Fix VSTHRD114 not firing for null in ternary conditional expressions (#1548)

* Initial plan

* Fix VSTHRD114: detect null in ternary/conditional expressions

Co-authored-by: AArnott <3548+AArnott@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: AArnott <3548+AArnott@users.noreply.github.com>

Author: Copilot

src/Microsoft.VisualStudio.Threading.Analyzers/AbstractVSTHRD114AvoidReturningNullTaskAnalyzer.cs

@@ -55,19 +55,42 @@ public override void Initialize(AnalysisContext context)
         };
     }
 
+    private static void CheckForNullValue(OperationAnalysisContext context, IOperation operation)
+    {
+        if (operation is IConditionalOperation conditionalOp)
+        {
+            if (conditionalOp.WhenTrue is { } whenTrue)
+            {
+                CheckForNullValue(context, whenTrue);
+            }
+
+            if (conditionalOp.WhenFalse is { } whenFalse)
+            {
+                CheckForNullValue(context, whenFalse);
+            }
+        }
+        else if (operation.ConstantValue is { HasValue: true, Value: null } &&
+            operation.Syntax is { } nullSyntax)
+        {
+            context.ReportDiagnostic(Diagnostic.Create(Descriptor, nullSyntax.GetLocation()));
+        }
+    }
+
     private void AnalyzerReturnOperation(OperationAnalysisContext context)
     {
         var returnOperation = (IReturnOperation)context.Operation;
 
-        if (returnOperation.ReturnedValue is { ConstantValue: { HasValue: true, Value: null } } && // could be null for implicit returns
-            returnOperation.ReturnedValue.Syntax is { } returnedValueSyntax &&
-            Utils.GetContainingFunctionBlock(returnOperation) is { } block &&
-            FindOwningSymbol(block, context.ContainingSymbol) is { } method &&
-            !method.IsAsync &&
-            Utils.IsTask(method.ReturnType) &&
-            !this.LanguageUtils.MethodReturnsNullableReferenceType(method))
+        // ReturnedValue is null for implicit/void returns
+        if (returnOperation.ReturnedValue is not { } returnedValue ||
+            Utils.GetContainingFunctionBlock(returnOperation) is not { } block ||
+            FindOwningSymbol(block, context.ContainingSymbol) is not { } owningMethod ||
+            owningMethod.IsAsync ||
+            !Utils.IsTask(owningMethod.ReturnType) ||
+            this.LanguageUtils.MethodReturnsNullableReferenceType(owningMethod))
         {
-            context.ReportDiagnostic(Diagnostic.Create(Descriptor, returnedValueSyntax.GetLocation()));
+            return;
         }
+
+        CheckForNullValue(context, returnedValue);
     }
 }

test/Microsoft.VisualStudio.Threading.Analyzers.Tests/VSTHRD114AvoidReturningNullTaskAnalyzerTests.cs

@@ -147,7 +147,7 @@ public Task<object> GetTaskObj()
     }
 
     [Fact]
-    public async Task NullInTernary_NoDiagnostic_FalseNegative()
+    public async Task NullInTernary_Diagnostic()
     {
         var test = @"
 using System.Threading.Tasks;
@@ -156,7 +156,7 @@ class Test
 {
     public Task<object> GetTaskObj(bool b)
     {
-        return b ? default(Task<object>) : null;
+        return b ? [|default(Task<object>)|] : [|null|];
     }
 }
 ";
@@ -166,6 +166,47 @@ public Task<object> GetTaskObj(bool b)
         }.RunAsync();
     }
 
+    [Fact]
+    public async Task NullInTernaryReturnStatement_Diagnostic()
+    {
+        var csharpTest = @"
+using System.Threading.Tasks;
+
+class Test
+{
+    public Task First(bool flag)
+    {
+        return flag
+            ? [|null|]
+            : Task.CompletedTask;
+    }
+
+    public Task Second(bool flag) =>
+        flag
+            ? [|null|]
+            : Task.CompletedTask;
+}
+";
+        await new CSVerify.Test
+        {
+            TestCode = csharpTest,
+        }.RunAsync();
+
+        var vbTest = @"
+Imports System.Threading.Tasks
+
+Friend Class Test
+    Public Function First(flag As Boolean) As Task
+        Return If(flag, [|Nothing|], Task.CompletedTask)
+    End Function
+End Class
+";
+        await new VerifyVB.Test
+        {
+            TestCode = vbTest,
+        }.RunAsync();
+    }
+
     [Fact]
     public async Task MultipleFaultyReturns_MultipleDiagnostics()
     {