c++: DR2237, cdtor and template-id tweaks [PR107126]

Since my r11-532 changes to implement DR2237, for this test:

  template<typename T>
  struct S {
    S<T>();
  };

in C++20 we emit the ugly:

q.C:3:8: error: expected unqualified-id before ')' token
    3 |   S<T>();

which doesn't explain what the problem is.  This patch improves that
diagnostic, reduces the error to a pedwarn, and adds a -Wc++20-compat
diagnostic.  We now say:

q.C:3:7: warning: template-id not allowed for constructor in C++20 [-Wtemplate-id-cdtor]
    3 |   S<T>();
q.C:3:7: note: remove the '< >'

This patch also fixes
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97202#c8>
where the C++20 diagnostic was missing altogether:  The problem was that I checked
for CPP_TEMPLATE_ID too early, at a point at which cp_parser_template_id may not
have been called yet.  So let's check for it at the end of the function, after
the tentative parse and rollback.

-Wc++20-compat triggered in libitm/; I sent a patch for that.

	DR 2237
	PR c++/107126
	PR c++/97202

gcc/c-family/ChangeLog:

	* c-opts.cc (c_common_post_options): In C++20 or with -Wc++20-compat,
	turn on -Wtemplate-id-cdtor.
	* c.opt (Wtemplate-id-cdtor): New.

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_unqualified_id): Downgrade the DR2237 error to
	a pedwarn.
	(cp_parser_constructor_declarator_p): Likewise.

gcc/ChangeLog:

	* doc/invoke.texi: Document -Wtemplate-id-cdtor.

gcc/testsuite/ChangeLog:

	* g++.dg/DRs/dr2237.C: Adjust dg-error.
	* g++.dg/parse/constructor2.C: Likewise.
	* g++.dg/template/error34.C: Likewise.
	* g++.old-deja/g++.pt/ctor2.C: Likewise.
	* g++.dg/DRs/dr2237-2.C: New test.
	* g++.dg/DRs/dr2237-3.C: New test.
	* g++.dg/DRs/dr2237-4.C: New test.
	* g++.dg/DRs/dr2237-5.C: New test.
	* g++.dg/warn/Wtemplate-id-cdtor-1.C: New test.
	* g++.dg/warn/Wtemplate-id-cdtor-2.C: New test.
	* g++.dg/warn/Wtemplate-id-cdtor-3.C: New test.
	* g++.dg/warn/Wtemplate-id-cdtor-4.C: New test.

gcc/c-family/c-opts.cc

@@ -998,6 +998,11 @@ c_common_post_options (const char **pfilename)
 		       warn_deprecated_enum_float_conv,
 		       cxx_dialect >= cxx20 && warn_deprecated);
 
+  /* -Wtemplate-id-cdtor is enabled by default in C++20.  */
+  SET_OPTION_IF_UNSET (&global_options, &global_options_set,
+		       warn_template_id_cdtor,
+		       cxx_dialect >= cxx20 || warn_cxx20_compat);
+
   /* Declone C++ 'structors if -Os.  */
   if (flag_declone_ctor_dtor == -1)
     flag_declone_ctor_dtor = optimize_size;

gcc/c-family/c.opt

@@ -1408,6 +1408,10 @@ Wtautological-compare
 C ObjC C++ ObjC++ Var(warn_tautological_compare) Warning LangEnabledBy(C ObjC C++ ObjC++,Wall)
 Warn if a comparison always evaluates to true or false.
 
+Wtemplate-id-cdtor
+C++ ObjC++ Var(warn_template_id_cdtor) Warning
+Warn about simple-template-id in a constructor or destructor.
+
 Wterminate
 C++ ObjC++ Warning Var(warn_terminate) Init(1)
 Warn if a throw expression will always result in a call to terminate().

gcc/cp/parser.cc

@@ -6720,12 +6720,19 @@ cp_parser_unqualified_id (cp_parser* parser,
 
 	/* 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 && declarator_p
-	    && cxx_dialect >= cxx20)
+	if (token->type == CPP_TEMPLATE_ID && declarator_p)
 	  {
-	    if (!cp_parser_simulate_error (parser))
-	      error_at (tilde_loc, "template-id not allowed for destructor");
-	    return error_mark_node;
+	    auto_diagnostic_group d;
+	    bool w = false;
+	    if (cxx_dialect >= cxx20 && !cp_parser_simulate_error (parser))
+	      w = pedwarn (tilde_loc, OPT_Wtemplate_id_cdtor,
+			   "template-id not allowed for destructor in C++20");
+	    else if (cxx_dialect < cxx20
+		     && !cp_parser_uncommitted_to_tentative_parse_p (parser))
+	      w = warning_at (tilde_loc, OPT_Wtemplate_id_cdtor,
+			      "template-id not allowed for destructor in C++20");
+	    if (w)
+	      inform (tilde_loc, "remove the %qs", "< >");
 	  }
 
 	/* If there was an explicit qualification (S::~T), first look
@@ -32332,9 +32339,7 @@ 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
-      /* 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))
+      && next_token->type != CPP_TEMPLATE_ID)
     return false;
 
   /* Parse tentatively; we are going to roll back all of the tokens
@@ -32553,6 +32558,18 @@ cp_parser_constructor_declarator_p (cp_parser *parser, cp_parser_flags flags,
   /* We did not really want to consume any tokens.  */
   cp_parser_abort_tentative_parse (parser);
 
+  /* DR 2237 (C++20 only): A simple-template-id is no longer valid as the
+     declarator-id of a constructor or destructor.  */
+  if (constructor_p
+      && cp_lexer_peek_token (parser->lexer)->type == CPP_TEMPLATE_ID)
+    {
+      auto_diagnostic_group d;
+      if (emit_diagnostic (cxx_dialect >= cxx20 ? DK_PEDWARN : DK_WARNING,
+			   input_location, OPT_Wtemplate_id_cdtor,
+			   "template-id not allowed for constructor in C++20"))
+	inform (input_location, "remove the %qs", "< >");
+    }
+
   return constructor_p;
 }
 

gcc/doc/invoke.texi

@@ -270,7 +270,7 @@ in the following sections.
 -Wno-non-template-friend  -Wold-style-cast
 -Woverloaded-virtual  -Wno-pmf-conversions -Wself-move -Wsign-promo
 -Wsized-deallocation  -Wsuggest-final-methods
--Wsuggest-final-types  -Wsuggest-override
+-Wsuggest-final-types  -Wsuggest-override  -Wno-template-id-cdtor
 -Wno-terminate  -Wno-vexing-parse  -Wvirtual-inheritance
 -Wno-virtual-move-assign  -Wvolatile  -Wzero-as-null-pointer-constant}
 
@@ -4604,6 +4604,23 @@ namespaces, and this may be used to enforce that rule.  The warning is
 inactive inside a system header file, such as the STL, so one can still
 use the STL.  One may also use using directives and qualified names.
 
+@opindex Wtemplate-id-cdtor
+@opindex Wno-template-id-cdtor
+@item -Wno-template-id-cdtor @r{(C++ and Objective-C++ only)}
+Disable the warning about the use of simple-template-id as the declarator-id
+of a constructor or destructor, which became invalid in C++20 via DR 2237.
+For example:
+
+@smallexample
+template<typename T> struct S @{
+  S<T>(); // should be S();
+  ~S<T>();  // should be ~S();
+@};
+@end smallexample
+
+@option{-Wtemplate-id-cdtor} is enabled by default with
+@option{-std=c++20}; it is also enabled by @option{-Wc++20-compat}.
+
 @opindex Wterminate
 @opindex Wno-terminate
 @item -Wno-terminate @r{(C++ and Objective-C++ only)}

gcc/testsuite/g++.dg/DRs/dr2237-2.C

@@ -0,0 +1,9 @@
+// DR 2237 - Can a template-id name a constructor?
+// { dg-options "" }
+
+template<class T>
+struct X {
+  X<T>(); // { dg-warning "template-id not allowed for constructor" "" { target c++20 } }
+  X(int); // OK, injected-class-name used
+  ~X<T>(); // { dg-warning "template-id not allowed for destructor" "" { target c++20 } }
+};

gcc/testsuite/g++.dg/DRs/dr2237-3.C

@@ -0,0 +1,16 @@
+// PR c++/107126
+// { dg-options "" }
+
+template<typename T>
+struct C
+{
+    ~C();
+};
+template<typename T>
+C<T>::~C<T>()	      // { dg-warning "template-id not allowed for destructor" "" { target c++20 } }
+{
+}
+int main()
+{
+    C<int> c;;
+}

gcc/testsuite/g++.dg/DRs/dr2237-4.C

@@ -0,0 +1,11 @@
+// PR c++/97202
+// { dg-options "" }
+
+template<typename T>
+struct F
+{
+  F<T>();  // { dg-warning "template-id not allowed for constructor" "" { target c++20 } }
+};
+
+template<typename T>
+inline F<T>::F() { }

gcc/testsuite/g++.dg/DRs/dr2237-5.C

@@ -0,0 +1,7 @@
+// PR c++/97202
+// { dg-options "" }
+
+template<typename Base> struct S : Base {
+  inline S<Base>() {} // { dg-warning "template-id not allowed for constructor" "" { target c++20 } }
+  inline ~S<Base>() {} // { dg-warning "template-id not allowed for destructor" "" { target c++20 } }
+};

gcc/testsuite/g++.dg/DRs/dr2237.C

@@ -2,7 +2,7 @@
 
 template<class T>
 struct X {
-  X<T>(); // { dg-error "expected" "" { target c++20 } }
+  X<T>(); // { dg-error "template-id not allowed for constructor" "" { target c++20 } }
   X(int); // OK, injected-class-name used
   ~X<T>(); // { dg-error "template-id not allowed for destructor" "" { target c++20 } }
 };

gcc/testsuite/g++.dg/parse/constructor2.C

@@ -1,11 +1,11 @@
 // PR c++/14260
 
-template <class TClass> 
-class T 
-{ 
-public: 
-  T(short,short f=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 } }
-}; 
+template <class TClass>
+class T
+{
+public:
+  T(short,short f=0) {}
+  T<TClass>(int f) {} // { dg-error "template-id not allowed for constructor" "" { target c++20 } }
+  T<TClass>(int f=0,const char* b=0) {} // { dg-error "template-id not allowed for constructor" "" { target c++20 } }
+};
 

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\\)|expected" }
+  A<__builtin_offsetof(T, x)>();	// { dg-error "type/value mismatch|offsetof\\(T, x\\)|template-id" }
 };
 
 template<typename T> struct B
 {
-  B<__builtin_offsetof(T, x.y)>();	// { dg-error "type/value mismatch|offsetof\\(T, x.y\\)|expected" }
+  B<__builtin_offsetof(T, x.y)>();	// { dg-error "type/value mismatch|offsetof\\(T, x.y\\)|template-id" }
 };
 
 template<typename T> struct C
 {
-  C<__builtin_offsetof(T, x[6])>();	// { dg-error "type/value mismatch|offsetof\\(T, x\\\[6\\\]\\)|expected" }
+  C<__builtin_offsetof(T, x[6])>();	// { dg-error "type/value mismatch|offsetof\\(T, x\\\[6\\\]\\)|template-id" }
 };
 
 template<typename T> struct D
 {
-  D<__builtin_offsetof(T, x.y[6].z)>();	// { dg-error "type/value mismatch|offsetof\\(T, x.y\\\[6\\\].z\\)|expected" }
+  D<__builtin_offsetof(T, x.y[6].z)>();	// { dg-error "type/value mismatch|offsetof\\(T, x.y\\\[6\\\].z\\)|template-id" }
 };
 
 struct E { int x; };
 
 template<typename T> struct F
 {
-  F<__builtin_offsetof(E, x)>();	// { dg-error "type/value mismatch|offsetof\\(E, x\\)|expected" }
+  F<__builtin_offsetof(E, x)>();	// { dg-error "type/value mismatch|offsetof\\(E, x\\)|template-id" }
 };

gcc/testsuite/g++.dg/warn/Wtemplate-id-cdtor-1.C

@@ -0,0 +1,9 @@
+// PR c++/107126
+// { dg-do compile }
+// { dg-options "-Wc++20-compat" }
+
+template<class T>
+struct X {
+  X<T>(); // { dg-warning "template-id not allowed for constructor" }
+  ~X<T>(); // { dg-warning "template-id not allowed for destructor" }
+};

gcc/testsuite/g++.dg/warn/Wtemplate-id-cdtor-2.C

@@ -0,0 +1,9 @@
+// PR c++/107126
+// { dg-do compile }
+// { dg-options "-Wtemplate-id-cdtor" }
+
+template<class T>
+struct X {
+  X<T>(); // { dg-warning "template-id not allowed for constructor" }
+  ~X<T>(); // { dg-warning "template-id not allowed for destructor" }
+};

gcc/testsuite/g++.dg/warn/Wtemplate-id-cdtor-3.C

@@ -0,0 +1,9 @@
+// PR c++/107126
+// { dg-do compile }
+// { dg-options "-Wc++20-compat -Wno-template-id-cdtor" }
+
+template<class T>
+struct X {
+  X<T>(); // { dg-bogus "template-id not allowed for constructor" }
+  ~X<T>(); // { dg-bogus "template-id not allowed for destructor" }
+};

gcc/testsuite/g++.dg/warn/Wtemplate-id-cdtor-4.C

@@ -0,0 +1,9 @@
+// PR c++/107126
+// { dg-do compile }
+// { dg-options "-Wtemplate-id-cdtor -Wno-c++20-compat" }
+
+template<class T>
+struct X {
+  X<T>(); // { dg-warning "template-id not allowed for constructor" }
+  ~X<T>(); // { dg-warning "template-id not allowed for destructor" }
+};

gcc/testsuite/g++.old-deja/g++.pt/ctor2.C

@@ -4,7 +4,7 @@
 
 template <class T>
 struct A {
-  A<T>(); // { dg-error "expected" "" { target c++20 } }
+  A<T>(); // { dg-error "template-id" "" { target c++20 } }
 };
 
 template <class T>