Warn when FSharp.Core major version exceeds SDK compiler version

src/FSharp.Build/Microsoft.FSharp.Core.NetSdk.props

@@ -20,10 +20,14 @@ WARNING:  DO NOT MODIFY this file unless you are knowledgeable about MSBuild and
     <MSBuildAllProjects>$(MSBuildAllProjects);$(MSBuildThisFileFullPath)</MSBuildAllProjects>
   </PropertyGroup>
 
-  <PropertyGroup Condition="'$(_FSCorePackageVersionSet)' != 'true'">
-    <_FSCorePackageVersionSet>true</_FSCorePackageVersionSet>
+  <PropertyGroup Condition="'$(FSCorePackageVersionSet)' != 'true'">
+    <FSCorePackageVersionSet>true</FSCorePackageVersionSet>
     <FSCorePackageVersion>{{FSCorePackageVersionValue}}</FSCorePackageVersion>
     <_FSharpCoreLibraryPacksFolder Condition="'$(_FSharpCoreLibraryPacksFolder)' == ''">$([MSBuild]::EnsureTrailingSlash('$(MSBuildThisFileDirectory)'))library-packs</_FSharpCoreLibraryPacksFolder>
   </PropertyGroup>
 
+  <PropertyGroup Condition="'$(FSCorePackageVersionSet)' == 'true' and '$(FSCorePackageVersion)' != '' and !$(FSCorePackageVersion.Contains('{'))">
+    <FSharpCoreMaximumMajorVersion Condition="'$(FSharpCoreMaximumMajorVersion)' == ''">$([System.Version]::Parse('$(FSCorePackageVersion.Split('-')[0])').Major)</FSharpCoreMaximumMajorVersion>
+  </PropertyGroup>
+
 </Project>

src/FSharp.Build/Microsoft.FSharp.NetSdk.targets

@@ -219,4 +219,29 @@ WARNING:  DO NOT MODIFY this file unless you are knowledgeable about MSBuild and
       <PathMap>@(_TopLevelSourceRoot->'%(EscapedKey)=%(EscapedValue)', ','),$(PathMap)</PathMap>
     </PropertyGroup>
   </Target>
+
+  <!--
+    Warn if resolved FSharp.Core major version exceeds SDK's compiler version.
+  -->
+  <Target Name="_CheckForUnsupportedFSharpCoreVersion" 
+          AfterTargets="ResolvePackageAssets"
+          Condition="'$(ValidateFSharpCoreVersion)' != 'false' and '$(FSharpCoreMaximumMajorVersion)' != ''">
+
+    <ItemGroup>
+      <_ResolvedFSharpCore Include="@(ReferencePath)" 
+                            Condition="'%(ReferencePath.NuGetPackageId)' == 'FSharp.Core'" />
+    </ItemGroup>
+
+    <PropertyGroup Condition="'@(_ResolvedFSharpCore->Count())' == '1'">
+      <_ResolvedFSharpCoreVersion>@(_ResolvedFSharpCore->'%(NuGetPackageVersion)')</_ResolvedFSharpCoreVersion>
+      <_ResolvedFSharpCoreMajor>$([System.Version]::Parse('$(_ResolvedFSharpCoreVersion.Split('-')[0])').Major)</_ResolvedFSharpCoreMajor>
+      <_FSharpCoreVersionTooHigh Condition="$(_ResolvedFSharpCoreMajor) &gt; $(FSharpCoreMaximumMajorVersion)">true</_FSharpCoreVersionTooHigh>
+    </PropertyGroup>
+
+    <Warning 
+      Condition="'$(_FSharpCoreVersionTooHigh)' == 'true'"
+      Code="FSSDK0001"
+      Text="The referenced FSharp.Core $(_ResolvedFSharpCoreVersion) has a higher major version ($(_ResolvedFSharpCoreMajor)) than this SDK supports ($(FSharpCoreMaximumMajorVersion)). This may cause compilation errors or runtime failures." />
+
+  </Target>
 </Project>

tests/fsharp/SDKTests/tests/FSharpCoreVersion - HigherVersionWarns.proj

@@ -0,0 +1,12 @@
+<Project ToolsVersion="4.0" DefaultTargets="Test" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <PropertyGroup>
+    <TestFSCorePackageVersion>8.0.100</TestFSCorePackageVersion>
+  </PropertyGroup>
+  <Import Project="FSharpCoreVersionTest.props" />
+  <PropertyGroup>
+    <TestResolvedVersion>9.0.0</TestResolvedVersion>
+    <ExpectedMaxMajor>8</ExpectedMaxMajor>
+    <ExpectWarning>true</ExpectWarning>
+  </PropertyGroup>
+  <Import Project="FSharpCoreVersionTest.targets" />
+</Project>

tests/fsharp/SDKTests/tests/FSharpCoreVersion - LowerVersionNoWarning.proj

@@ -0,0 +1,12 @@
+<Project ToolsVersion="4.0" DefaultTargets="Test" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <PropertyGroup>
+    <TestFSCorePackageVersion>9.0.100</TestFSCorePackageVersion>
+  </PropertyGroup>
+  <Import Project="FSharpCoreVersionTest.props" />
+  <PropertyGroup>
+    <TestResolvedVersion>8.0.100</TestResolvedVersion>
+    <ExpectedMaxMajor>9</ExpectedMaxMajor>
+    <ExpectWarning>false</ExpectWarning>
+  </PropertyGroup>
+  <Import Project="FSharpCoreVersionTest.targets" />
+</Project>

tests/fsharp/SDKTests/tests/FSharpCoreVersion - PrereleaseParsing.proj

@@ -0,0 +1,12 @@
+<Project ToolsVersion="4.0" DefaultTargets="Test" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <PropertyGroup>
+    <TestFSCorePackageVersion>9.0.100-preview.1</TestFSCorePackageVersion>
+  </PropertyGroup>
+  <Import Project="FSharpCoreVersionTest.props" />
+  <PropertyGroup>
+    <TestResolvedVersion>10.0.0-beta.1</TestResolvedVersion>
+    <ExpectedMaxMajor>9</ExpectedMaxMajor>
+    <ExpectWarning>true</ExpectWarning>
+  </PropertyGroup>
+  <Import Project="FSharpCoreVersionTest.targets" />
+</Project>

tests/fsharp/SDKTests/tests/FSharpCoreVersion - SameVersionNoWarning.proj

@@ -0,0 +1,12 @@
+<Project ToolsVersion="4.0" DefaultTargets="Test" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <PropertyGroup>
+    <TestFSCorePackageVersion>9.0.100</TestFSCorePackageVersion>
+  </PropertyGroup>
+  <Import Project="FSharpCoreVersionTest.props" />
+  <PropertyGroup>
+    <TestResolvedVersion>9.0.200</TestResolvedVersion>
+    <ExpectedMaxMajor>9</ExpectedMaxMajor>
+    <ExpectWarning>false</ExpectWarning>
+  </PropertyGroup>
+  <Import Project="FSharpCoreVersionTest.targets" />
+</Project>

tests/fsharp/SDKTests/tests/FSharpCoreVersionTest.props

@@ -0,0 +1,23 @@
+<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+
+  <Import Project="$(FSharpRepositoryPath)\eng\Versions.props" />
+
+  <PropertyGroup>
+    <TargetFramework Condition="'$(TargetFramework)' == ''">net10.0</TargetFramework>
+    <_TargetFrameworkVersionWithoutV>10.0</_TargetFrameworkVersionWithoutV>
+    <TargetFrameworkIdentifier>.NETCoreApp</TargetFrameworkIdentifier>
+    <Configuration Condition="'$(Configuration)' == ''">Release</Configuration>
+    <TargetsDirectory>$(FSharpRepositoryPath)\artifacts\bin\FSharpSuite.Tests\$(Configuration)\$(TARGETFRAMEWORK)</TargetsDirectory>
+    <Platform Condition="'$(Platform)' == ''">AnyCPU</Platform>
+    <OutputPath>$(MSBuildThisFileDirectory)</OutputPath>
+  </PropertyGroup>
+
+  <!-- Override SDK version BEFORE import -->
+  <PropertyGroup Condition="'$(TestFSCorePackageVersion)' != ''">
+    <FSCorePackageVersion>$(TestFSCorePackageVersion)</FSCorePackageVersion>
+    <FSCorePackageVersionSet>true</FSCorePackageVersionSet>
+  </PropertyGroup>
+
+  <Import Project="$(TargetsDirectory)\Microsoft.FSharp.NetSdk.props" />
+
+</Project>

tests/fsharp/SDKTests/tests/FSharpCoreVersionTest.targets

@@ -0,0 +1,27 @@
+<Project ToolsVersion="4.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+
+  <Import Project="$(TargetsDirectory)\Microsoft.FSharp.NetSdk.targets" />
+
+  <Target Name="Test">
+    <Message Importance="High" Text="Testing: $(MSBuildProjectName)" />
+
+    <!-- Simulate version comparison -->
+    <PropertyGroup Condition="'$(TestResolvedVersion)' != ''">
+      <_TestResolvedMajor>$([System.Version]::Parse('$(TestResolvedVersion.Split('-')[0])').Major)</_TestResolvedMajor>
+      <_TestShouldWarn Condition="$(_TestResolvedMajor) &gt; $(FSharpCoreMaximumMajorVersion)">true</_TestShouldWarn>
+    </PropertyGroup>
+
+    <!-- Assertions -->
+    <Error Condition="'$(ExpectedMaxMajor)' != '' and '$(FSharpCoreMaximumMajorVersion)' != '$(ExpectedMaxMajor)'" 
+           Text="FSharpCoreMaximumMajorVersion: expected '$(ExpectedMaxMajor)' actual '$(FSharpCoreMaximumMajorVersion)'" />
+
+    <Error Condition="'$(ExpectWarning)' == 'true' and '$(_TestShouldWarn)' != 'true'" 
+           Text="Expected warning but didn't get one" />
+
+    <Error Condition="'$(ExpectWarning)' == 'false' and '$(_TestShouldWarn)' == 'true'" 
+           Text="Did not expect warning but got one" />
+
+    <Message Importance="High" Text="PASSED: $(MSBuildProjectName)" />
+  </Target>
+
+</Project>