c++: Improve diagnostics for template args terminated with >= or >>= [PR104319]

As mentioned in the PR, for C++98 we have diagnostics that expect
>> terminating template arguments to be a mistake for > > (C++11
said it has to be treated that way), while if user trying to spare the
spacebar doesn't separate > from following = or >> from following =,
the diagnostics is confusing, while clang suggests adding space in between.

The following patch does that for >= and >>= too.

For some strange reason the error recovery emits further errors,
not really sure what's going on because I overwrite the token->type
like the code does for the C++11 >> case or for the C++98 >> cases,
but at least the first error is nicer (well, for the C++98 nested
template case and >>= I need to overwrite it to > and so the = is lost,
so perhaps some follow-up errors are needed for that case).

2022-04-29  Jakub Jelinek  <jakub@redhat.com>

	PR c++/104319
	* parser.cc (cp_parser_template_argument): Treat >= like C++98 >>
	after a type id by setting maybe_type_id and aborting tentative
	parse.
	(cp_parser_enclosed_template_argument_list): Handle
	CPP_GREATER_EQ like misspelled CPP_GREATER CPP_RQ and
	CPP_RSHIFT_EQ like misspelled CPP_GREATER CPP_GREATER_EQ
	or CPP_RSHIFT CPP_EQ or CPP_GREATER CPP_GREATER CPP_EQ.
	(cp_parser_next_token_ends_template_argument_p): Return true
	also for CPP_GREATER_EQ and CPP_RSHIFT_EQ.

	* g++.dg/parse/template28.C: Adjust expected diagnostics.
	* g++.dg/parse/template30.C: New test.

3 files changed, 98 insertions, 6 deletions

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 2235da1..ee69934 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -18818,8 +18818,13 @@ cp_parser_template_argument (cp_parser* parser)
      In C++0x, the '>>' will be considered two separate '>'
      tokens.  */
   if (!cp_parser_error_occurred (parser)
-      && cxx_dialect == cxx98
-      && cp_lexer_next_token_is (parser->lexer, CPP_RSHIFT))
+      && ((cxx_dialect == cxx98
+	   && cp_lexer_next_token_is (parser->lexer, CPP_RSHIFT))
+	  /* Similarly for >= which
+	     cp_parser_next_token_ends_template_argument_p treats for
+	     diagnostics purposes as mistyped > =, but can be valid
+	     after a type-id.  */
+	  || cp_lexer_next_token_is (parser->lexer, CPP_GREATER_EQ)))
     {
       maybe_type_id = true;
       cp_parser_abort_tentative_parse (parser);
@@ -32045,7 +32050,9 @@ cp_parser_enclosed_template_argument_list (cp_parser* parser)
   cp_evaluated ev;
   /* Parse the template-argument-list itself.  */
   if (cp_lexer_next_token_is (parser->lexer, CPP_GREATER)
-      || cp_lexer_next_token_is (parser->lexer, CPP_RSHIFT))
+      || cp_lexer_next_token_is (parser->lexer, CPP_RSHIFT)
+      || cp_lexer_next_token_is (parser->lexer, CPP_GREATER_EQ)
+      || cp_lexer_next_token_is (parser->lexer, CPP_RSHIFT_EQ))
     arguments = NULL_TREE;
   else
     arguments = cp_parser_template_argument_list (parser);
@@ -32102,6 +32109,38 @@ cp_parser_enclosed_template_argument_list (cp_parser* parser)
 		    "a template argument list");
 	}
     }
+  /* Similarly for >>= and >=.  */
+  else if (cp_lexer_next_token_is (parser->lexer, CPP_GREATER_EQ)
+	   || cp_lexer_next_token_is (parser->lexer, CPP_RSHIFT_EQ))
+    {
+      cp_token *token = cp_lexer_consume_token (parser->lexer);
+      gcc_rich_location richloc (token->location);
+      enum cpp_ttype new_type;
+      const char *replacement;
+      if (token->type == CPP_GREATER_EQ)
+	{
+	  replacement = "> =";
+	  new_type = CPP_EQ;
+	}
+      else if (!saved_greater_than_is_operator_p)
+	{
+	  if (cxx_dialect != cxx98)
+	    replacement = ">> =";
+	  else
+	    replacement = "> > =";
+	  new_type = CPP_GREATER;
+	}
+      else
+	{
+	  replacement = "> >=";
+	  new_type = CPP_GREATER_EQ;
+	}
+      richloc.add_fixit_replace (replacement);
+      error_at (&richloc, "%qs should be %qs to terminate a template "
+		"argument list",
+		cpp_type2name (token->type, token->flags), replacement);
+      token->type = new_type;
+    }
   else
     cp_parser_require_end_of_template_parameter_list (parser);
   /* The `>' token might be a greater-than operator again now.  */
@@ -33180,7 +33219,11 @@ cp_parser_next_token_ends_template_argument_p (cp_parser *parser)
   return (token->type == CPP_COMMA
           || token->type == CPP_GREATER
           || token->type == CPP_ELLIPSIS
-	  || ((cxx_dialect != cxx98) && token->type == CPP_RSHIFT));
+	  || ((cxx_dialect != cxx98) && token->type == CPP_RSHIFT)
+	  /* For better diagnostics, treat >>= like that too, that
+	     shouldn't appear non-nested in template arguments.  */
+	  || token->type == CPP_GREATER_EQ
+	  || token->type == CPP_RSHIFT_EQ);
 }
 
 /* Returns TRUE iff the n-th token is a "<", or the n-th is a "[" and the
diff --git a/gcc/testsuite/g++.dg/parse/template28.C b/gcc/testsuite/g++.dg/parse/template28.C
index bfd8af1..fc65a54 100644
--- a/gcc/testsuite/g++.dg/parse/template28.C
+++ b/gcc/testsuite/g++.dg/parse/template28.C
@@ -2,8 +2,8 @@
 
 template<class> struct A {};
 
-template<class T> void foo(A<T>=A<T>()) {} // { dg-error "24:variable or field .foo. declared void" }
-// { dg-error "template" "" { target *-*-* } .-1 }
+template<class T> void foo(A<T>=A<T>()) {} // { dg-error "'>=' should be '> =' to terminate a template argument list" }
+// { dg-error "expected" "" { target *-*-* } .-1 }
 
 void bar()
 {
diff --git a/gcc/testsuite/g++.dg/parse/template30.C b/gcc/testsuite/g++.dg/parse/template30.C
new file mode 100644
index 0000000..fa89889
--- /dev/null
+++ b/gcc/testsuite/g++.dg/parse/template30.C
@@ -0,0 +1,49 @@
+// PR c++/104319
+// { dg-do compile }
+// { dg-options "" }
+
+template<typename T> struct A {};
+template<typename T> int z;	// { dg-warning "variable templates only available with" "" { target c++11_down } }
+template<int N> int w;		// { dg-warning "variable templates only available with" "" { target c++11_down } }
+
+void
+foo ()
+{
+  z<int>=0;			// { dg-error "'>=' should be '> =' to terminate a template argument list" }
+}				// { dg-error "expected ';' before numeric constant" "" { target *-*-* } .-1 }
+
+int
+bar ()
+{
+  return z<int>>0;		// { dg-error "spurious '>>', use '>' to terminate a template argument list" "" { target c++98_only } }
+}				// { dg-error "expected ';' before numeric constant" "" { target c++98_only } .-1 }
+
+int
+baz ()
+{
+  return z<int>>=0;		// { dg-error "'>>=' should be '> >=' to terminate a template argument list" }
+}				// { dg-error "expected ';' before numeric constant" "" { target *-*-* } .-1 }
+
+int
+qux ()
+{
+  return z<A<int>>=0;		// { dg-error "'>>=' should be '>> =' to terminate a template argument list" "" { target c++11 } }
+}				// { dg-error "'>>=' should be '> > =' to terminate a template argument list" "" { target c++98_only } .-1 }
+				// { dg-error "parse error in template argument list" "" { target *-*-* } .-2 }
+				// { dg-error "template argument 1 is invalid" "" { target *-*-* } .-3 }
+
+void
+quux ()
+{
+  w<5>=0>=6>=8> = 5;
+}
+
+#if __cplusplus >= 201103L
+struct B { constexpr bool operator >= (int) { return true; } };
+
+void
+corge ()
+{
+  w<B()>=5> = 5;
+}
+#endif