(With Visual Studio 17.14.13) Steps to reproduce (sorry this is a bit long but these are the fewest steps I could find):

1. From the command line, create the solution folder:
   • md test1
   • cd test1
2. Create an empty solution:
   • dotnet new sln --name test1 --format slnx
3. Create a razor class library project:
   • dotnet new razorclasslib -n test1 -o .
4. Add the razor class library project to solution:
   • dotnet sln add test1.csproj
5. Add TypeScript support:
   • npm install typescript --save-dev
6. Open the test1 solution.
7. Add a typescript file to the test1 project:
   1. Right-click test1 project and select "Add | New Item"
   2. Select "Web | Scripts" on the left.
   3. Select "TypeScript File" on the right.
   4. Click "Add" to use the default name of "file1.ts".
   5. Add the following TypeScript code to "file1.ts":

   export function showAlert(message: string): void
   {
      alert(message);
   }
   

8. Add the typescript NuGet package to the "test1.csproj" project file:

   <PackageReference Include="Microsoft.TypeScript.MSBuild" Version="5.9.2">
     <PrivateAssets>all</PrivateAssets>
     <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
   </PackageReference>
   

9. Add a tsconfig.json configuration file to the root of the project.
   (Menu item is under Add -> New Item, C# Items -> Web -> Scripts -> TypeScript JSON Configuration File.)
   The file contents are:

   {
     "compilerOptions": {
       "target": "ES6",
       "module": "ES6",
       "outDir": "./wwwroot/js",
       "strict": true
     },
     "include": [ "*.ts" ]
   }
   

Build/Rebuild

Now select "Build | Rebuild test1" twice.

The first rebuild succeeds, but the second rebuild results in an error message:

System.InvalidOperationException: No file exists for the asset at either location 'D:code\test1\wwwroot\js\file1.js' or 'wwwroot\js\file1.js'. at Microsoft.AspNetCore.StaticWebAssets.Tasks.StaticWebAsset.ResolveFile(String identity, String originalItemSpec) at Microsoft.AspNetCore.StaticWebAssets.Tasks.DefineStaticWebAssets.ResolveFileDetails(String originalItemSpec, String identity) at Microsoft.AspNetCore.StaticWebAssets.Tasks.DefineStaticWebAssets.Execute()

I can then repeatedly select "Build | Rebuild test1" and it will fail on alternate rebuilds.

My questions are:

1. What have I done wrong?
2. How do I fix it?
3. Can anyone else even reproduce this?