C++ Language Updates in MSVC in Visual Studio 2022 17.14

Introduction

In this update, we continue the tradition of bucketing bugs into helpful categories for you all to filter through along with even more notes from the compiler team as to what, exactly, was fixed. This blog is also complemented by the recent Pure Virtual C++ pre-conference talk by RanDair Porter; so please check out RanDair’s talk, “MSVC C++23 Conformance”, if you have not already to get a better view of MSVC’s conformance status for Visual Studio 2022 overall.

17.13 notes for reference.

C++23 Features

Note: C++23 features can be used by either adding /std:c++latest or
/std:c++23preview to the command line. In addition, features used from unfinished standards may not
have full IntelliSense support.

• Add support for P1102R2
  (Down with ()!) which makes a parameter-list an optional part of a lambda-declarator.

auto lambda = [] constexpr { }; // C++23: can now omit the '()' after the capture list.

• Implemented P1938R3:
  if consteval for C++23.

constexpr int f(int) {
  if consteval { // C++23: previously this would be 'if (std::is_constant_evaluated()) { ... }'.
    return 42;
  }
  else {
    return 1337;
  }
}
int main(int argc, const char**) {
  static_assert(f() == 42);
  int x = f(argc);
  if (x != 1337) // Since 'f' cannot be evaluated at compile-time, the value here is '1337'
                 // due to taking the 'else' branch in 'f'.
    return 1;
}

• Implement P1401R5 –
  Narrowing contextual conversions to bool, making the following well-formed as of C++23:

enum Flags { Zero, One, Two };
static_assert(One | Two);

• Add support of P2173R1 –
  attributes for lambda-expressions.

auto lambda = [] [[nodiscard]] [[deprecated]] { return 10; };
void f() {
  lambda(); // C++23: Attributes here will fire a deprecation warning and a discard of return value warning.
}

• Implement P1169R4 which adds
  support for static operator().
• Implement P2589R1 which adds
  support for static operator[].

struct Functor {
  static constexpr int operator()() { return 42; }
  static constexpr int operator[](int) { return 42; }
};
using T1 = decltype(Functor::operator()());  // C++23: Deduces to 'int'.
static_assert(Functor::operator()() == 42); // C++23: Can be used without creating a 'Functor' object.

using T2 = decltype(Functor::operator[](10));  // C++23: Deduces to 'int'.
static_assert(Functor::operator[](10) == 42); // C++23: Can be used without creating a 'Functor' object.

Smaller C++23 Items

• Implement CWG2428 – Deprecating a concept.

Compiler improvements for 17.14

C++/CLI

• Fix bug where std::optional could not be used with managed types.
  • Regression
    in 17.12.0: Substitution Failure Is An Error C3699 when compiling with /CLR
• Fix an issue related to nested lambdas and pack-expansions and stop emitting a warning related to
  ‘volatile’ if the class type is imported.
  • [Version
    16.7.0 Preview 4.0] [C++/CLI] Warning C5220 is issued for non-static data members of classes
    imported from metadata
  • MSVC
    rejects valid pack expansion of lambda expressions
• Fix regression in C++/CLI which causes a crash or bad diagnostics when calling functions with parameter-arrays
  and compiling for Net Core.
  • Error
    with String::Format in C++ CLR Class Library (.NET) project
• Compilation
  Error Occurs When Using std::shared_ptr in C++/CLI Code

Diagnostics

• Add a warning to detect undefined behavior mentioned in [meta.reqmts].
  • C++
    std::is_signed_v is implemented wrong.
• Fixes the lack of a diagnostic on an initialization getting skipped over in a switch statement.
  • Variable
    Declaration Allowed Inside Switch Case Without Scoping Operator
• Fix missing error diagnostic for explicit constructor initialization
  • list-init
    of aggregate with explicitly-default-constructible member incorrectly accepted

constexpr

• Prevent binding constexpr references to local temporaries
  • constexpr
    reference should bind to global variable
• Fix spurious error where if consteval would not properly be considered in consetxpr functions.
  • error
    C3615 “constexpr function ‘f’ cannot result in a constant expression” does not
    properly recognize early return in if consteval
• Fix an issue with combing a non-type template parameter of reference type with a template argument that is a
  function call that returns by reference.
  • std::is_constant_evaluated()
    fails in function returning reference
• Fix a bug in immediate function folding that resulted in bad string data
  • user
    literals causes crash if it’s consteval but not if it’s
    constexpr.
• Fix incorrect C7595 errors in constructor member initializers and as the object expression of member access
  expressions
  • Error
    calling consteval function from an ordinary constructor
• We now run destructors during constant evaluation that appear in the init-stmt of a for loop
  • MSVC
    doesn’t properly destroy a loop variable in constant evaluation
• Fixed constexpr __builtin_memcmp comparing unsigned chars
  • [regression]
    v17.10: __builtin_memcmp() yields erroneous results if evaluated at compile time
• Fix an incorrect rejection of an out-of-line static constexpr data member definition
  • C2370
    is falsely emitted when a static const member is defined with the constexpr specifier outside a
    template class
• Assigning
  value to field of base class in constexpr virtual function gennerates ICE
• Labeled
  statements is incorrectly rejected during constant evaluation

C++ Modules

• Fix issue where the compiler would sometimes issue a syntax error when parsing template-ids from a module.
  • MSVC
    C++20 modules: export using template alias
• Fixed an issue where users would sometimes see linker errors with certain inline functions from a module and
  various other compiler issues when building with header units enabled for Unreal Engine 5.
  • Unreal
    does not build with header units enabled
• Fix ICE in the compiler when importing a type with an apparent cyclic dependency on itself
  • C++/CLI
    internal compiler error importing types from C# dll
• Fixed an issue where custom-defined data sections were not persisted into a C++ module.
  • C2341
    is incorrectly raised if the segment is defined in a header unit module
• Fixed a case where C5260 was firing spuriously
  • When
    building a header unit module, using a static const variable in a template causes C5260
    to incorrectly fire
• Fixed an issue where using a class marked as __declspec(dllimport) would sometimes issue a linker
  error when imported through a module.
  • unresolved
    external symbol local vftable when using modules, dll, virtual destructor and MSVC 14.36.17.6
• Fixed an issue where the compiler would sometimes crash when loading a PCH with a module import.
  • Internal
    Compiler Error if std::shared_ptr is used both with and without modules
• Fixed a bug where functions returning auto would sometimes not be resolved from a module.
  • Error
    C3779 using C++20 modules due to unrecognized function definition
  • Duplicate
    COMDAT error when using C++20 modules
• Fixed compiler crash in modules when using lambdas in static member functions of a class.
  • False
    positive C2028 error with c++ modules
• Fix a compiler crash when trying to match explicit object member functions which originate outside a C++ module.
  • C2028
    caused by weird interaction of modules and a virtual function calling a deducing this function.
• Fix an issue with mdspan and modules.
  • Cannot
    use C++23 multi-dimensional operator[] in concepts in modules due to “Sorry: not yet
    implemented” internal compiler error
• Fixed an issue when using std::ranges in modules where the compiler would issue a syntax error.
  • A
    custom formatter and a module raise errors in __msvc_formatter.hpp
• Fix issue where __vectorcall functions in a module would sometimes not compile.
  • __vectorcall
    conflicts with C++ modules

Conformance

• Implement CWG 1213: Array subscripting and xvalues
  • Subscripting
    an rvalue array does not generate an rvalue in VS2019
• Added /Zc:u8EscapeEncoding for conformant encoding of escape sequences like u8″xFF”
  • Hex
    escaping in strings can’t be compiled in v15.6.5
  • escape
    sequences in unicode string literals are overencoded (non conforming => compiler bug)
  • VC++
    is incorrectly encoding UTF-8 x escape chars as u escape chars
• Fix missing diagnostic for mixing ‘friend’ and ‘static’.
  • [accepts
    invalid] cl accepts static friend functions
• Fix issue where lookup-related diagnostics are missing in fold expressions
  • Fold
    expressions unreliable in 17.1 with c++20
• Fixed an issue where the compiler did not conform to P1779R3.
  • Assertion
    failed: !XMVector3IsInfinite(EyeDirection) when there is no implementation unit for a module which
    includes DirectXMath.h

Reliability

• Fix a compiler crash when using d1initall and constant evaluation
  • ICE
    when using d1initall in a constexpr context
• Fix a compiler crash caused by a pointer-to-member
  • Internal
    compiler error (CRTP, multiple inheritance)
• Fix a silent compiler crash on certain malformed explicit destructor calls.
  • Compiler
    crashes (but with no ICE apparently) on this code
• Fix a compiler crash related to default function parameters
  • Internal
    compiler error with VS17.12.3 when compiling a function with a default parameter involving multiple
    vector operations
• Fix an issue with name-lookup of a conversion-function-id and an issue related to the pack-expansion of
  a nested lambda.
  • MSVC
    v19 fatal error C1001: Internal compiler error.
• Fix an ICE in certain alias template definitions with multi-component, dependent nested names.
  • Internal
    compiler error for some template aliases
• Fix ICE when C11 _Atomic is used with PCH
  • Adding
    #including generates C1001: Internal compiler error: ‘msc1.cpp’, line 1599 in
    Visual Studio 2022 Version 17.12.0 Preview 5.0
• Fix an ICE with certain noexcept-specifiers
  • ICE
    when having virtual noexcept(constexpr) function with argument
• Fix a crash in constant evaluation with base-to-derived conversions of pointers to array elements
  • ICE
    when calling CRTP function with member access from constexpr array member
• Fix an issue relating to explicit lambda template parameters and also an issue related to a multidimensional
  subscript operator
  • ICE
    using lambda template parameter as size for an array
  • MSVC
    rejects a template argument with multidimensional subscript operator
• Fixes extreme memory use for some zero-initialized arrays (and aggregates thereof)
  • Compiler
    memory usage explodes with lambda returning large structure
• Fix an ICE on a decltype containing a noexcept expression
  • Internal
    compiler error if use noexcept inside decltype
• Fix an issue with class template argument deduction for alias templates
  • ICE
    with Eigen 3.4.0 when missing template argument
• Fix an ICE with some ill-formed qualified names.
  • CL
    /std:c++latest – INTERNAL COMPILER ERROR C1001 tested in CL v19.42.34435 and CL v19.43.34604
• Fix an issue with dependent base-classes and class template argument deduction for aggregates.
  • ICE
    on CTAD for aggregate derived from a local class type
• Fix a crash with brace-initialization of an object containing a const reference member
  • Internal
    Compiler Error (ICE) when an object is brace initialized using another object that contains a
    reference
• fatal
  error C1001: Internal compiler error on atomic operation with struct

Correctness

• Fix incorrect error given when a base class name referenced in a template is found on multiple base class paths.
  • error
    C2385: ambiguous access of ‘IUnknown’ referring to common base class in VS2017.7 Preview
    3
• Correct an issue with using directives not being respected in some nested lambdas.
  • Nested template lambda
    accidentally ignores the using-directives of the parent scope, and thus the
    non-ADL user-defined literals to fail.
• Add missing diagnostic for ill-formed pure-specifier and fix issue with combination of ‘volatile’ and
  copy-assignment operators.
  • [accepts
    invalid] VS accepts invalid pure specifier
  • Possible
    C++ compiler bug: copy assignment operator with volatile qualifier
• Fix an issue related to the backing array for a std::initializer_list being destroyed too soon.
  • Optimization
    removes initialization for std::initializer_list of trivially copyable objects
• Fix an issue with lambda captures within a decltype-specifier, and an issue with a static_assert within a
  concept.
  • Error
    C2327 in decltype context within lambda referencing member variable (C++20)
  • MSVC
    ignores static_assert inside concept definition
• Fix a case where the compiler dropped pointer dereference of a cast of a pointer to an array in some cases
  • Incorrect
    code generated with convoluted pointer casting
• Fix incorrect access checks in certain alias templates
  • MSVC
    fails to compile template member function full specialization with private type and type alias
    arguments
• Fix an issue where a volatile reference could bind to an rvalue of a derived class
  • [_MSC_VER
    = 1940] MSVC allows binding a const volatile lvalue reference to an rvalue under
    /permissive-
• Fix an issue with the initialization of a variable of class type that is volatile.
  • MSVC
    discards volatile variable
• Fix incorrect error C3770 when certain alias template specializations are used as base classes
  • Const
    base class rejected when used through alias template
• Fix an issue with capturing ‘this’ in a lambda that is part of an init-capture and an issue in which the
  compiler to did not correctly handle an abstract class type during template argument substitution.
  • Unable
    to directly access member variable inside lambda nested in capture list of lambda
  • Call
    to method template with abstract class parameter may fail depending on call point
• Fix an issue in which a variable with an ‘auto’ type was referenced before its type had been deduced and
  an issue in which a valid goto-statement within an if-constexpr statement was flagged as an error.
  • ICE
    when recursively calling a lambda template
  • Incorrect
    C3907 in C++20 only
• Fix an issue in which warning C4100, “unused parameter”, was not emitted,
  • variable
    passed by value and warning 4100
• Fix an invalid error C2210 with some alias template specializations
  • error C2210
    Compile error
• Fix an issue with lambdas in a class with virtual bases.
  • Compilation
    error in lambda expressions in hierarchies with virtual inheritance
• Fix an issue with the zero-initialization of arrays with /d1initAll
  • /d1initall1023
    /d1initAll:FillPatternE2 options causes incorrect initialization results in constant evaluation
• Fix an issue with decltype and calls to static member functions with an object and an issue with warnings and
  the conditional operator
  • Illegal
    use of undefined type
  • C4365
    incorrect warning and diagnostic location
• Address an issue in which C4100, unused parameter, was not emitted.
  • variable
    passed by value and warning 4100
• Fix an issue with aggregate initialization when a non-static data member is a self-referential reference type.
  • “too
    many initializers” error for valid code
• Fix an issue with hidden friend functions that are declared to return ‘auto’
  • Unable
    to compile this piece of code with constexpr and auto since 17.12.0
• Fix a regression that a deduction guide has incorrect return type.
  • CTAD
    failure with deduction guide in the abbreviated function template form
• Correct invalid implicit move of extern or thread_local variables
  • MSVC
    incorrectly implicitly moves a non-local variable when the function contains its non-defining
    declaration
• Fix issue where braced initialization in a non-type template argument would confuse the compiler.
  • Syntax
    error when using non-type template parameters in templated class member function
• Fix an issue with a template-id that had a template-argument that used a decltype-specifier and fix an issue
  with a variable template + class template argument deduction + lambdas
  • Failure
    with concept containing decltype and lambda
  • Rejects
    valid variable template CTAD for aggregate
• Fix an issue with ‘[[no_unique_address]]’ and trivial copy/move assignment operators.
  • std::set
    broken under VS 17.12.4 with “no_unique_address” comparator
• Fix and issue with a compiler requires-expression in an if-constexpr statement.
  • The
    ‘requires requires’ doesn’t short-circuit when used in ‘if constexpr’
• Fix incorrect template specialization involving certain qualified-names and dependent base classes.
  • C++
    compilation error on using Boost.Range
• Fix incorrect access checks in certain alias templates
  • MSVC
    fails to compile template member function full specialization with private type and type alias
    arguments
• Fix an incorrect type-deduction failure involving certain template specializations and empty packs.
  • formatter
    for empty tuples is broken in VS 17.13
• Fixed a bug where a user-provided operator, would sometimes not be called if there was a cast
  involved on one of the operands.
  • Silent
    bad codegen: Operand to overloaded operator comma is wrongly turned into pointer
• Fix an issue with undecorating cv-qualified nullptr.
  • c++
    Bug in type_info.name() for TMyClass_const std::nullptrt
• Reduced implementation divergence between clang and gcc when C++23 explicit object functions are called in
  various cases.
  • C++23
    Deducing This build error for Cat::Meow(this, float) in VS2022 Preview 2.1 (Version 17.2.0)
• Fix an issue related to pack-expansion and an issue related to the use of ‘decltype(auto)’ within a
  lambda.
  • Compiler
    error expanding parameter pack
  • Compilation
    Failure with decltype(auto) in Lambda Capture on MSVC, Works on GCC/Clang
• Fix an issue causing temporaries to be required in certain conditional expressions.
  • MSVC
    incorrectly attempts to call the copy constructor for xvalue conditional expression
• Add missing checks for deduction guide and trailing return type.
  • decorated
    auto accepted despite a trailing-return-type
  • Const
    in deduction guide
• Fix an issue with the compiler emitting the IL for a special member function when it was not ODR used.
  • std::vector
    tries to default construct an object from constructor accepting std::initializer_list
• C++20
  compile error with ATL::CComPtr<>
• Cannot
  capture local constexpr closure object in lambda in C++20 mode
• VisualCppTools
  / ppltasks.h race condition bug in when_any
• Write
  to base class member through virtual function called by another function fails to compile.
• vcruntime
  nothrow array operator new falls back on the wrong function
• 17.12.5
  internal preview fails to scan PreFast on cppcorecheck plugin
• MSVC
  instantiates constexpr destructors too early when data member initializers are involved
• VC++ 2017:
  Member enumeration of class template cannot be defined outside of the class template definition.
• MSVC
  19.43.34808 Preprocessor Bug: /EP Adds Unwanted #pragma warning Directives
  (Introduced in VS 2022 v17.13.x, Corrupts Preprocessed Files)
• Requires
  clause rejected in lambda initializing a variable template
• False
  positive lifetime code analysis warning C26849: Don’t dereference an invalid pointer: (lifetime.1).
• C26822
  false positive with not_null
• can’t compile
  with valid CTAD
• Type
  always prefered over value when using qualified identifiers
• std::bit_cast
  does not work for bit-fields equal to whole type
• Bit
  fields with widths computed via template are treated as 1-bit fields instead starting with MSVC 19.40
• OpenJDK
  (jdk17u) build failure with VS2022: error C2244: ‘TestEnableIfNested::sub1’: unable to match
  function definition to an existing declaration
• C26815
  false positive on “return *this;” with assignment of std::optional member in member function.
• Trivial
  template argument expressions broken in 14.44
• False
  positive lifetime code analysis warning C26847: Don’t return an invalid pointer (lifetime.4).
• Inconsistent
  ESP analysis defects after 17.10 update (Office)
• Breakpoint
  line mismatch due to VC++ compiler parse the line number wrongly, when a template class with alignas and
  comma
• Microsoft
  cl compiler successfully compile code that has unnecessary template-id specified.

Correctness (C compiler)

• C Make char types unique for _Generic operator
  • _Generic
    char, signed char, unsigned char are not distinct in C11
• C dropped pointer dereference of a cast of a pointer to an array in some cases
  • Incorrect
    code generated with convoluted pointer casting
• C compiler stack overflow on large array initialization
  • VS
    2022 cl.exe stack overflows when compiling zig2.c (~185 MB, ~4.1 M lines)

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