From ac33e3657ac0d22f3ab5c9636872260717bd3418 Mon Sep 17 00:00:00 2001
From: Rikki Gibson <rigibson@microsoft.com>
Date: Wed, 4 Dec 2019 11:31:38 -0800
Subject: [PATCH 1/2] Don't crash on self-referencing attribute in nullable

---
 .../Portable/FlowAnalysis/NullableWalker.cs   |  2 +-
 .../Semantics/NullableReferenceTypesTests.cs  | 19 +++++++++++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs b/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs
index cf034a5fe6643..bbcaaeccfeea4 100644
--- a/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs
+++ b/src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs
@@ -3607,7 +3607,7 @@ private ImmutableArray<VisitArgumentResult> VisitArguments(
                 }
             }
 
-            if (method is object && (method.FlowAnalysisAnnotations & FlowAnalysisAnnotations.DoesNotReturn) == FlowAnalysisAnnotations.DoesNotReturn)
+            if (!IsAnalyzingAttribute && method is object && (method.FlowAnalysisAnnotations & FlowAnalysisAnnotations.DoesNotReturn) == FlowAnalysisAnnotations.DoesNotReturn)
             {
                 SetUnreachable();
             }
diff --git a/src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs b/src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs
index d7f1ea5dd022e..5386191d28763 100644
--- a/src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs
+++ b/src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs
@@ -7235,6 +7235,25 @@ class C { }
                 );
         }
 
+        [Fact, WorkItem(40136, "https://github.com/dotnet/roslyn/issues/40136")]
+        public void SelfReferencingAttribute()
+        {
+            var source = @"
+using System;
+
+[AttributeUsage(AttributeTargets.All)]
+[ExplicitCrossPackageInternal]
+internal sealed class ExplicitCrossPackageInternalAttribute : Attribute
+{
+    [ExplicitCrossPackageInternal]
+    internal ExplicitCrossPackageInternalAttribute()
+    {
+    }
+}";
+            var comp = CreateNullableCompilation(source);
+            comp.VerifyDiagnostics();
+        }
+
         [Fact]
         public void NullableAndConditionalOperators()
         {

From 93435156b112b0ae172890b64ad265a2cdaed259 Mon Sep 17 00:00:00 2001
From: Rikki Gibson <rigibson@microsoft.com>
Date: Tue, 10 Dec 2019 12:31:48 -0800
Subject: [PATCH 2/2] Cover a few more self-referencing cases

---
 .../Semantics/NullableReferenceTypesTests.cs         | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs b/src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs
index 5386191d28763..1d7b000e33e16 100644
--- a/src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs
+++ b/src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs
@@ -7242,13 +7242,19 @@ public void SelfReferencingAttribute()
 using System;
 
 [AttributeUsage(AttributeTargets.All)]
-[ExplicitCrossPackageInternal]
+[ExplicitCrossPackageInternal(ExplicitCrossPackageInternalAttribute.s)]
 internal sealed class ExplicitCrossPackageInternalAttribute : Attribute
 {
-    [ExplicitCrossPackageInternal]
-    internal ExplicitCrossPackageInternalAttribute()
+    internal const string s = """";
+
+    [ExplicitCrossPackageInternal(s)]
+    internal ExplicitCrossPackageInternalAttribute([ExplicitCrossPackageInternal(s)] string prop)
     {
     }
+
+    [return: ExplicitCrossPackageInternal(s)]
+    [ExplicitCrossPackageInternal(s)]
+    internal void Method() { }
 }";
             var comp = CreateNullableCompilation(source);
             comp.VerifyDiagnostics();