Re: [PATCH] c++: C++20 DR 2237, disallow simple-template-id in cdtor.

[Marek Polacek <polacek@redhat.com>]

On Sun, Apr 05, 2020 at 09:46:09PM -0400, Jason Merrill wrote:
> On 4/4/20 7:30 PM, Marek Polacek wrote:
> > This patch implements DR 2237 which says that a simple-template-id is
> > no longer valid as the declarator-id of a constructor or destructor;
> > see <https://eel.is/c++draft/diff.cpp17.class#2>.  It is not explicitly
> > stated but out-of-line destructors with a simple-template-id are also
> > meant to be ill-formed now.  (Out-of-line constructors like that are
> > invalid since DR1435 I think.)  This change only applies to C++20; it
> > is not a DR against C++17.
> > 
> > I'm not crazy about the diagnostic in constructors but ISTM that
> > cp_parser_constructor_declarator_p shouldn't print errors.
> > 
> > Does it seem reasonable to apply this now or should I defer to GCC 11?
> 
> A new error should wait for GCC 11.

Coming back to this now that we're again in stage 1.

> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > 
> > 2020-04-04  Marek Polacek  <polacek@redhat.com>
> > 
> > 	DR 2237
> > 	* parser.c (cp_parser_unqualified_id): Reject simple-template-id as
> > 	the declarator-id of a destructor.
> > 	(cp_parser_constructor_declarator_p): Reject simple-template-id as
> > 	the declarator-id of a constructor.
> > 
> > 	* g++.dg/DRs/dr2237.C: New test.
> > 	* g++.dg/parse/constructor2.C: Add dg-error for C++20.
> > 	* g++.dg/parse/dtor12.C: Likewise.
> > 	* g++.dg/parse/dtor4.C: Likewise.
> > 	* g++.dg/template/dtor4.C: Adjust dg-error.
> > 	* g++.dg/template/error34.C: Likewise.
> > 	* g++.old-deja/g++.other/inline15.C: Only run for C++17 and lesses.
> > 	* g++.old-deja/g++.pt/ctor2.C: Add dg-error for C++20.
> > ---
> >   gcc/cp/parser.c                                | 16 ++++++++++++++++
> >   gcc/testsuite/g++.dg/DRs/dr2237.C              | 18 ++++++++++++++++++
> >   gcc/testsuite/g++.dg/parse/constructor2.C      |  4 ++--
> >   gcc/testsuite/g++.dg/parse/dtor12.C            |  2 +-
> >   gcc/testsuite/g++.dg/parse/dtor4.C             |  2 +-
> >   gcc/testsuite/g++.dg/template/dtor4.C          |  2 +-
> >   gcc/testsuite/g++.dg/template/error34.C        | 10 +++++-----
> >   .../g++.old-deja/g++.other/inline15.C          |  2 +-
> >   gcc/testsuite/g++.old-deja/g++.pt/ctor2.C      |  2 +-
> >   9 files changed, 46 insertions(+), 12 deletions(-)
> >   create mode 100644 gcc/testsuite/g++.dg/DRs/dr2237.C
> > 
> > diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
> > index 7e5921e039f..810edfa87a9 100644
> > --- a/gcc/cp/parser.c
> > +++ b/gcc/cp/parser.c
> > @@ -6114,6 +6114,16 @@ cp_parser_unqualified_id (cp_parser* parser,
> >   	    return build_min_nt_loc (loc, BIT_NOT_EXPR, make_auto ());
> >   	  }
> > +	/* DR 2237 (C++20 only): A simple-template-id is no longer valid as the
> > +	   declarator-id of a constructor or destructor.  */
> > +	if (token->type == CPP_TEMPLATE_ID && cxx_dialect >= cxx2a)
> > +	  {
> > +	    if (!cp_parser_uncommitted_to_tentative_parse_p (parser))
> > +	      error_at (tilde_loc, "template-id not allowed for destructor");
> > +	    cp_parser_simulate_error (parser);
> 
> The usual pattern is
> 
> if (!cp_parser_simulate_error (parser))
>   error...

Fixed.

> > +	    return error_mark_node;
> > +	  }
> > +
> >   	/* If there was an explicit qualification (S::~T), first look
> >   	   in the scope given by the qualification (i.e., S).
> > @@ -28675,6 +28685,12 @@ cp_parser_constructor_declarator_p (cp_parser *parser, cp_parser_flags flags,
> >         if (!constructor_name_p (id, nested_name_specifier))
> >   	constructor_p = false;
> >       }
> > +  /* DR 2237 (C++20 only): A simple-template-id is no longer valid as the
> > +     declarator-id of a constructor or destructor.  */
> > +  else if (constructor_p
> > +	   && cxx_dialect >= cxx2a
> > +	   && cp_lexer_next_token_is (parser->lexer, CPP_TEMPLATE_ID))
> > +    constructor_p = false;
> >     /* If we still think that this might be a constructor-declarator,
> >        look for a class-name.  */
> >     else if (constructor_p)
> 
> Do you also want to exclude CPP_TEMPLATE_ID from the test at the top of the
> function for C++20?

In fact we can get away with only excluding CPP_TEMPLATE_ID in C++20 there,
because for e.g. X<T>::X<T>(); we'll give the "names the constructor, not the
type" error, so we won't even get here.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

-- >8 --
This patch implements DR 2237 which says that a simple-template-id is
no longer valid as the declarator-id of a constructor or destructor;
see [diff.cpp17.class]#2.  It is not explicitly stated but out-of-line
destructors with a simple-template-id are also meant to be ill-formed
now.  (Out-of-line constructors like that are invalid since DR1435 I
think.)  This change only applies to C++20; it is not a DR against C++17.

I'm not crazy about the diagnostic in constructors but ISTM that
cp_parser_constructor_declarator_p shouldn't print errors.

	DR 2237
	* parser.c (cp_parser_unqualified_id): Reject simple-template-id as
	the declarator-id of a destructor.
	(cp_parser_constructor_declarator_p): Reject simple-template-id as
	the declarator-id of a constructor.

	* g++.dg/DRs/dr2237.C: New test.
	* g++.dg/parse/constructor2.C: Add dg-error for C++20.
	* g++.dg/parse/dtor12.C: Likewise.
	* g++.dg/parse/dtor4.C: Likewise.
	* g++.dg/template/dtor4.C: Adjust dg-error.
	* g++.dg/template/error34.C: Likewise.
	* g++.old-deja/g++.other/inline15.C: Only run for C++17 and lesses.
	* g++.old-deja/g++.pt/ctor2.C: Add dg-error for C++20.
---
 gcc/cp/parser.c                                | 13 ++++++++++++-
 gcc/testsuite/g++.dg/DRs/dr2237.C              | 18 ++++++++++++++++++
 gcc/testsuite/g++.dg/parse/constructor2.C      |  4 ++--
 gcc/testsuite/g++.dg/parse/dtor12.C            |  2 +-
 gcc/testsuite/g++.dg/parse/dtor4.C             |  2 +-
 gcc/testsuite/g++.dg/template/dtor4.C          |  2 +-
 gcc/testsuite/g++.dg/template/error34.C        | 10 +++++-----
 .../g++.old-deja/g++.other/inline15.C          |  2 +-
 gcc/testsuite/g++.old-deja/g++.pt/ctor2.C      |  2 +-
 9 files changed, 42 insertions(+), 13 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/DRs/dr2237.C

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 591f44f4934..593c45b5888 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -6114,6 +6114,15 @@ cp_parser_unqualified_id (cp_parser* parser,
 	    return build_min_nt_loc (loc, BIT_NOT_EXPR, make_auto ());
 	  }
 
+	/* DR 2237 (C++20 only): A simple-template-id is no longer valid as the
+	   declarator-id of a constructor or destructor.  */
+	if (token->type == CPP_TEMPLATE_ID && cxx_dialect >= cxx2a)
+	  {
+	    if (!cp_parser_simulate_error (parser))
+	      error_at (tilde_loc, "template-id not allowed for destructor");
+	    return error_mark_node;
+	  }
+
 	/* If there was an explicit qualification (S::~T), first look
 	   in the scope given by the qualification (i.e., S).
 
@@ -28636,7 +28645,9 @@ cp_parser_constructor_declarator_p (cp_parser *parser, cp_parser_flags flags,
   if (next_token->type != CPP_NAME
       && next_token->type != CPP_SCOPE
       && next_token->type != CPP_NESTED_NAME_SPECIFIER
-      && next_token->type != CPP_TEMPLATE_ID)
+      /* DR 2237 (C++20 only): A simple-template-id is no longer valid as the
+	 declarator-id of a constructor or destructor.  */
+      && (next_token->type != CPP_TEMPLATE_ID || cxx_dialect >= cxx2a))
     return false;
 
   /* Parse tentatively; we are going to roll back all of the tokens
diff --git a/gcc/testsuite/g++.dg/DRs/dr2237.C b/gcc/testsuite/g++.dg/DRs/dr2237.C
new file mode 100644
index 00000000000..17abdf179c1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/DRs/dr2237.C
@@ -0,0 +1,18 @@
+// DR 2237 - Can a template-id name a constructor?
+
+template<class T>
+struct X {
+  X<T>(); // { dg-error "expected" "" { target c++2a } }
+  X(int); // OK, injected-class-name used
+  ~X<T>(); // { dg-error "template-id not allowed for destructor" "" { target c++2a } }
+};
+
+// ill-formed since DR1435
+template<typename T> X<T>::X<T>() {} // { dg-error "names the constructor|as no template constructors" }
+template<typename T> X<T>::~X<T>() {} // { dg-error "template-id not allowed for destructor" "" { target c++2a } }
+
+struct Q {
+  // ill-formed since DR1435
+  template<typename T> friend X<T>::X<T>(); // { dg-error "names the constructor|as no template constructors" }
+  template<typename T> friend X<T>::~X<T>(); // { dg-error "template-id not allowed for destructor" "" { target c++2a } }
+};
diff --git a/gcc/testsuite/g++.dg/parse/constructor2.C b/gcc/testsuite/g++.dg/parse/constructor2.C
index e514e9397e9..87007e9c715 100644
--- a/gcc/testsuite/g++.dg/parse/constructor2.C
+++ b/gcc/testsuite/g++.dg/parse/constructor2.C
@@ -5,7 +5,7 @@ class T
 { 
 public: 
   T(short,short f=0) {} 
-  T<TClass>(int f) {} 
-  T<TClass>(int f=0,const char* b=0) {} 
+  T<TClass>(int f) {} // { dg-error "expected" "" { target c++2a } }
+  T<TClass>(int f=0,const char* b=0) {} // { dg-error "expected" "" { target c++2a } }
 }; 
 
diff --git a/gcc/testsuite/g++.dg/parse/dtor12.C b/gcc/testsuite/g++.dg/parse/dtor12.C
index 1acdfa36b69..d758d3e861d 100644
--- a/gcc/testsuite/g++.dg/parse/dtor12.C
+++ b/gcc/testsuite/g++.dg/parse/dtor12.C
@@ -2,5 +2,5 @@
 
 template <class T> class a
 {
-  ~a<T>();
+  ~a<T>(); // { dg-error "template-id not allowed for destructor" "" { target c++2a } }
 };
diff --git a/gcc/testsuite/g++.dg/parse/dtor4.C b/gcc/testsuite/g++.dg/parse/dtor4.C
index 729ee2fa130..1059c189eff 100644
--- a/gcc/testsuite/g++.dg/parse/dtor4.C
+++ b/gcc/testsuite/g++.dg/parse/dtor4.C
@@ -7,4 +7,4 @@ template <int N> struct X {
 };
 
 template <int N>
-X<N>::~X<N>(){}
+X<N>::~X<N>(){} // { dg-error "template-id not allowed for destructor" "" { target c++2a } }
diff --git a/gcc/testsuite/g++.dg/template/dtor4.C b/gcc/testsuite/g++.dg/template/dtor4.C
index 4f277b248bd..917681fc13e 100644
--- a/gcc/testsuite/g++.dg/template/dtor4.C
+++ b/gcc/testsuite/g++.dg/template/dtor4.C
@@ -5,5 +5,5 @@
 
 template<int> struct A
 {
-  ~A<0>(); // { dg-error "parse error|declaration" }
+  ~A<0>(); // { dg-error "parse error|declaration|template-id" }
 };
diff --git a/gcc/testsuite/g++.dg/template/error34.C b/gcc/testsuite/g++.dg/template/error34.C
index d0cbcaef28e..ab688d9ba8c 100644
--- a/gcc/testsuite/g++.dg/template/error34.C
+++ b/gcc/testsuite/g++.dg/template/error34.C
@@ -3,27 +3,27 @@
 
 template<typename T> struct A
 {
-  A<__builtin_offsetof(T, x)>();	// { dg-error "type/value mismatch|offsetof\\(T, x\\)" }
+  A<__builtin_offsetof(T, x)>();	// { dg-error "type/value mismatch|offsetof\\(T, x\\)|expected" }
 };
 
 template<typename T> struct B
 {
-  B<__builtin_offsetof(T, x.y)>();	// { dg-error "type/value mismatch|offsetof\\(T, x.y\\)" }
+  B<__builtin_offsetof(T, x.y)>();	// { dg-error "type/value mismatch|offsetof\\(T, x.y\\)|expected" }
 };
 
 template<typename T> struct C
 {
-  C<__builtin_offsetof(T, x[6])>();	// { dg-error "type/value mismatch|offsetof\\(T, x\\\[6\\\]\\)" }
+  C<__builtin_offsetof(T, x[6])>();	// { dg-error "type/value mismatch|offsetof\\(T, x\\\[6\\\]\\)|expected" }
 };
 
 template<typename T> struct D
 {
-  D<__builtin_offsetof(T, x.y[6].z)>();	// { dg-error "type/value mismatch|offsetof\\(T, x.y\\\[6\\\].z\\)" }
+  D<__builtin_offsetof(T, x.y[6].z)>();	// { dg-error "type/value mismatch|offsetof\\(T, x.y\\\[6\\\].z\\)|expected" }
 };
 
 struct E { int x; };
 
 template<typename T> struct F
 {
-  F<__builtin_offsetof(E, x)>();	// { dg-error "type/value mismatch|offsetof\\(E, x\\)" }
+  F<__builtin_offsetof(E, x)>();	// { dg-error "type/value mismatch|offsetof\\(E, x\\)|expected" }
 };
diff --git a/gcc/testsuite/g++.old-deja/g++.other/inline15.C b/gcc/testsuite/g++.old-deja/g++.other/inline15.C
index 43f7f5e3025..a31c60960a5 100644
--- a/gcc/testsuite/g++.old-deja/g++.other/inline15.C
+++ b/gcc/testsuite/g++.old-deja/g++.other/inline15.C
@@ -1,4 +1,4 @@
-// { dg-do assemble  }
+// { dg-do assemble { target c++17_down } }
 // { dg-options "-O1" }
 // Origin: Jakub Jelinek <jakub@redhat.com>
 
diff --git a/gcc/testsuite/g++.old-deja/g++.pt/ctor2.C b/gcc/testsuite/g++.old-deja/g++.pt/ctor2.C
index a8be91ddbb9..4159c84ff56 100644
--- a/gcc/testsuite/g++.old-deja/g++.pt/ctor2.C
+++ b/gcc/testsuite/g++.old-deja/g++.pt/ctor2.C
@@ -4,7 +4,7 @@
 
 template <class T>
 struct A {
-  A<T>();
+  A<T>(); // { dg-error "expected" "" { target c++2a } }
 };
 
 template <class T>

base-commit: 2b2d298ff845ab7a07ffbd51da79473736da3324
-- 
Marek Polacek • Red Hat, Inc. • 300 A St, Boston, MA