Keep the whole NoWarn list, and each target framework once

src/DiffEngine.Tests/BuildFileTests.cs

@@ -0,0 +1,58 @@
+/// <summary>
+/// Two things about the build files that a build cannot tell anyone. A second property of the same
+/// name replaces the first rather than adding to it, and a target framework listed twice is
+/// collapsed, so both mistakes build clean and say nothing.
+/// </summary>
+public class BuildFileTests
+{
+    /// <summary>
+    /// There were two, so only the later list was in force and CS0649, NU1608 and NU1109 were
+    /// suppressed nowhere - which under TreatWarningsAsErrors is a build failure waiting for the
+    /// first unassigned field.
+    /// </summary>
+    [Test]
+    public async Task NoWarn_is_declared_once()
+    {
+        var props = await File.ReadAllTextAsync(Path.Combine(Source(), "Directory.Build.props"));
+
+        var declarations = props.Split(["<NoWarn>"], StringSplitOptions.None).Length - 1;
+
+        await Assert.That(declarations).IsEqualTo(1);
+    }
+
+    [Test]
+    public async Task No_target_framework_is_listed_twice()
+    {
+        var project = await File.ReadAllTextAsync(Path.Combine(Source(), "DiffEngine", "DiffEngine.csproj"));
+
+        var listed = project
+            .Split('\n')
+            .Where(_ => _.Contains("<TargetFrameworks"))
+            .SelectMany(_ => _[(_.IndexOf('>') + 1)..^"</TargetFrameworks>".Length].Split(';'))
+            .Where(_ => _.StartsWith("net", StringComparison.Ordinal))
+            .ToList();
+
+        await Assert.That(listed).IsNotEmpty();
+        await Assert.That(listed.Distinct()).IsEquivalentTo(listed);
+    }
+
+    /// <summary>
+    /// The src directory, found by walking up from the test output rather than by counting
+    /// directories, which differs per target framework and configuration.
+    /// </summary>
+    static string Source()
+    {
+        var directory = new DirectoryInfo(AppContext.BaseDirectory);
+        while (directory != null)
+        {
+            if (File.Exists(Path.Combine(directory.FullName, "Directory.Build.props")))
+            {
+                return directory.FullName;
+            }
+
+            directory = directory.Parent;
+        }
+
+        throw new("Could not find Directory.Build.props above the test output.");
+    }
+}

src/DiffEngine/DiffEngine.csproj

@@ -1,7 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT'">net462;net472;net48;net9.0;net10.0</TargetFrameworks>
-    <TargetFrameworks>$(TargetFrameworks);net6.0;net7.0;net8.0;net9.0;net10.0</TargetFrameworks>
+    <TargetFrameworks>net6.0;net7.0;net8.0;net9.0;net10.0</TargetFrameworks>
+    <!-- The .NET Framework legs, which only Windows can build -->
+    <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT'">net462;net472;net48;$(TargetFrameworks)</TargetFrameworks>
     <AllowUnsafeBlocks Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net7.0'))">true</AllowUnsafeBlocks>
     <IsAotCompatible Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net7.0'))">true</IsAotCompatible>
   </PropertyGroup>

src/Directory.Build.props

@@ -1,7 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project>
   <PropertyGroup>
-    <NoWarn>CS1591;CS0649;NU1608;NU1109</NoWarn>
     <Version>20.0.0-beta.30</Version>
     <AssemblyVersion>1.0.0</AssemblyVersion>
     <PackageTags>Testing, Snapshot, Diff, Compare</PackageTags>
@@ -14,7 +13,12 @@
     <CheckEolTargetFramework>false</CheckEolTargetFramework>
     <SuppressTfmSupportBuildWarnings>true</SuppressTfmSupportBuildWarnings>
     <ResolveAssemblyReferencesSilent>true</ResolveAssemblyReferencesSilent>
-    <NoWarn>CA1416;CS1591</NoWarn>
+    <!--
+    One NoWarn, because a second property of the same name replaces the first rather than adding to
+    it. There were two, so CS0649, NU1608 and NU1109 were not suppressed at all - and with
+    TreatWarningsAsErrors below they would have failed a build rather than warned in one.
+    -->
+    <NoWarn>CS1591;CS0649;NU1608;NU1109;CA1416</NoWarn>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
     <SuppressNETCoreSdkPreviewMessage>true</SuppressNETCoreSdkPreviewMessage>