Fix crash in `EventAssignmentExpression.EventInstance` in VB

Which is caused by invalid code.

src/Compilers/Core/CodeAnalysisTest/Diagnostics/OperationTestAnalyzer.cs

@@ -935,6 +935,14 @@ public class MemberReferenceAnalyzer : DiagnosticAnalyzer
             DiagnosticSeverity.Warning,
             isEnabledByDefault: true);
 
+        public static readonly DiagnosticDescriptor InvalidEventDescriptor = new DiagnosticDescriptor(
+            "InvalidEvent",
+            "Invalid Event",
+            "A EventAssignmentExpression with invalid event found.",
+            "Testing",
+            DiagnosticSeverity.Warning,
+            isEnabledByDefault: true);
+
         public static readonly DiagnosticDescriptor HandlerAddedDescriptor = new DiagnosticDescriptor(
             "HandlerAdded",
             "Handler Added",
@@ -975,7 +983,14 @@ public class MemberReferenceAnalyzer : DiagnosticAnalyzer
             DiagnosticSeverity.Warning,
             isEnabledByDefault: true);
 
-        public sealed override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics => ImmutableArray.Create(EventReferenceDescriptor, HandlerAddedDescriptor, HandlerRemovedDescriptor, PropertyReferenceDescriptor, FieldReferenceDescriptor, MethodBindingDescriptor);
+        public sealed override ImmutableArray<DiagnosticDescriptor> SupportedDiagnostics => 
+            ImmutableArray.Create(EventReferenceDescriptor, 
+                HandlerAddedDescriptor, 
+                HandlerRemovedDescriptor, 
+                PropertyReferenceDescriptor, 
+                FieldReferenceDescriptor, 
+                MethodBindingDescriptor,
+                InvalidEventDescriptor);
 
         public sealed override void Initialize(AnalysisContext context)
         {
@@ -991,6 +1006,15 @@ public sealed override void Initialize(AnalysisContext context)
                  {
                      IEventAssignmentExpression eventAssignment = (IEventAssignmentExpression)operationContext.Operation;
                      operationContext.ReportDiagnostic(Diagnostic.Create(eventAssignment.Adds ? HandlerAddedDescriptor : HandlerRemovedDescriptor, operationContext.Operation.Syntax.GetLocation()));
+
+                     if (eventAssignment.Event == null)
+                     {
+                         if (eventAssignment.EventInstance == null && eventAssignment.IsInvalid)
+                         {
+                             // report inside after checking for null to make sure it does't crash.
+                             operationContext.ReportDiagnostic(Diagnostic.Create(InvalidEventDescriptor, eventAssignment.Syntax.GetLocation()));
+                         }
+                     }
                  },
                  OperationKind.EventAssignmentExpression);
 

src/Compilers/VisualBasic/Portable/BoundTree/Statement.vb

@@ -1545,7 +1545,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
 
             Public ReadOnly Property EventInstance As IOperation Implements IEventAssignmentExpression.EventInstance
                 Get
-                    If [Event].IsStatic Then
+                    If [Event] Is Nothing OrElse [Event].IsStatic Then
                         Return Nothing
                     End If
 

src/Compilers/VisualBasic/Test/Semantic/Diagnostics/OperationAnalyzerTests.vb

@@ -1724,5 +1724,84 @@ End Module
                 Diagnostic(ForLoopConditionCrashVBTestAnalyzer.ForLoopConditionCrashDescriptor.Id, "Boo").WithLocation(41, 24),
                 Diagnostic(ForLoopConditionCrashVBTestAnalyzer.ForLoopConditionCrashDescriptor.Id, "10").WithLocation(19, 40))
         End Sub
+
+        <WorkItem(9012, "https://github.com/dotnet/roslyn/issues/9012")>
+        <Fact>
+        Public Sub InvalidEventInstanceVisualBasic()
+            Dim source = <compilation>
+                             <file name="c.vb">
+                                 <![CDATA[
+Imports System
+Imports System.Collections.Generic
+
+Module Program
+    Sub Main(args As String())
+        AddHandler Function(ByVal x) x
+    End Sub
+End Module
+
+Class TestClass
+
+    Event TestEvent As Action
+
+    Shared Sub Test(receiver As TestClass)
+        AddHandler receiver?.TestEvent, AddressOf Main
+    End Sub
+
+    Shared Sub Main()
+    End Sub
+End Class
+
+Module Module1
+    Sub Main()
+        Dim x = {Iterator sub() yield, new object}
+        Dim y = {Iterator sub() yield 1, Iterator sub() yield, new object}
+        Dim z = {Sub() AddHandler, New Object}
+        g0(Iterator sub() Yield)
+        g1(Iterator Sub() Yield, 5)
+    End Sub
+
+    Sub g0(ByVal x As Func(Of IEnumerator))
+    End Sub
+    Sub g1(ByVal x As Func(Of IEnumerator), ByVal y As Integer)
+    End Sub
+
+    Iterator Function f() As IEnumerator
+        Yield
+    End Function
+End Module
+]]>
+                             </file>
+                         </compilation>
+
+            Dim comp = CompilationUtils.CreateCompilationWithMscorlibAndVBRuntime(source)
+            comp.VerifyDiagnostics(
+                Diagnostic(ERRID.ERR_ExpectedComma, "").WithLocation(6, 39),
+                Diagnostic(ERRID.ERR_ExpectedExpression, "").WithLocation(6, 39),
+                Diagnostic(ERRID.ERR_ExpectedExpression, "").WithLocation(24, 38),
+                Diagnostic(ERRID.ERR_ExpectedExpression, "").WithLocation(25, 62),
+                Diagnostic(ERRID.ERR_ExpectedExpression, "").WithLocation(26, 34),
+                Diagnostic(ERRID.ERR_ExpectedExpression, "").WithLocation(27, 32),
+                Diagnostic(ERRID.ERR_ExpectedExpression, "").WithLocation(28, 32),
+                Diagnostic(ERRID.ERR_ExpectedExpression, "").WithLocation(37, 14),
+                Diagnostic(ERRID.ERR_TooFewGenericArguments1, "IEnumerator").WithArguments("System.Collections.Generic.IEnumerator(Of Out T)").WithLocation(31, 31),
+                Diagnostic(ERRID.ERR_TooFewGenericArguments1, "IEnumerator").WithArguments("System.Collections.Generic.IEnumerator(Of Out T)").WithLocation(33, 31),
+                Diagnostic(ERRID.ERR_TooFewGenericArguments1, "IEnumerator").WithArguments("System.Collections.Generic.IEnumerator(Of Out T)").WithLocation(36, 30),
+                Diagnostic(ERRID.ERR_AddOrRemoveHandlerEvent, "receiver?.TestEvent").WithLocation(15, 20),
+                Diagnostic(ERRID.ERR_AddOrRemoveHandlerEvent, "Function(ByVal x) x").WithLocation(6, 20),
+                Diagnostic(ERRID.ERR_BadIteratorReturn, "sub").WithLocation(24, 27),
+                Diagnostic(ERRID.ERR_BadIteratorReturn, "sub").WithLocation(25, 27),
+                Diagnostic(ERRID.ERR_BadIteratorReturn, "sub").WithLocation(25, 51),
+                Diagnostic(ERRID.ERR_BadIteratorReturn, "sub").WithLocation(27, 21),
+                Diagnostic(ERRID.ERR_BadIteratorReturn, "Sub").WithLocation(28, 21),
+                Diagnostic(ERRID.HDN_UnusedImportStatement, "Imports System.Collections.Generic").WithLocation(2, 1))
+            comp.VerifyAnalyzerDiagnostics({New MemberReferenceAnalyzer}, Nothing, Nothing, False,
+                Diagnostic(MemberReferenceAnalyzer.HandlerAddedDescriptor.Id, "AddHandler, New Object").WithLocation(26, 24),
+                Diagnostic(MemberReferenceAnalyzer.InvalidEventDescriptor.Id, "AddHandler, New Object").WithLocation(26, 24),
+                Diagnostic(MemberReferenceAnalyzer.HandlerAddedDescriptor.Id, "AddHandler receiver?.TestEvent, AddressOf Main").WithLocation(15, 9),
+                Diagnostic(MemberReferenceAnalyzer.InvalidEventDescriptor.Id, "AddHandler receiver?.TestEvent, AddressOf Main").WithLocation(15, 9),
+                Diagnostic(MemberReferenceAnalyzer.HandlerAddedDescriptor.Id, "AddHandler Function(ByVal x) x").WithLocation(6, 9),
+                Diagnostic(MemberReferenceAnalyzer.InvalidEventDescriptor.Id, "AddHandler Function(ByVal x) x").WithLocation(6, 9))
+        End Sub
     End Class
 End Namespace