[clang] Convert a default argument expression to the parameter type...

...before checking that the default argument is valid with
CheckDefaultArgumentVisitor.

Currently the restrictions on a default argument are checked with the visitor
CheckDefaultArgumentVisitor in ActOnParamDefaultArgument before
performing the conversion to the parameter type in SetParamDefaultArgument.

This was fine before the previous patch but now some valid code post-CWG 2346
is rejected:

void test() {
  const int i2 = 0;
  extern void h2a(int x = i2);     // FIXME: ok, not odr-use
  extern void h2b(int x = i2 + 0); // ok, not odr-use
}

This is because the reference to i2 in h2a has not been marked yet with
NOUR_Constant. i2 is marked NOUR_Constant when the conversion to the parameter
type is done, which is done just after.

The solution is to do the conversion to the parameter type before checking
the restrictions on default arguments with CheckDefaultArgumentVisitor.
This has the side-benefit of improving some diagnostics.

Differential Revision: https://reviews.llvm.org/D81616

Reviewed By: rsmith

Author: riccibruno

clang/include/clang/Sema/Sema.h

@@ -2381,11 +2381,13 @@ class Sema final {
   void ActOnParamDefaultArgument(Decl *param,
                                  SourceLocation EqualLoc,
                                  Expr *defarg);
-  void ActOnParamUnparsedDefaultArgument(Decl *param,
-                                         SourceLocation EqualLoc,
+  void ActOnParamUnparsedDefaultArgument(Decl *param, SourceLocation EqualLoc,
                                          SourceLocation ArgLoc);
   void ActOnParamDefaultArgumentError(Decl *param, SourceLocation EqualLoc);
-  bool SetParamDefaultArgument(ParmVarDecl *Param, Expr *DefaultArg,
+  ExprResult ConvertParamDefaultArgument(const ParmVarDecl *Param,
+                                         Expr *DefaultArg,
+                                         SourceLocation EqualLoc);
+  void SetParamDefaultArgument(ParmVarDecl *Param, Expr *DefaultArg,
                                SourceLocation EqualLoc);
 
   // Contexts where using non-trivial C union types can be disallowed. This is

clang/lib/Sema/SemaDeclCXX.cpp

@@ -254,14 +254,12 @@ void Sema::ImplicitExceptionSpecification::CalledStmt(Stmt *S) {
     ComputedEST = EST_None;
 }
 
-bool
-Sema::SetParamDefaultArgument(ParmVarDecl *Param, Expr *Arg,
-                              SourceLocation EqualLoc) {
+ExprResult Sema::ConvertParamDefaultArgument(const ParmVarDecl *Param,
+                                             Expr *Arg,
+                                             SourceLocation EqualLoc) {
   if (RequireCompleteType(Param->getLocation(), Param->getType(),
-                          diag::err_typecheck_decl_incomplete_type)) {
-    Param->setInvalidDecl();
+                          diag::err_typecheck_decl_incomplete_type))
     return true;
-  }
 
   // C++ [dcl.fct.default]p5
   //   A default argument expression is implicitly converted (clause
@@ -282,7 +280,12 @@ Sema::SetParamDefaultArgument(ParmVarDecl *Param, Expr *Arg,
   CheckCompletedExpr(Arg, EqualLoc);
   Arg = MaybeCreateExprWithCleanups(Arg);
 
-  // Okay: add the default argument to the parameter
+  return Arg;
+}
+
+void Sema::SetParamDefaultArgument(ParmVarDecl *Param, Expr *Arg,
+                                   SourceLocation EqualLoc) {
+  // Add the default argument to the parameter
   Param->setDefaultArg(Arg);
 
   // We have already instantiated this parameter; provide each of the
@@ -296,8 +299,6 @@ Sema::SetParamDefaultArgument(ParmVarDecl *Param, Expr *Arg,
     // We're done tracking this parameter's instantiations.
     UnparsedDefaultArgInstantiations.erase(InstPos);
   }
-
-  return false;
 }
 
 /// ActOnParamDefaultArgument - Check whether the default argument
@@ -341,13 +342,18 @@ Sema::ActOnParamDefaultArgument(Decl *param, SourceLocation EqualLoc,
     return;
   }
 
+  ExprResult Result = ConvertParamDefaultArgument(Param, DefaultArg, EqualLoc);
+  if (Result.isInvalid())
+    return Fail();
+
+  DefaultArg = Result.getAs<Expr>();
+
   // Check that the default argument is well-formed
   CheckDefaultArgumentVisitor DefaultArgChecker(*this, DefaultArg);
   if (DefaultArgChecker.Visit(DefaultArg))
     return Fail();
 
-  if (SetParamDefaultArgument(Param, DefaultArg, EqualLoc))
-    return Fail();
+  SetParamDefaultArgument(Param, DefaultArg, EqualLoc);
 }
 
 /// ActOnParamUnparsedDefaultArgument - We've seen a default

clang/lib/Sema/SemaTemplateInstantiate.cpp

@@ -2405,7 +2405,12 @@ ParmVarDecl *Sema::SubstParmVarDecl(ParmVarDecl *OldParm,
       if (NewArg.isUsable()) {
         // It would be nice if we still had this.
         SourceLocation EqualLoc = NewArg.get()->getBeginLoc();
-        SetParamDefaultArgument(NewParm, NewArg.get(), EqualLoc);
+        ExprResult Result =
+            ConvertParamDefaultArgument(NewParm, NewArg.get(), EqualLoc);
+        if (Result.isInvalid())
+          return nullptr;
+
+        SetParamDefaultArgument(NewParm, Result.getAs<Expr>(), EqualLoc);
       }
     } else {
       // FIXME: if we non-lazily instantiated non-dependent default args for

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

@@ -6,8 +6,7 @@ void h() {
   // expected-error@-1 {{default argument references local variable 'i1' of enclosing function}}
 
   const int i2 = 0;
-  extern void h2a(int x = i2); // FIXME: ok, not odr-use
-  // expected-error@-1 {{default argument references local variable 'i2' of enclosing function}}
+  extern void h2a(int x = i2);     // ok, not odr-use
   extern void h2b(int x = i2 + 0); // ok, not odr-use
 
   const int i3 = 0;

clang/test/CXX/dcl.decl/dcl.meaning/dcl.fct.default/p8.cpp

@@ -6,5 +6,10 @@ class A {
 };
 
 void A::test() {
-  void g(int = this); // expected-error {{default argument references 'this'}}
+  void g(int = this);
+  // expected-error@-1 {{cannot initialize a parameter of type 'int' with an rvalue of type 'A *'}}
+  // expected-note@-2 {{passing argument to parameter here}}
+
+  void h(int = ((void)this,42));
+  // expected-error@-1 {{default argument references 'this'}}
 }

clang/test/SemaCXX/vartemplate-lambda.cpp

@@ -6,10 +6,7 @@ template <typename> void foo0() { fn0<char>(); }
 template<typename T> auto fn1 = [](auto a) { return a + T(1); };
 template<typename T> auto v1 = [](int a = T()) { return a; }();
 // expected-error@-1{{cannot initialize a parameter of type 'int' with an rvalue of type 'int *'}}
-// expected-error@-2{{no matching function for call}}
-// expected-note@-3{{passing argument to parameter 'a' here}}
-// expected-note@-4{{candidate function not viable}}
-// expected-note@-5{{conversion candidate of type 'int (*)(int)'}}
+// expected-note@-2{{passing argument to parameter 'a' here}}
 
 struct S {
   template<class T>

clang/test/SemaTemplate/instantiate-local-class.cpp

@@ -462,18 +462,15 @@ namespace rdar23721638 {
     struct Inner { // expected-note {{in instantiation}}
       void operator()(T a = "") {} // expected-error {{conversion function from 'const char [1]' to 'rdar23721638::A' invokes a deleted function}}
       // expected-note@-1 {{passing argument to parameter 'a' here}}
-      // expected-note@-2 {{candidate function not viable}}
     };
-    Inner()(); // expected-error {{no matching function}}
+    Inner()(); // expected-error {{type 'Inner' does not provide a call operator}}
   }
   template void foo<A>(); // expected-note 2 {{in instantiation}}
 
   template <typename T> void bar() {
     auto lambda = [](T a = "") {}; // expected-error {{conversion function from 'const char [1]' to 'rdar23721638::A' invokes a deleted function}}
       // expected-note@-1 {{passing argument to parameter 'a' here}}
-      // expected-note@-2 {{candidate function not viable}}
-      // expected-note@-3 {{conversion candidate of type}}
-    lambda(); // expected-error {{no matching function}}
+    lambda();
   }
   template void bar<A>(); // expected-note {{in instantiation}}
 }
@@ -494,9 +491,6 @@ namespace PR45000 {
   void f(int x = [](T x = nullptr) -> int { return x; }());
   // expected-error@-1 {{cannot initialize a parameter of type 'int' with an rvalue of type 'nullptr_t'}}
   // expected-note@-2 {{passing argument to parameter 'x' here}}
-  // expected-error@-3 {{no matching function for call}}
-  // expected-note@-4 {{candidate function not viable: requires single argument 'x', but no arguments were provided}}
-  // expected-note@-5 {{conversion candidate of type 'auto (*)(int) -> int'}}
 
   void g() { f<int>(); }
   // expected-note@-1 {{in instantiation of default function argument expression for 'f<int>' required here}}