescript: typeof ts,
+  config: TypeScriptPatchConfig
+) {
+  const {
+    createEmitAndSemanticDiagnosticsBuilderProgram: originalCreateEmitAndSemanticDiagnosticsBuilderProgram
+  } = typescript;
+
+  const patchSkipGetSyntacticDiagnostics: Pick<
+    typeof ts,
+    'createEmitAndSemanticDiagnosticsBuilderProgram'
+  > = {
+    createEmitAndSemanticDiagnosticsBuilderProgram(...args: any[]) {
+      const program = originalCreateEmitAndSemanticDiagnosticsBuilderProgram.apply(
+        typescript,
+        args as any
+      );
+      program.getSyntacticDiagnostics = () => [];
+      return program;
+    }
+  };
+
+  // directly patch the typescript object
+  Object.assign(
+    typescript,
+    config.skipGetSyntacticDiagnostics ? patchSkipGetSyntacticDiagnostics : {}
+  );
+}

src/service.ts

@@ -13,6 +13,7 @@ import {
 } from './NormalizedMessageFactories';
 import { RpcProvider } from 'worker-rpc';
 import { RunPayload, RunResult, RUN } from './RpcTypes';
+import { TypeScriptPatchConfig, patchTypescript } from './patchTypescript';
 
 const rpc = new RpcProvider(message => {
   try {
@@ -25,6 +26,13 @@ const rpc = new RpcProvider(message => {
 process.on('message', message => rpc.dispatch(message));
 
 const typescript: typeof ts = require(process.env.TYPESCRIPT_PATH!);
+const patchConfig: TypeScriptPatchConfig = {
+  skipGetSyntacticDiagnostics:
+    process.env.USE_INCREMENTAL_API === 'true' &&
+    process.env.CHECK_SYNTACTIC_ERRORS !== 'true'
+};
+
+patchTypescript(typescript, patchConfig);
 
 // message factories
 export const createNormalizedMessageFromDiagnostic = makeCreateNormalizedMessageFromDiagnostic(