How can the type of braces influence object lifetime in C++?

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Gcc is correct. The lifetime of the temporary will be extended only when using list-initialization syntax (i.e. using braces) in initialization of an aggregate.

(since C++20) a temporary bound to a reference in a reference element of an
aggregate initialized using direct-initialization syntax (parentheses)
as opposed to list-initialization syntax (braces) exists until the end
of the full expression containing the initializer.

struct A {
  int&& r;
};
A a1{7}; // OK, lifetime is extended
A a2(7); // well-formed, but dangling reference

For direct initialization:

(emphasis mine)

otherwise, if the destination type is a (possibly cv-qualified) aggregate class, it is initialized as described in aggregate initialization except that narrowing conversions are permitted, designated initializers are not allowed, a temporary bound to a reference does not have its lifetime extended, there is no brace elision, and any elements without an initializer are value-initialized. (since C++20)

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