Implements CWG 2082 Referring to parameters in unevaluated operands of default arguments

[Mordante]

Summary

This implements the current standard wording for [dcl.fct.default]p9 and [dcl.fct.default]p7. This has been changed by CWG 2082.

Note: I don't have access to the paper therefore I assume it retroactively applies to all C++ standards.

[rsmith]

clang/lib/Sema/SemaDeclCXX.cpp

103–105

Please add tests for the distinction between "potentially-evaluated" and "odr-used" here, for example:

void f() {
  const int n = 123;
  void g(int k = n); // ok, not an odr-use
}

clang/www/cxx_dr_status.html

3

Note that this is an auto-generated file. To update it, you need to add a test to the relevant file (test/CXX/drs/dr20xx.cpp) with a suitable comment (// dr2082: 10 to mark this implemented in Clang 10), grab a recent cwg_index.html file, and run the make_cxx_dr_status script.

[Mordante]

clang/lib/Sema/SemaDeclCXX.cpp

103–105

I added the test but unfortunately clang disagrees with you and considers n ODR used.
I'll have look how to teach clang n is not ODR used.

clang/www/cxx_dr_status.html

3

Thanks for the info, I'll have a look at it after I fix the ODR used part.

[Mordante]

Updated the unit tests as requested. This required the Sema::ActOnParamDefaultArgument to delay a part of the ODR validation until the default argument has been 'instantiated'.
As discussed on IRC; the up to date cwg_index.html is not public, so I only updated the unit test and removed the changes to cxx_dr_status.html.

[rsmith]

clang/lib/Sema/SemaDeclCXX.cpp

154–159

Instead of visiting the default argument twice, can we call SetParamDefaultArgument first before doing the checks?

clang/lib/Sema/SemaDeclCXX.cpp

namespace {
 
   /// VisitDeclRefExpr - Visit a reference to a declaration, to
   /// determine whether this declaration can be used in the default
   /// argument expression.
   bool CheckDefaultArgumentVisitor::VisitDeclRefExpr(DeclRefExpr *DRE) {
     NamedDecl *Decl = DRE->getDecl();
     if (ParmVarDecl *Param = dyn_cast<ParmVarDecl>(Decl)) {
       // C++ [dcl.fct.default]p9
-      //   Default arguments are evaluated each time the function is
-      //   called. The order of evaluation of function arguments is
-      //   unspecified. Consequently, parameters of a function shall not
-      //   be used in default argument expressions, even if they are not
-      //   evaluated. Parameters of a function declared before a default
-      //   argument expression are in scope and can hide namespace and
-      //   class member names.
+      //   A default argument is evaluated each time the function is called
+      //   with no argument for the corresponding parameter. A parameter shall
+      //   not appear as a potentially-evaluated expression in a default
+      //   argument. Parameters of a function declared before a default
+      //   argument are in scope and can hide namespace and class member
+      //   names.
+      if (DRE->isNonOdrUse() == NOUR_Unevaluated)
+        return false;
+
       return S->Diag(DRE->getBeginLoc(),
                      diag::err_param_default_argument_references_param)
              << Param->getDeclName() << DefaultArg->getSourceRange();
     } else if (VarDecl *VDecl = dyn_cast<VarDecl>(Decl)) {
       // C++ [dcl.fct.default]p7
-      //   Local variables shall not be used in default argument
-      //   expressions.
+      //   A local variable cannot be odr-used (6.2) in a default argument.
+      if (DRE->isNonOdrUse() != NOUR_None)
+        return false;
+
       if (VDecl->isLocalVarDecl())
         return S->Diag(DRE->getBeginLoc(),
                        diag::err_param_default_argument_references_local)
                << VDecl->getDeclName() << DefaultArg->getSourceRange();
     }
 
     return false;
   }
bool CheckDefaultArgumentVisitor::VisitLambdaExpr(LambdaExpr *Lambda) {
     //   implicitly or explicitly capture any entity.
     if (Lambda->capture_begin() == Lambda->capture_end())
       return false;
 
     return S->Diag(Lambda->getBeginLoc(), diag::err_lambda_capture_default_arg);
   }
 }
 
 void
 Sema::ImplicitExceptionSpecification::CalledDecl(SourceLocation CallLoc,
                                                  const CXXMethodDecl *Method) {
   // If we have an MSAny spec already, don't bother.
   if (!Method || ComputedEST == EST_MSAny)
     return;
 
   const FunctionProtoType *Proto
     = Method->getType()->getAs<FunctionProtoType>();
   Proto = Self->ResolveExceptionSpec(CallLoc, Proto);
   if (!Proto)
     return;
 
   ExceptionSpecificationType EST = Proto->getExceptionSpecType();

clang/test/CXX/dcl.decl/dcl.fct.def/dcl.fct.def.default/p9.cpp

+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+int a;
+int f(int a, int b = a); // expected-error {{default argument references parameter 'a'}}
+typedef int I;
+int g(float I, int b = I(2)); // expected-error {{called object type 'float' is not a function or function pointer}}
+int h(int a, int b = sizeof(a));
+
+int b;
+class X {
+  int a;
+  int mem1(int i = a); // expected-error {{invalid use of non-static data member 'a'}}
+  int mem2(int i = b);
+  static int b;
+};
+
+int f(int = 0);
+void h() {
+  int j = f(1);
+  int k = f();
+}
+int (*p1)(int) = &f;
+int (*p2)() = &f; // expected-error {{cannot initialize a variable of type 'int (*)()' with an rvalue of type 'int (*)(int)': different number of parameters (0 vs 1)}}

clang/test/CXX/dcl.decl/dcl.meaning/dcl.fct.default/p7.cpp

 // RUN: %clang_cc1 -fsyntax-only -verify %s
 
-void h()
+void f()
 {
   int i;
-  extern void h2(int x = sizeof(i)); // expected-error {{default argument references local variable 'i' of enclosing function}}
+  extern void g(int x = i); // expected-error {{default argument references local variable 'i' of enclosing function}}
+  extern void h(int x = sizeof(i));
 }

clang/www/cxx_dr_status.html

 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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 <html>
 <head>
   <META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
   <title>Clang - C++ Defect Report Status</title>
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   <link type="text/css" rel="stylesheet" href="content.css">
   <style type="text/css">
     .none { background-color: #FFCCCC }
<tr id="2081">
     <td>DRWP</td>
     <td>Deduced return type in redeclaration or specialization of function template</td>
     <td class="none" align="center">Unknown</td>
   </tr>
   <tr id="2082">
     <td><a href="http://wg21.link/cwg2082">2082</a></td>
     <td>CD4</td>
     <td>Referring to parameters in unevaluated operands of default arguments</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="svn" align="center">SVN</td>
   </tr>
   <tr id="2083">
     <td><a href="http://wg21.link/cwg2083">2083</a></td>
     <td>DR</td>
     <td>Incorrect cases of odr-use</td>
     <td class="partial" align="center">Partial</td>
   </tr>
   <tr id="2084">