[diff.cpp17.dcl.dcl]

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 revision of C++.

[Example 1: typedef struct { void f() {} / ill-formed; previously well-formed } S; — end example]

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 revision of C++, with no diagnostic required.

[Example 2: / 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 — end example]

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 revision of C++.

[Example 3: 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 — end example]

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 revision of C++.

[Example 4: bool y[] = { "bc" }; / ill-formed; previously well-formed — end example]