Properly generate type syntax for tuples with less than arity-2.

src/EditorFeatures/CSharpTest/ImplementInterface/ImplementInterfaceTests.cs

@@ -141,6 +141,35 @@ class Class : IInterface
 }");
         }
 
+        [WorkItem(16793, "https://github.com/dotnet/roslyn/issues/16793")]
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsImplementInterface)]
+        public async Task TestMethodWithValueTupleArity1()
+        {
+            await TestWithAllCodeStyleOptionsOffAsync(
+@"
+using System;
+interface I
+{
+    ValueTuple<object> F();
+}
+class C : [|I|]
+{
+}",
+@"
+using System;
+interface I
+{
+    ValueTuple<object> F();
+}
+class C : I
+{
+    public ValueTuple<object> F()
+    {
+        throw new NotImplementedException();
+    }
+}");
+        }
+
         [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsImplementInterface)]
         public async Task TestExpressionBodiedMethod1()
         {

src/Workspaces/CSharp/Portable/Extensions/ITypeSymbolExtensions.TypeSyntaxGeneratorVisitor.cs

@@ -100,6 +100,11 @@ public TypeSyntax CreateSimpleTypeSyntax(INamedTypeSymbol symbol)
                     }
                 }
 
+                if (symbol.IsTupleType && symbol.TupleUnderlyingType != null && !symbol.Equals(symbol.TupleUnderlyingType))
+                {
+                    return CreateSimpleTypeSyntax(symbol.TupleUnderlyingType);
+                }
+
                 if (symbol.Name == string.Empty || symbol.IsAnonymousType)
                 {
                     return CreateSystemObject();
@@ -145,7 +150,7 @@ private TypeSyntax TryCreateSpecializedNamedTypeSyntax(INamedTypeSymbol symbol)
                     return SyntaxFactory.PredefinedType(SyntaxFactory.Token(SyntaxKind.VoidKeyword));
                 }
 
-                if (symbol.IsTupleType)
+                if (symbol.IsTupleType && symbol.TupleElements.Length >= 2)
                 {
                     return CreateTupleTypeSyntax(symbol);
                 }

src/Workspaces/VisualBasic/Portable/Extensions/TypeSyntaxGeneratorVisitor.vb

@@ -102,7 +102,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic.Extensions
                     Return SyntaxFactory.QualifiedName(SyntaxFactory.IdentifierName("System"), SyntaxFactory.IdentifierName("DateTime"))
             End Select
 
-            If symbol.IsTupleType Then
+            If symbol.IsTupleType AndAlso symbol.TupleElements.Length >= 2 Then
                 Return CreateTupleTypeSyntax(symbol)
             End If