VB EE: Properly handle syntax errors in synthesized assignments (#22647)

* VB EE: Properly handle syntax errors in synthesized assignments

* Fix test

src/ExpressionEvaluator/Core/Source/ExpressionCompiler/ExpressionCompiler.cs

@@ -173,7 +173,10 @@ private static ImmutableArray<Alias> GetAliases(DkmClrRuntimeInstance runtimeIns
                     },
                     out error);
 
-                Debug.Assert((r.ResultProperties.Flags & DkmClrCompilationResultFlags.PotentialSideEffect) == DkmClrCompilationResultFlags.PotentialSideEffect);
+                Debug.Assert(
+                    r.CompileResult == null && r.ResultProperties.Flags == default ||
+                    (r.ResultProperties.Flags & DkmClrCompilationResultFlags.PotentialSideEffect) == DkmClrCompilationResultFlags.PotentialSideEffect);
+
                 result = r.CompileResult.ToQueryResult(this.CompilerId, r.ResultProperties, runtimeInstance);
             }
             catch (Exception e) when (ExpressionEvaluatorFatalError.CrashIfFailFastEnabled(e))

src/ExpressionEvaluator/VisualBasic/Source/ExpressionCompiler/SyntaxHelpers.vb

@@ -38,6 +38,10 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.ExpressionEvaluator
             Dim syntaxTree = expression.CreateSyntaxTree()
             diagnostics.AddRange(syntaxTree.GetDiagnostics())
 
+            If diagnostics.HasAnyErrors Then
+                Return Nothing
+            End If
+
             ' Any Diagnostic spans produced in binding will be offset by the length of the "target" expression text.
             ' If we want to support live squiggles in debugger windows, SemanticModel, etc, we'll want to address this.
             Dim targetSyntax = SyntaxHelpers.ParseDebuggerExpressionInternal(SourceText.From(target), consumeFullText:=True)

src/ExpressionEvaluator/VisualBasic/Test/ExpressionCompiler/ExpressionCompilerTests.vb

@@ -471,6 +471,27 @@ End Class
             End If
         End Sub
 
+        <Fact, WorkItem(18531, "https://github.com/dotnet/roslyn/issues/18531")>
+        Public Sub SyntaxErrors()
+            Const source = "
+Class C
+    Shared Function F(x As String) As Object
+        Return x
+    End Function
+End Class
+"
+            Dim comp = CreateCompilationWithMscorlib({source}, options:=TestOptions.DebugDll)
+            WithRuntimeInstance(comp,
+                Sub(runtime)
+                    Dim context = CreateMethodContext(runtime, methodName:="C.F")
+                    Dim errorMessage As String = Nothing
+
+                    Dim result = context.CompileAssignment("x", "", errorMessage, formatter:=DebuggerDiagnosticFormatter.Instance)
+                    Assert.Null(result)
+                    Assert.Equal("error BC30201: Expression expected.", errorMessage)
+                End Sub)
+        End Sub
+
         ''' <summary>
         ''' Locals in the generated method should account for temporary slots
         ''' in the original method.  Also, some temporaries may not be included