MicrosoftDocs · colin-home · Oct 19, 2021 · Oct 19, 2021 · Oct 19, 2021
diff --git a/docs/_breadcrumb/toc.yml b/docs/_breadcrumb/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: Docs
   tocHref: /
   topicHref: /

diff --git a/docs/assembler/toc.yml b/docs/assembler/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: Compiler intrinsics and assembly language
   href: ../intrinsics/index.yml
 - name: Compiler intrinsics

diff --git a/docs/atl-mfc-shared/toc.yml b/docs/atl-mfc-shared/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: ATL-MFC shared classes
   href: ../atl-mfc-shared/atl-mfc-shared-classes.md
 - name: ATL-MFC concepts

diff --git a/docs/atl/toc.yml b/docs/atl/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: Active Template Library (ATL)
   href: atl-com-desktop-components.md
 - name: Active Template Library (ATL) concepts

diff --git a/docs/build-insights/toc.yml b/docs/build-insights/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: "C++ Build Insights"
   href: ../build-insights/index.yml
 - name: "Get started with C++ Build Insights"

diff --git a/docs/build/toc.yml b/docs/build/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: Projects and build systems
   href: ../build/projects-and-build-systems-cpp.md
 - name: Visual Studio projects - C++

diff --git a/docs/c-language/toc.yml b/docs/c-language/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: C language documentation
   href: ../c-language/index.yml
 - name: C language reference

diff --git a/docs/c-runtime-library/toc.yml b/docs/c-runtime-library/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: C runtime library (CRT) reference
   href: ../c-runtime-library/c-run-time-library-reference.md
 - name: CRT library features

diff --git a/docs/code-quality/toc.yml b/docs/code-quality/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: Code analysis documentation
   href: ../code-quality/index.yml
 - name: Overview

diff --git a/docs/cpp/toc.yml b/docs/cpp/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: C++ language documentation
   href: ../cpp/index.yml
 - name: C++ language reference
@@ -69,9 +70,9 @@
           href: ../cpp/void-cpp.md
         - name: bool
           href: ../cpp/bool-cpp.md
-        - name: false
+        - name: "false"
           href: ../cpp/false-cpp.md
-        - name: true
+        - name: "true"
           href: ../cpp/true-cpp.md
         - name: char, wchar_t, char8_t, char16_t, char32_t
           href: ../cpp/char-wchar-t-char16-t-char32-t.md
@@ -419,8 +420,6 @@
           href: ../cpp/special-member-functions.md
         - name: Static members
           href: ../cpp/static-members-cpp.md
-        - name: C++ classes as value types
-          href: ../cpp/value-types-modern-cpp.md
         - name: User-defined type conversions
           href: ../cpp/user-defined-type-conversions-cpp.md
         - name: Mutable data members
@@ -481,8 +480,6 @@
           href: ../cpp/how-to-create-and-use-weak-ptr-instances.md
         - name: "How to: Create and use CComPtr and CComQIPtr instances"
           href: ../cpp/how-to-create-and-use-ccomptr-and-ccomqiptr-instances.md
-        - name: Pimpl for compile-time encapsulation
-          href: ../cpp/pimpl-for-compile-time-encapsulation-modern-cpp.md  
     - name: Exception handling in C++
       items: 
         - name: Exception handling in C++

diff --git a/docs/cppcx/toc.yml b/docs/cppcx/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: UWP development
   href: ../cppcx/universal-windows-apps-cpp.md
 - name: C++/CX language reference

diff --git a/docs/cross-platform/toc.yml b/docs/cross-platform/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: Cross-platform mobile development with C++
   href: ../cross-platform/index.yml
 - name: Overview

diff --git a/docs/data/toc.yml b/docs/data/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: Data access in Visual C++
   href: ../data/data-access-in-cpp.md
 - name: Data access programming (MFC-ATL)

diff --git a/docs/dotnet/toc.yml b/docs/dotnet/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: .NET programming with C++/CLI
   href: ../dotnet/dotnet-programming-with-cpp-cli-visual-cpp.md
 - name: "Walkthrough: Compile a C++/CLI program that targets the CLR"

diff --git a/docs/error-messages/compiler-errors-2/compiler-error-c2666.md b/docs/error-messages/compiler-errors-2/compiler-error-c2666.md
@@ -1,7 +1,7 @@
 ---
 description: "Learn more about: Compiler Error C2666"
 title: "Compiler Error C2666"
-ms.date: "11/04/2016"
+ms.date: 10/18/2021
 f1_keywords: ["C2666"]
 helpviewer_keywords: ["C2666"]
 ms.assetid: 78364d15-c6eb-439a-9088-e04a0176692b
@@ -30,6 +30,52 @@ int main() {
 }
 ```
 
+This error can be generated as a result of compiler conformance work that was done for Visual Studio 2019 version 16.1:
+
+- A conversion that promotes an enumeration whose underlying type is fixed to its underlying type is better than one that promotes to the promoted underlying type, if the two are different.
+
+The following example demonstrates how compiler behavior changes in Visual Studio 2019 version 16.1 and later versions:
+
+```cpp
+#include <type_traits>
+
+enum E : unsigned char { e };
+
+int f(unsigned int)
+{
+    return 1;
+}
+
+int f(unsigned char)
+{
+    return 2;
+}
+
+struct A {};
+struct B : public A {};
+
+int f(unsigned int, const B&)
+{
+    return 3;
+}
+
+int f(unsigned char, const A&)
+{
+    return 4;
+}
+
+int main()
+{
+    // Calls f(unsigned char) in 16.1 and later. Called f(unsigned int) in earlier versions.
+    // The conversion from 'E' to the fixed underlying type 'unsigned char' is better than the
+    // conversion from 'E' to the promoted type 'unsigned int'.
+    f(e);
+
+    // Error C2666. This call is ambiguous, but previously called f(unsigned int, const B&). 
+    f(e, B{});
+}
+```
+
 This error can also be generated as a result of compiler conformance work that was done for Visual Studio .NET 2003:
 
 - binary operators and user-defined conversions to pointer types

diff --git a/docs/error-messages/toc.yml b/docs/error-messages/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: C/C++ Compiler and build tools errors and warnings
   href: compiler-errors-1/c-cpp-build-errors.md
 - name: BSCMAKE errors and warnings

diff --git a/docs/extensions/toc.yml b/docs/extensions/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: Component Extensions for .NET and UWP
   href: ../extensions/component-extensions-for-runtime-platforms.md
 - name: Tracking reference operator

diff --git a/docs/get-started/breadcrumb/toc.yml b/docs/get-started/breadcrumb/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: Docs
   tocHref: /
   topicHref: /

diff --git a/docs/ide/toc.yml b/docs/ide/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: Read and write C++ code in Visual Studio
   expanded: true
   items: 

diff --git a/docs/intrinsics/toc.yml b/docs/intrinsics/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: Compiler intrinsics and assembly language
   href: ../intrinsics/index.yml
 - name: Compiler intrinsics

diff --git a/docs/linux/toc.yml b/docs/linux/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: Linux with Visual Studio C++ documentation
   href: ../linux/index.yml
 - name: Download, install, and set up the Linux Development workload

diff --git a/docs/mfc/toc.yml b/docs/mfc/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: MFC desktop applications
   href: mfc-desktop-applications.md
 - name: MFC concepts

diff --git a/docs/overview/cpp-conformance-improvements.md b/docs/overview/cpp-conformance-improvements.md
@@ -1,7 +1,7 @@
 ---
 title: "C++ conformance improvements in Visual Studio 2019"
 description: "Microsoft C++ in Visual Studio is progressing toward full conformance with the C++20 language standard."
-ms.date: 06/11/2021
+ms.date: 10/18/2021
 ms.technology: "cpp-language"
 ---
 # C++ Conformance improvements, behavior changes, and bug fixes in Visual Studio 2019
@@ -523,6 +523,58 @@ void f() {
 
 [P0329R4](https://wg21.link/p0329r4) (C++20) *Designated initialization* allows specific members to be selected in aggregate initialization by using the `Type t { .member = expr }` syntax. Requires **`/std:c++latest`** (or **`/std:c++20`** starting in Visual Studio 2019 version 16.11).
 
+### Ranking of enum conversion to its fixed underlying type
+
+The compiler now ranks enum conversions according to [N4800](https://wg21.link/n4800) 11.3.3.2 Ranking implicit conversion sequences (4.2):
+
+- A conversion that promotes an enumeration whose underlying type is fixed to its underlying type is better than one that promotes to the promoted underlying type, if the two are different.
+
+This conversion ranking wasn't implemented correctly before Visual Studio 2019 version 16.1. The conforming behavior may change overload resolution behavior or expose an ambiguity where one previously was not detected.
+
+This compiler behavior change applies to all **`/std`** modes and is both a source and binary breaking change.
+
+The following example demonstrates how compiler behavior changes in 16.1 and later versions:
+
+```cpp
+#include <type_traits>
+
+enum E : unsigned char { e };
+
+int f(unsigned int)
+{
+    return 1;
+}
+
+int f(unsigned char)
+{
+    return 2;
+}
+
+struct A {};
+struct B : public A {};
+
+int f(unsigned int, const B&)
+{
+    return 3;
+}
+
+int f(unsigned char, const A&)
+{
+    return 4;
+}
+
+int main()
+{
+    // Calls f(unsigned char) in 16.1 and later. Called f(unsigned int) in earlier versions.
+    // The conversion from 'E' to the fixed underlying type 'unsigned char' is better than the
+    // conversion from 'E' to the promoted type 'unsigned int'.
+    f(e);
+
+    // Error C2666. This call is ambiguous, but previously called f(unsigned int, const B&). 
+    f(e, B{});
+}
+```
+
 ### New and updated standard library functions (C++20)
 
 - `starts_with()` and `ends_with()` for `basic_string` and `basic_string_view`.
@@ -2462,6 +2514,12 @@ struct S {
 };
 ```
 
+## <a name="improvements_16b"></a> Conformance improvements in Visual Studio 2019 version 16.11
+
+### `/std:c++20` compiler mode
+
+Starting in Visual Studio 2019 version 16.11, the compiler now supports the [`/std:c++20`](../build/reference/std-specify-language-standard-version.md) compiler mode. Previously, C++20 features were available only in [`/std:c++latest`](../build/reference/std-specify-language-standard-version.md) mode in Visual Studio 2019. Features that originally required **`/std:c++latest`** mode now work in **`/std:c++20`** mode or later in the latest versions of Visual Studio.
+
 ## See also
 
 [Microsoft C/C++ language conformance](visual-cpp-language-conformance.md)
diff --git a/docs/overview/toc.yml b/docs/overview/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: Microsoft C/C++ in Visual Studio
   expanded: false
   items: 

diff --git a/docs/parallel/toc.yml b/docs/parallel/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: Parallel programming in MSVC
   href: ../parallel/parallel-programming-in-visual-cpp.md
 - name: Auto-parallelization and auto-vectorization

diff --git a/docs/porting/toc.yml b/docs/porting/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: Microsoft C++ Porting and Upgrade Guide
   href: ../porting/visual-cpp-porting-and-upgrading-guide.md
 - name: Upgrade projects from earlier versions

diff --git a/docs/preprocessor/toc.yml b/docs/preprocessor/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: C/C++ preprocessor reference
   href: ../preprocessor/c-cpp-preprocessor-reference.md
 - name: Preprocessor

diff --git a/docs/sanitizers/toc.yml b/docs/sanitizers/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: "Code Sanitizers"
   href: ../sanitizers/index.yml
 - name: "AddressSanitizer"

diff --git a/docs/standard-library/toc.yml b/docs/standard-library/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: C++ Standard Library reference
   href: cpp-standard-library-reference.md
 - name: C++ Standard Library header files

diff --git a/docs/text/toc.yml b/docs/text/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: Text and strings in MSVC
   href: ../text/text-and-strings-in-visual-cpp.md
 - name: Unicode and MBCS

diff --git a/docs/windows/toc.yml b/docs/windows/toc.yml
@@ -1,3 +1,4 @@
+items: 
 - name: Overview of Windows programming in C++
   href: ../windows/overview-of-windows-programming-in-cpp.md
 - name: Desktop Applications in C++