clang | Run Clang in WebAssembly | Binary Executable Format library

The boolean conversion operator for std::basic_istream is explicit. This means that instances of the type will not implicitly become a bool but can be converted to one explicitly, for instance by typing bool(infile).

Explicit boolean conversion operators are considered for conditional statements, i.e. the expression parts of if, while etc. More info about contextual conversions here.

However, a return statement will not consider the explicit conversion operators or constructors. So you have to explicitly convert that to a boolean for a return.

Without a declaration of field, this isn’t even valid syntax: the < can’t begin a template argument list, and expressions aren’t allowed there in a member-declaration. (With a suitable declaration, it could be an invalid declaration with two types and no variables.) Definitely diagnosable, and definitely a Clang bug.

From the C18 standard in 6.7.2.2:

Each enumerated type shall be compatible with char, a signed integer type, or an unsigned integer type. The choice of type is implementation-defined, but shall be capable of representing the values of all the members of the enumeration.

So yes enum { a } e = 1; is valid. e is a 'integer' type so it can take the value 1. The fact that 1 is not present as an enumeration value is no issue. The enumeration members only give handy identifiers for some of possible values.

Seems a bug of openssl itself. https://github.com/openssl/openssl/issues/16487

~~What about export SDKROOT="/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk "?~~

Homebrew pre-build packages for some versions of macOS. But it keep dropping this pre-building support for old macOS. On macOS 10.12, you're building openssl from the source code and Xcode command line tool is needed.

[temp.names]/5 says that a name prefixed by template must be a template-id, meaning that it must have a template argument list. (Or it can refer to a class/alias template without template argument list, but this is deprecated in the current draft as a result of P1787R6 authored by @DavisHerring.)

There is even an example almost identical to yours under it, identifying your use of template as ill-formed.

The requirement and example comes from CWG defect report 96, in which the possible ambiguity without the requirement is considered.

Open GCC bug report for this is here. I was not able to find a Clang bug report, but searching for it isn't that easy. Its implementation status page for defect reports however does list the defect report as unimplemented.

It's all slightly mysterious. gcc behaves the same as clang.

The standard has this to say (emphasis mine):

Absent default member initializers, if any non-static data member of a union has a non-trivial default constructor, copy constructor, move constructor, copy assignment operator, move assignment operator, or destructor, the corresponding member function of the union must be user-provided or it will be implicitly deleted for the union.

But I think the wording is a bit wooly here. I think what they actually mean is that you must provide an initialiser for the member that has (in your example) a non-trivial constructor, like so:

Clang is wrong.

[dcl.fct.def.general]

2 The type of a parameter or the return type for a function definition shall not be a (possibly cv-qualified) class type that is incomplete or abstract within the function body unless the function is deleted ([dcl.fct.def.delete]).

That's pretty clear I think. A deleted definition allows for an incomplete class type. It's not like the function can actually be called in a well-formed program, or the body is actually using the incomplete type in some way. The function is a placeholder to signify an invalid result to overload resolution.

Granted, the parameter types are more interesting in the case of actual overload resolution (and the return type can be anything), but there is no reason to restrict the return type into being complete here either.

Looks like a bug in Visual Studio, according to the standard [except.handle]:

A handler is a match for an exception object of type E if

[...]

• the handler is of type cv T or const T& where T is a pointer or pointer-to->member type and E is std​::​nullptr_t.

[basic.start.main]/4:

If std​::​exit is called to end a program during the destruction of an object with static or thread storage duration, the program has undefined behavior.