Use RoslynToolchain if dotnet SDK is not installed.

Author: timcassell

src/BenchmarkDotNet/Toolchains/CsProj/CsProjClassicNetToolchain.cs

@@ -47,12 +47,9 @@ public override IEnumerable<ValidationError> Validate(BenchmarkCase benchmarkCas
                     $"Classic .NET toolchain is supported only for Windows, benchmark '{benchmarkCase.DisplayInfo}' will not be executed",
                     benchmarkCase);
             }
-            else if (IsCliPathInvalid(customDotNetCliPath: null, benchmarkCase, out _))
+            else if (IsCliPathInvalid(customDotNetCliPath: null, benchmarkCase, out var invalidCliError))
             {
-                yield return new ValidationError(true,
-                    "CsProjClassicNetToolchain requires dotnet SDK to be installed or path to local dotnet cli provided in explicit way using `--cli` argument." +
-                    $" You may also use RoslynToolchain instead. Benchmark '{benchmarkCase.DisplayInfo}' will not be executed",
-                    benchmarkCase);
+                yield return invalidCliError;
             }
         }
     }

src/BenchmarkDotNet/Toolchains/ToolchainExtensions.cs

@@ -34,9 +34,12 @@ internal static IToolchain GetToolchain(this Runtime runtime, Descriptor descrip
             switch (runtime)
             {
                 case ClrRuntime clrRuntime:
-                    // Integration tests take too much time, because each benchmark run rebuilds the test suite and BenchmarkDotNet itself.
-                    // To reduce the total duration of the CI workflows, we just use RoslynToolchain.
-                    if (RuntimeInformation.IsFullFramework && XUnitHelper.IsIntegrationTest.Value)
+                    if (RuntimeInformation.IsFullFramework &&
+                        // If dotnet SDK is not installed, we use RoslynToolchain (CsProjClassicNetToolchain currently does not support custom cli path).
+                        (!HostEnvironmentInfo.GetCurrent().IsDotNetCliInstalled()
+                        // Integration tests take too much time, because each benchmark run rebuilds the test suite and BenchmarkDotNet itself.
+                        // To reduce the total duration of the CI workflows, we just use RoslynToolchain.
+                        || XUnitHelper.IsIntegrationTest.Value))
                         return RoslynToolchain.Instance;
 
                     // Default to CsProjClassicNetToolchain, even if the host is Full Framework.