Sdk: Warnings are not reported for repeated builds

[svick]

Steps to reproduce

1. dotnet new classlib

2. Change Class1.cs to:

   c# class C { async void M() {} }

3. dotnet build

4. dotnet build

Expected behavior

The second dotnet build prints the CS1998 warning, just like the first one.

Actual behavior

The second dotnet build does not print any warnings:

> dotnet build
Microsoft (R) Build Engine version 15.6.82.30579 for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.

  Restore completed in 87,92 ms for C:\code\tmp\hwapp\hwapp.csproj.
Class1.cs(3,16): warning CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread. [C:\code\tmp\hwapp\hwapp.csproj]
  hwapp -> C:\code\tmp\hwapp\bin\Debug\netstandard2.0\hwapp.dll

Build succeeded.

Class1.cs(3,16): warning CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread. [C:\code\tmp\hwapp\hwapp.csproj]
    1 Warning(s)
    0 Error(s)

Time Elapsed 00:00:02.37
> dotnet build
Microsoft (R) Build Engine version 15.6.82.30579 for .NET Core
Copyright (C) Microsoft Corporation. All rights reserved.

  Restore completed in 81,94 ms for C:\code\tmp\hwapp\hwapp.csproj.
  hwapp -> C:\code\tmp\hwapp\bin\Debug\netstandard2.0\hwapp.dll

Build succeeded.
    0 Warning(s)
    0 Error(s)

Time Elapsed 00:00:01.96

Environment data

dotnet --info output:

.NET Command Line Tools (2.1.300-preview2-008251)

Product Information:
Version: 2.1.300-preview2-008251
Commit SHA-1 hash: 94fd3fd392

Runtime Environment:
OS Name: Windows
OS Version: 10.0.16299
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\2.1.300-preview2-008251\

Microsoft .NET Core Shared Framework Host

Version : 2.1.0-preview2-26131-06
Build : b13a0d5c331f374afd35ded57b9a4b4ab128864c

[tannergooding]

@svick, this is because, the second time you run 'dotnet build', various up-to-date checks succeed and the target that produced the warning does not run.

[svick]

@tannergooding Then maybe warnings from previous runs should be saved somewhere and if some target doesn't run because it's up to date, its old warnings are still printed?

[tannergooding]

@svick, that seems like a general MSBuild feature request (which would impact both Desktop and .NET Core), rather than something CLI should be doing.

If you feel the same, could you open a bug against https://github.com/Microsoft/MSBuild?

[svick]

@tannergooding Done.

[petermorlion]

I think you want to use the --no-incremental option, i.e.:

dotnet build --no-incremental

This does, however, mean a full rebuild.