Build failure in nostd/span.h

[VivekSubr]

Describe your environment
subramaniamv@a7e76fbbc22b:/src/cna$ clang --version
clang version 16.0.0
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

Steps to reproduce
Install opentelemetry, and build using this compiler... you will get error,

In file included from /usr/include/affirmed/logging/genTracer.h:3:
In file included from /usr/include/affirmed/logging/genSpan.h:4:
In file included from /usr/include/opentelemetry/sdk/trace/tracer.h:11:
In file included from /usr/include/opentelemetry/sdk/instrumentationscope/instrumentation_scope.h:8:
In file included from /usr/include/opentelemetry/common/key_value_iterable_view.h:9:
In file included from /usr/include/opentelemetry/common/key_value_iterable.h:6:
In file included from /usr/include/opentelemetry/common/attribute_value.h:8:
/usr/include/opentelemetry/nostd/span.h:160:64: error: expected body of lambda expression
                                    std::is_convertible<U (*)[], T (*)[]>::value>::type * = nullptr>

Additional context
Looks like compound statements in enable_if is not getting accepted by clang, this patch gets it working ->

diff -urNp opentelemetry-cpp-1.8.1/api/include/opentelemetry/nostd/span.h opentelemetry-cpp-1.8.1-patched/api/include/opentelemetry/nostd/span.h
--- opentelemetry-cpp-1.8.1/api/include/opentelemetry/nostd/span.h	2022-12-04 12:11:21.000000000 -0600
+++ opentelemetry-cpp-1.8.1-patched/api/include/opentelemetry/nostd/span.h	2023-01-27 06:02:20.660992000 -0600
@@ -135,9 +135,7 @@ public:
   }
 
   template <class U,
-            size_t N,
-            typename std::enable_if<N == Extent &&
-                                    std::is_convertible<U (*)[], T (*)[]>::value>::type * = nullptr>
+            size_t N>
   span(const span<U, N> &other) noexcept : data_{other.data()}
   {}
 
@@ -216,8 +214,7 @@ public:
   {}
 
   template <class U,
-            size_t N,
-            typename std::enable_if<std::is_convertible<U (*)[], T (*)[]>::value>::type * = nullptr>
+            size_t N>
   span(const span<U, N> &other) noexcept : extent_{other.size()}, data_{other.data()}
   {}

[marcalff]

Investigating.

A couple of comments:

The syntax std::is_convertible<U (*)[], T (*)[]> looks legitimate.

This is for example what I have in my /usr/include/c++/12/type_traits header with g++12

  // helper trait for unique_ptr<T[]>, shared_ptr<T[]>, and span<T, N>
  template<typename _ToElementType, typename _FromElementType>
    using __is_array_convertible
      = is_convertible<_FromElementType(*)[], _ToElementType(*)[]>;

Please indicate:

• which C++ mode is used (C++11/14/17/20/23) when building.
• the setting for WITH_STL, I assume it is OFF since nostd::span is used

Also, please indicate which .cc file fails to build, and whether:

• this fails when compiling opentelemetry-cpp itself (in which cc file ?),
• or if it fails when compiling some application code using opentelemetry-cpp (what is /usr/include/affirmed/logging/genSpan.h ?)

In both cases, show the calling code that uses a span, causing this.

Also, note that the patch is incorrect.

It may build, but is unsafe, and will assign spans of incompatible objects U and size N to a span<T, Extent>, causing runtime crashes.

[marcalff]

Trying the following code, which is extracted from nostd::span in opentelemetry-cpp:

#include <cstddef>
#include <type_traits>

template <class T, size_t Extent>
class span {
public:
    span(T* data);

  template <class U,
            size_t N,
            typename std::enable_if<N == Extent &&
                                    std::is_convertible<U (*)[], T (*)[]>::value>::type * = nullptr>
  span(const span<U, N> &other) noexcept : data_{other.data()}
  {}

private:
  T* data_;
};

int main() {
    int *d1 = new int[10];
    double *d2 = new double[20];
    span<int, 10> s1(d1);
    span<double, 20> s2(d2);

    span<int, 10> s3(s1);

    // Fails as expected
    // span<int, 20> s4(s2);
}

in compiler explorer (https://godbolt.org/) with Clang 16.0.0.

The code builds properly for valid spans (s3), and fails as expected for invalid ones (s4).

[VivekSubr]

Thanks for the help @marcalff!

"which C++ mode is used (C++11/14/17/20/23) when building."
C++17, we build otel as,

cmake .. -DCMAKE_INSTALL_RPATH_USE_LINK_PATH=ON -DCMAKE_VERBOSE_MAKEFILE=ON -DCMAKE_CXX_STANDARD=17 \
         -DWITH_STL=ON -DBUILD_SHARED_LIBS=ON -DWITH_OTLP=ON -DWITH_OTLP_GRPC=ON -DBUILD_TESTING=OFF 

Since span is not in C++17, it still uses nostd.

"Also, please indicate which .cc file fails to build, and whether:"
Application code fails when including the header, syntax error

In file included from ../common/pfcpUtils.cc:18:
In file included from /src/cna/cna/cna-pfcp/interface/pfcpLib/common/pfcpUtils.h:26:
In file included from /src/cna/cna/cna-pfcp/interface/pfcpLib/common/pfcpCommon.h:35:
In file included from /src/cna/cna/cna-pfcp/interface/pfcpLib/common/pfcpTracer.h:20:
In file included from /usr/include/affirmed/logging/genTracer.h:3:
In file included from /usr/include/affirmed/logging/genSpan.h:4:
In file included from /usr/include/opentelemetry/sdk/trace/tracer.h:9:
In file included from /usr/include/opentelemetry/sdk/resource/resource.h:6:
In file included from /usr/include/opentelemetry/common/attribute_value.h:8:

In genSpan.h line 4, we have an include for tracer.h, and this class also creates an object of type std::shared_ptrbase::GenSpan... but the compiler throws a syntax error well before this.

[VivekSubr]

@esigo... what information is needed?

[lalitb]

@VivekSubr Based on the conversation during the community meeting, and indicated by @marcalff in above comment, the error couldn't be replicated.

[marcalff]

Closing, can not reproduce.