P1766R1 Mitigating minor modules maladies

Author: jensmaurer

source/basic.tex

@@ -276,7 +276,9 @@
 \rSec1[basic.def.odr]{One-definition rule}
 
 \pnum
-A variable, function, class type, enumeration type, or template
+A variable, function, class type, enumeration type, template,
+default argument for a parameter (for a function in a given scope), or
+default template argument
 shall not be defined where a prior definition is necessarily reachable\iref{module.reach};
 no diagnostic is required if the prior declaration is in another translation unit.
 
@@ -548,10 +550,12 @@
 non-static function template\iref{temp.fct},
 concept\iref{temp.concept},
 static data member of a class template\iref{temp.static}, member
-function of a class template\iref{temp.mem.func}, or template
+function of a class template\iref{temp.mem.func}, template
 specialization for which some template parameters are not
-specified~(\ref{temp.spec}, \ref{temp.class.spec}) in a program
-provided that
+specified~(\ref{temp.spec}, \ref{temp.class.spec}),
+default argument for a parameter (for a function in a given scope), or
+default template argument
+in a program provided that
 no prior definition is necessarily reachable\iref{module.reach}
 at the point where a definition appears, and
 provided the definitions satisfy the following requirements.

source/compatibility.tex

@@ -1922,6 +1922,34 @@
 
 \rSec2[diff.cpp17.dcl.dcl]{\ref{dcl.dcl}: declarations}
 
+\diffref{dcl.typedef}
+\change Unnamed classes with a typedef name for linkage purposes
+can only contain C-compatible constructs.
+\rationale Necessary for implementability.
+\effect Valid C++ 2017 code may be ill-formed in this International Standard.
+\begin{codeblock}
+typedef struct {
+  void f() {}           // ill-formed; previously well-formed
+} S;
+\end{codeblock}
+
+\diffref{dcl.fct.default}
+\change A function cannot have different default arguments
+in different translation units.
+\rationale Required for modules support.
+\effect Valid C++ 2017 code may be ill-formed in this International Standard,
+with no diagnostic required.
+\begin{codeblock}
+// 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{codeblock}
+
 \diffref{dcl.init.aggr}
 \change A class that has user-declared constructors is never an aggregate.
 \rationale Remove potentially error-prone aggregate initialization

source/declarations.tex

@@ -736,6 +736,24 @@
 typedef decltype([]{}) C;       // the closure type has no name for linkage purposes
 \end{codeblock}
 \end{example}
+An unnamed class with a typedef name for linkage purposes shall not
+\begin{itemize}
+\item
+  declare any members
+  other than non-static data members, member enumerations, or member classes,
+\item
+  have any base classes or default member initializers, or
+\item
+  contain a \grammarterm{lambda-expression},
+\end{itemize}
+and all member classes shall also satisfy these requirements (recursively).
+\begin{example}
+\begin{codeblock}
+typedef struct {
+  int f() {}
+} X;                            // error: struct with typedef name for linkage has member functions
+\end{codeblock}
+\end{example}
 
 \rSec2[dcl.friend]{The \tcode{friend} specifier}%
 \indextext{specifier!\idxcode{friend}}
@@ -3777,11 +3795,12 @@
 supplied in this or a previous declaration,
 unless the parameter was expanded from a parameter pack,
 or shall be a function parameter pack.
+\begin{note}
 A default argument
-shall not be redefined by a later declaration (not even to the
-same value).
+cannot be redefined by a later declaration
+(not even to the same value)\iref{basic.def.odr}.
+\end{note}
 \begin{example}
-
 \begin{codeblock}
 void g(int = 0, ...);           // OK, ellipsis is not a parameter so it can follow
                                 // a parameter with a default argument
@@ -3809,8 +3828,7 @@
 \end{example}
 For a given inline function defined in different translation units,
 the accumulated sets of default arguments at the end of the
-translation units shall be the same;
-see~\ref{basic.def.odr}.
+translation units shall be the same; no diagnostic is required.
 If a friend declaration specifies a default argument expression,
 that declaration shall be a definition and shall be the only
 declaration of the function or function template in the translation unit.