Clang incorrectly rejects default construction of union with nontrivial member

[miscco]

Bugzilla Link	49072
Version	trunk
OS	Windows NT
CC	@CaseyCarter,@zygoloid

Extended Description

Consider the following valid TU:

struct TriviallyConstructible {};
struct BadNontrivialClass {
    BadNontrivialClass() {}
};

union U {
    TriviallyConstructible first{};
    BadNontrivialClass second;
};

U u;

Currently clang rejects this with:

<source>:11:3: error: call to implicitly-deleted default constructor of 'U'
U u;
  ^
<source>:8:24: note: default constructor of 'U' is implicitly deleted because variant field 'second' has a non-trivial default constructor
    BadNontrivialClass second;

However, according to the standard (https://eel.is/c++draft/class.union#general-4) the member initializer of first makes this a valid construct.

[shafik]

Notes are non-normative, so you want to refer to class.default.ctor to back up this assertion.

[llvmbot]

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