c++: warning for dependent template members [PR70417]

Add a helpful warning message for when the user forgets to
include the "template" keyword after ., -> or :: when
accessing a member in a dependent context, where the member is a
template.

	PR c++/70417

gcc/c-family/ChangeLog:

	* c.opt: Added -Wmissing-template-keyword.

gcc/cp/ChangeLog:

	* parser.c (cp_parser_id_expression): Handle
	-Wmissing-template-keyword.
	(struct saved_token_sentinel): Add modes to control what happens
	on destruction.
	(cp_parser_statement): Adjust.
	(cp_parser_skip_entire_template_parameter_list): New function that
	skips an entire template parameter list.
	(cp_parser_require_end_of_template_parameter_list): Rename old
	cp_parser_skip_to_end_of_template_parameter_list.
	(cp_parser_skip_to_end_of_template_parameter_list): Refactor to be
	called from one of the above two functions.
	(cp_parser_lambda_declarator_opt)
	(cp_parser_explicit_template_declaration)
	(cp_parser_enclosed_template_argument_list): Adjust.

gcc/ChangeLog:

	* doc/invoke.texi: Documentation for Wmissing-template-keyword.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/variadic-mem_fn2.C: Catch warning about missing
	template keyword.
	* g++.dg/template/dependent-name17.C: New test.
	* g++.dg/template/dependent-name18.C: New test.

Co-authored-by: Jason Merrill <jason@redhat.com>

1 files changed, 33 insertions, 0 deletions

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 6b84228..5504971 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -8963,6 +8963,39 @@ type @samp{T}.
 
 This warning can be disabled with @option{-Wno-missing-requires}.
 
+@item -Wno-missing-template-keyword
+@opindex Wmissing-template-keyword
+@opindex Wno-missing-template-keyword
+
+The member access tokens ., -> and :: must be followed by the @code{template}
+keyword if the parent object is dependent and the member being named is a
+template.
+
+@smallexample
+template <class X>
+void DoStuff (X x)
+@{
+  x.template DoSomeOtherStuff<X>(); // Good.
+  x.DoMoreStuff<X>(); // Warning, x is dependent.
+@}
+@end smallexample
+
+In rare cases it is possible to get false positives. To silence this, wrap
+the expression in parentheses. For example, the following is treated as a
+template, even where m and N are integers:
+
+@smallexample
+void NotATemplate (my_class t)
+@{
+  int N = 5;
+
+  bool test = t.m < N > (0); // Treated as a template.
+  test = (t.m < N) > (0); // Same meaning, but not treated as a template.
+@}
+@end smallexample
+
+This warning can be disabled with @option{-Wno-missing-template-keyword}.
+
 @item -Wno-multichar
 @opindex Wno-multichar
 @opindex Wmultichar