Implement type parameter inference from constraints

.github/memory/testing/compiler.md

@@ -27,6 +27,11 @@ public class MyTests : CSharpTestBase
 
 - **Unit tests** target individual compiler phases (lexing, parsing); **compilation
   tests** create `Compilation` objects and verify symbols/diagnostics.
+- **Version-specific C# features** use the matching
+  `src/Compilers/CSharp/Test/CSharpN/` project. Use `TestOptions.RegularNext`
+  for the upcoming language version and the latest stable `RegularN` option for
+  the disabled-feature boundary; retain an explicit `RegularPreview` case when
+  testing the feature's language-version gate.
 - **Cross-language patterns**: many test patterns work for both C# and VB with
   minor syntax changes.
 - **Verification baselines**: when helpers like `VerifyDiagnostics`,

Compilers.slnf

@@ -6,6 +6,7 @@
       "src\\Compilers\\CSharp\\Portable\\Microsoft.CodeAnalysis.CSharp.csproj",
       "src\\Compilers\\CSharp\\Test\\CommandLine\\Microsoft.CodeAnalysis.CSharp.CommandLine.UnitTests.csproj",
       "src\\Compilers\\CSharp\\Test\\CSharp15\\Microsoft.CodeAnalysis.CSharp.CSharp15.UnitTests.csproj",
+      "src\\Compilers\\CSharp\\Test\\CSharp16\\Microsoft.CodeAnalysis.CSharp.CSharp16.UnitTests.csproj",
       "src\\Compilers\\CSharp\\Test\\Emit3\\Microsoft.CodeAnalysis.CSharp.Emit3.UnitTests.csproj",
       "src\\Compilers\\CSharp\\Test\\Emit2\\Microsoft.CodeAnalysis.CSharp.Emit2.UnitTests.csproj",
       "src\\Compilers\\CSharp\\Test\\Emit\\Microsoft.CodeAnalysis.CSharp.Emit.UnitTests.csproj",

Ide.slnf

@@ -27,6 +27,7 @@
       "src\\Compilers\\CSharp\\Test\\Emit2\\Microsoft.CodeAnalysis.CSharp.Emit2.UnitTests.csproj",
       "src\\Compilers\\CSharp\\Test\\Emit3\\Microsoft.CodeAnalysis.CSharp.Emit3.UnitTests.csproj",
       "src\\Compilers\\CSharp\\Test\\CSharp15\\Microsoft.CodeAnalysis.CSharp.CSharp15.UnitTests.csproj",
+      "src\\Compilers\\CSharp\\Test\\CSharp16\\Microsoft.CodeAnalysis.CSharp.CSharp16.UnitTests.csproj",
       "src\\Compilers\\CSharp\\Test\\Emit\\Microsoft.CodeAnalysis.CSharp.Emit.UnitTests.csproj",
       "src\\Compilers\\CSharp\\Test\\EndToEnd\\Microsoft.CodeAnalysis.CSharp.EndToEnd.UnitTests.csproj",
       "src\\Compilers\\CSharp\\Test\\IOperation\\Microsoft.CodeAnalysis.CSharp.IOperation.UnitTests.csproj",

Roslyn.slnx

@@ -109,6 +109,7 @@
     <Project Path="src/Compilers/CSharp/Test/Symbol/Microsoft.CodeAnalysis.CSharp.Symbol.UnitTests.csproj" />
     <Project Path="src/Compilers/CSharp/Test/Syntax/Microsoft.CodeAnalysis.CSharp.Syntax.UnitTests.csproj" />
     <Project Path="src/Compilers/CSharp/Test/CSharp15/Microsoft.CodeAnalysis.CSharp.CSharp15.UnitTests.csproj" />
+    <Project Path="src/Compilers/CSharp/Test/CSharp16/Microsoft.CodeAnalysis.CSharp.CSharp16.UnitTests.csproj" />
     <Project Path="src/Compilers/CSharp/Test/WinRT/Microsoft.CodeAnalysis.CSharp.WinRT.UnitTests.csproj" />
     <Project Path="src/Compilers/Test/Utilities/CSharp/Microsoft.CodeAnalysis.CSharp.Test.Utilities.csproj" />
   </Folder>

docs/compilers/CSharp/compiler-breaking-changes-dotnet-12.md

@@ -0,0 +1,29 @@
+# Breaking changes in Roslyn after .NET 11.0.100 through .NET 12.0.100
+
+This document lists known breaking changes in Roslyn after .NET 11 general release (.NET SDK version 11.0.100) through .NET 12 general release (.NET SDK version 12.0.100).
+
+## Type parameter inference considers generic constraints
+
+***Introduced in .NET 12***
+
+In C# 16, method type inference can infer type parameters that occur only in the constraints of other inferred type parameters. As a result, a generic method that was previously excluded from overload resolution may become applicable. This can change the selected overload, introduce an ambiguity, or produce a more specific diagnostic from the newly applicable candidate.
+
+For example, the following call previously selected the non-generic overload because `U` could not be inferred. It now selects the generic overload because `T` is fixed to `string` and `U` is then inferred as `char` from the `IEnumerable<U>` constraint.
+
+```cs
+using System;
+using System.Collections.Generic;
+
+static void M(object value) => Console.WriteLine("non-generic");
+static void M<T, U>(T value) where T : IEnumerable<U> => Console.WriteLine("generic");
+
+M("test"); // C# 15: "non-generic"; C# 16: "generic"
+```
+
+To preserve the previous overload selection, explicitly convert the argument to the intended parameter type:
+
+```cs
+M((object)"test"); // "non-generic"
+```
+
+See [the type parameter inference from constraints proposal](https://github.com/dotnet/csharplang/issues/9453) and [the Roslyn implementation](https://github.com/dotnet/roslyn/pull/84655).

eng/build.ps1

@@ -383,6 +383,7 @@ function GetCompilerTestAssembliesIncludePaths() {
   $assemblies += " --include '^Microsoft\.CodeAnalysis\.CSharp\.Emit2\.UnitTests$'"
   $assemblies += " --include '^Microsoft\.CodeAnalysis\.CSharp\.Emit3\.UnitTests$'"
   $assemblies += " --include '^Microsoft\.CodeAnalysis\.CSharp\.CSharp15\.UnitTests$'"
+  $assemblies += " --include '^Microsoft\.CodeAnalysis\.CSharp\.CSharp16\.UnitTests$'"
   $assemblies += " --include '^Microsoft\.CodeAnalysis\.CSharp\.IOperation\.UnitTests$'"
   $assemblies += " --include '^Microsoft\.CodeAnalysis\.CSharp\.CommandLine\.UnitTests$'"
   $assemblies += " --include '^Microsoft\.CodeAnalysis\.VisualBasic\.Syntax\.UnitTests$'"

eng/build.sh

@@ -378,6 +378,7 @@ function GetCompilerTestAssembliesIncludePaths {
   assemblies+=" --include '^Microsoft\.CodeAnalysis\.CSharp\.Emit2\.UnitTests$'"
   assemblies+=" --include '^Microsoft\.CodeAnalysis\.CSharp\.Emit3\.UnitTests$'"
   assemblies+=" --include '^Microsoft\.CodeAnalysis\.CSharp\.CSharp15\.UnitTests$'"
+  assemblies+=" --include '^Microsoft\.CodeAnalysis\.CSharp\.CSharp16\.UnitTests$'"
   assemblies+=" --include '^Microsoft\.CodeAnalysis\.CSharp\.IOperation\.UnitTests$'"
   assemblies+=" --include '^Microsoft\.CodeAnalysis\.CSharp\.CommandLine\.UnitTests$'"
   assemblies+=" --include '^Microsoft\.CodeAnalysis\.VisualBasic\.Syntax\.UnitTests$'"