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

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no longer valid as the declarator-id of a constructor or destructor;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.

11 files changed, 62 insertions, 13 deletions

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 9ed4c58..cfe5cf0 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,5 +1,13 @@
 2020-05-20  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.
+
+2020-05-20  Marek Polacek  <polacek@redhat.com>
+
 	DR 2289
 	PR c++/94553
 	* cp-tree.h (SD_DECOMPOSITION): New flag.
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index a6a5d97..a8082d3 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -6108,6 +6108,15 @@ 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+	/* DR 2237 (C++20 only): A simple-template-id is no longer valid as the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 >= cxx20)
+	  {
+	    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).
 
@@ -28628,7 +28637,9 @@ 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 >= cxx20))
     return false;
 
   /* Parse tentatively; we are going to roll back all of the tokens
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 9e437af..e9d334d 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,5 +1,17 @@
 2020-05-20  Marek Polacek  <polacek@redhat.com>
 
+	DR 2237
+	* 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.
+
+2020-05-20  Marek Polacek  <polacek@redhat.com>
+
 	DR 2289
 	PR c++/94553
 	* g++.dg/cpp1z/decomp52.C: New test.
diff --git a/gcc/testsuite/g++.dg/DRs/dr2237.C b/gcc/testsuite/g++.dg/DRs/dr2237.C
new file mode 100644
index 0000000..f3d6d11
--- /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++20 } }
+  X(int); // OK, injected-class-name used
+  ~X<T>(); // { dg-error "template-id not allowed for destructor" "" { target c++20 } }
+};
+
+// 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++20 } }
+
+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++20 } }
+};
diff --git a/gcc/testsuite/g++.dg/parse/constructor2.C b/gcc/testsuite/g++.dg/parse/constructor2.C
index e514e93..d620f41 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++20 } }
+  T<TClass>(int f=0,const char* b=0) {} // { dg-error "expected" "" { target c++20 } }
 }; 
 
diff --git a/gcc/testsuite/g++.dg/parse/dtor12.C b/gcc/testsuite/g++.dg/parse/dtor12.C
index 1acdfa3..dda7388 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++20 } }
 };
diff --git a/gcc/testsuite/g++.dg/parse/dtor4.C b/gcc/testsuite/g++.dg/parse/dtor4.C
index 729ee2f..1a1a845 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++20 } }
diff --git a/gcc/testsuite/g++.dg/template/dtor4.C b/gcc/testsuite/g++.dg/template/dtor4.C
index 4f277b2..917681f 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 d0cbcae..ab688d9 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 43f7f5e..a31c609 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 a8be91d..bf418ba 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++20 } }
 };
 
 template <class T>