Use an explicit type

src/services/services.ts

@@ -1158,10 +1158,11 @@ namespace ts {
         function getValidSourceFile(fileName: string): SourceFile {
             const sourceFile = program.getSourceFile(fileName);
             if (!sourceFile) {
-                let error = new Error(`Could not find sourceFile: '${fileName}'.`);
+                const error: Error & PossibleProgramFileInfo = new Error(`Could not find sourceFile: '${fileName}'.`);
 
-                // Attach sidecar data for the server log
-                (error as any)["ProgramFiles"] = program && JSON.stringify(program.getSourceFiles().map(f => f.fileName));
+                // We've been having trouble debugging this, so attach sidecar data for the tsserver log.
+                // See https://github.com/microsoft/TypeScript/issues/30180.
+                error.ProgramFiles = program.getSourceFiles().map(f => f.fileName);
 
                 throw error;
             }

src/services/utilities.ts

@@ -967,6 +967,11 @@ namespace ts {
         readonly called: Identifier;
         readonly nTypeArguments: number;
     }
+
+    export interface PossibleProgramFileInfo {
+        ProgramFiles?: string[];
+    }
+
     // Get info for an expression like `f <` that may be the start of type arguments.
     export function getPossibleTypeArgumentsInfo(tokenIn: Node, sourceFile: SourceFile): PossibleTypeArgumentInfo | undefined {
         let token: Node | undefined = tokenIn;