Allow structures to implement nested interfaces as we do for classes. (#41711)

Fixes #41501.

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)

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();
+        }
     }
 }