Annex C (informative) Compatibility [diff]

C.1 C++ and ISO C++ 2017 [diff.cpp17]

C.1.4 [dcl.dcl]: declarations [diff.cpp17.dcl.dcl]

1
#
Affected subclause: [dcl.typedef]
Change: Unnamed classes with a typedef name for linkage purposes can contain only C-compatible constructs.

Rationale: Necessary for implementability.

Effect on original feature: Valid C++ 2017 code may be ill-formed in this International Standard.
typedef struct {
  void f() {}           // ill-formed; previously well-formed
} S;
2
#
Affected subclause: [dcl.fct.default]
Change: A function cannot have different default arguments in different translation units.

Rationale: Required for modules support.

Effect on original feature: Valid C++ 2017 code may be ill-formed in this International Standard, with no diagnostic required.
// Translation unit 1
int f(int a = 42);
int g() { return f(); }

// Translation unit 2
int f(int a = 76) { return a; }         // ill-formed, no diagnostic required; previously well-formed
int g();
int main() { return g(); }              // used to return 42
3
#
Affected subclause: [dcl.init.aggr]
Change: A class that has user-declared constructors is never an aggregate.

Rationale: Remove potentially error-prone aggregate initialization which may apply notwithstanding the declared constructors of a class.

Effect on original feature: Valid C++ 2017 code that aggregate-initializes a type with a user-declared constructor may be ill-formed or have different semantics in this International Standard.
struct A {              // not an aggregate; previously an aggregate
  A() = delete;
};

struct B {              // not an aggregate; previously an aggregate
  B() = default;
  int i = 0;
};

struct C {              // not an aggregate; previously an aggregate
  C(C&&) = default;
  int a, b;
};

A a{};                  // ill-formed; previously well-formed
B b = {1};              // ill-formed; previously well-formed
auto* c = new C{2, 3};  // ill-formed; previously well-formed

struct Y;

struct X {
  operator Y();
};

struct Y {              // not an aggregate; previously an aggregate
  Y(const Y&) = default;
  X x;
};

Y y{X{}};               // copy constructor call; previously aggregate-initialization
4
#
Affected subclause: [dcl.init.list]
Change: Boolean conversion from a pointer or pointer-to-member type is now a narrowing conversion.

Rationale: Catches bugs.

Effect on original feature: Valid C++ 2017 code may fail to compile in this International Standard.
For example: 
bool y[] = { "bc" };    // ill-formed; previously well-formed