GCC、clang 不同意 MSVC 对缩小转换的看法

Question

Consider the following program:

struct uint1 {
    unsigned x;
    uint1(unsigned x_) : x(x_) { }
};

struct foo { uint1 a; };

foo f(int v) { 
    return {v}; 
}

struct bar { unsigned a; };

bar g(int v) {
    return {v};
}

Now, where do we have narrowing conversions here?

Compiler	version	narrowing conversion in f() ?	narrowing conversion in g() ?
GCC	11.2	No	Yes
clang	13.0	No	Yes
MSVC	19.4	Yes	Yes

Which is right? Naively, it seems to me there's a narrowing conversion in both functions.

See it all on GodBolt .

Note: C++17 in case it matters.

Answer 1

It seems that GCC and Clang are right in not classing this as a narrowing conversion.

The initialisation in question is that of a foo object. Its constructor argument is enclosed in {} . There cannot be a narrowing conversion in this initialisation, because the argument of the constructor is not of a numeric type. It is struct uint1 .

So where does struct uint1 cone from? It is converted from v , but not by means of braced initialisation. The pair of braces around v is already occupied by the constructor of uint1 .

So this is equivalent to foo{uint1(v)} .

MSVC on the other hand seems to interpret this as foo(uint1{v}) , and I don't think this interpretation is justified.

Neither compiler complains about an explicit foo{uint1(v)} , as is required by the standard, because there is no narrowing at the top level.