Missing -Wuninitialized warning for construction of member with another member using a constructor

[Quuxplusone]

Bugzilla Link	PR19376
Status	NEW
Importance	P normal
Reported by	Argyrios Kyrtzidis (akyrtzi@gmail.com)
Reported on	2014-04-08 20:07:18 -0700
Last modified on	2014-04-08 22:38:58 -0700
Version	unspecified
Hardware	PC All
CC	dblaikie@gmail.com, llvm-bugs@lists.llvm.org, rtrieu@google.com
Fixed by commit(s)
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This test case:
-------------
struct Bar {
    int val = 0;
};

struct Foo {
    Foo(const Bar& b) : val(b.val)
    {
    }

    int val;
};

class MyClass1 {
public:
    MyClass1();
private:
    Bar x;
    Bar y;
};

MyClass1::MyClass1() : x(y)
{
}

class MyClass2 {
public:
    MyClass2();
private:
    Foo x;
    Bar y;
};

MyClass2::MyClass2() : x(y)
{
}
-----------------

Only gives only warning:
test.cpp:21:26: warning: field 'y' is uninitialized when used here
      [-Wuninitialized]
MyClass1::MyClass1() : x(y)
                         ^
1 warning generated.

It should warn on MyClass2 as well.

[Quuxplusone]

Also tracked by rdar://16544901

[Quuxplusone]

false positives may be difficult to deal with - there are benign cases where two members want to be constructed with references to each other but they don't do anything with those references until later (where later happens to be "after both objects are actually constructed")

but, yeah, experiment and find out how often that comes up (just not sure what we'll suggest users do to rewrite their code when it does - but it might be there are more readable, less error-prone ways to write the code anyway)