From bff51642eb5b329a8821f26027b38097f74793d1 Mon Sep 17 00:00:00 2001
From: AlekseyTs <AlekseyTs@users.noreply.github.com>
Date: Fri, 14 Feb 2020 18:47:05 -0800
Subject: [PATCH] Allow structures to implement nested interfaces as we do for
 classes. (#41711)

Fixes #41501.
---
 .../Source/SourceNamedTypeSymbol_Bases.cs     |  8 ++---
 .../CSharp/Test/Symbol/Symbols/TypeTests.cs   | 32 +++++++++++++++++++
 2 files changed, 36 insertions(+), 4 deletions(-)

diff --git a/src/Compilers/CSharp/Portable/Symbols/Source/SourceNamedTypeSymbol_Bases.cs b/src/Compilers/CSharp/Portable/Symbols/Source/SourceNamedTypeSymbol_Bases.cs
index 9d2e7f43a2f..6e4af9daf05 100644
--- a/src/Compilers/CSharp/Portable/Symbols/Source/SourceNamedTypeSymbol_Bases.cs
+++ b/src/Compilers/CSharp/Portable/Symbols/Source/SourceNamedTypeSymbol_Bases.cs
@@ -571,12 +571,12 @@ private ImmutableArray<NamedTypeSymbol> MakeAcyclicInterfaces(ConsList<TypeSymbo
             }
 
             var declaredInterfaces = GetDeclaredInterfaces(basesBeingResolved: basesBeingResolved);
-            bool isClass = (typeKind == TypeKind.Class);
+            bool isInterface = (typeKind == TypeKind.Interface);
 
-            ArrayBuilder<NamedTypeSymbol> result = isClass ? null : ArrayBuilder<NamedTypeSymbol>.GetInstance();
+            ArrayBuilder<NamedTypeSymbol> result = isInterface ? ArrayBuilder<NamedTypeSymbol>.GetInstance() : null;
             foreach (var t in declaredInterfaces)
             {
-                if (!isClass)
+                if (isInterface)
                 {
                     if (BaseTypeAnalysis.TypeDependsOn(depends: t, on: this))
                     {
@@ -611,7 +611,7 @@ private ImmutableArray<NamedTypeSymbol> MakeAcyclicInterfaces(ConsList<TypeSymbo
                 }
             }
 
-            return isClass ? declaredInterfaces : result.ToImmutableAndFree();
+            return isInterface ? result.ToImmutableAndFree() : declaredInterfaces;
         }
 
         private NamedTypeSymbol MakeAcyclicBaseType(DiagnosticBag diagnostics)
diff --git a/src/Compilers/CSharp/Test/Symbol/Symbols/TypeTests.cs b/src/Compilers/CSharp/Test/Symbol/Symbols/TypeTests.cs
index 4e97087dd87..31c2cefe0f6 100644
--- a/src/Compilers/CSharp/Test/Symbol/Symbols/TypeTests.cs
+++ b/src/Compilers/CSharp/Test/Symbol/Symbols/TypeTests.cs
@@ -2292,5 +2292,37 @@ public interface I1
 
             Assert.True(i1.IsExplicitDefinitionOfNoPiaLocalType);
         }
+
+        [Fact]
+        [WorkItem(41501, "https://github.com/dotnet/roslyn/issues/41501")]
+        public void ImplementNestedInterface_01()
+        {
+            var text = @"
+public struct TestStruct : TestStruct.IInnerInterface
+{
+    public interface IInnerInterface
+    {
+    }
+}
+";
+            var compilation = CreateCompilation(text);
+            compilation.VerifyDiagnostics();
+        }
+
+        [Fact]
+        [WorkItem(41501, "https://github.com/dotnet/roslyn/issues/41501")]
+        public void ImplementNestedInterface_02()
+        {
+            var text = @"
+public class TestClass : TestClass.IInnerInterface
+{
+    public interface IInnerInterface
+    {
+    }
+}
+";
+            var compilation = CreateCompilation(text);
+            compilation.VerifyDiagnostics();
+        }
     }
 }
-- 
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