From 93819e3d61e077f3498143456a2818589c7bcca4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tom=C3=A1=C5=A1=20Matou=C5=A1ek?=
 <tmat@users.noreply.github.com>
Date: Mon, 20 Nov 2017 17:52:02 -0800
Subject: [PATCH] VB EE: Properly handle syntax errors in synthesized
 assignments (#22647)

* VB EE: Properly handle syntax errors in synthesized assignments

* Fix test
---
 .../ExpressionCompiler/ExpressionCompiler.cs  |  5 ++++-
 .../ExpressionCompiler/SyntaxHelpers.vb       |  4 ++++
 .../ExpressionCompilerTests.vb                | 21 +++++++++++++++++++
 3 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/src/ExpressionEvaluator/Core/Source/ExpressionCompiler/ExpressionCompiler.cs b/src/ExpressionEvaluator/Core/Source/ExpressionCompiler/ExpressionCompiler.cs
index ab2657bea47..8ef028988ac 100644
--- a/src/ExpressionEvaluator/Core/Source/ExpressionCompiler/ExpressionCompiler.cs
+++ b/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))
diff --git a/src/ExpressionEvaluator/VisualBasic/Source/ExpressionCompiler/SyntaxHelpers.vb b/src/ExpressionEvaluator/VisualBasic/Source/ExpressionCompiler/SyntaxHelpers.vb
index 60eee98926b..93f27b31b44 100644
--- a/src/ExpressionEvaluator/VisualBasic/Source/ExpressionCompiler/SyntaxHelpers.vb
+++ b/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)
diff --git a/src/ExpressionEvaluator/VisualBasic/Test/ExpressionCompiler/ExpressionCompilerTests.vb b/src/ExpressionEvaluator/VisualBasic/Test/ExpressionCompiler/ExpressionCompilerTests.vb
index c7044d42aa0..0cd7c0fa527 100644
--- a/src/ExpressionEvaluator/VisualBasic/Test/ExpressionCompiler/ExpressionCompilerTests.vb
+++ b/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
-- 
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