`.template` not followed by template arguments is incorrectly accepted

[HolyBlackCat]

Consider this example:

#include <iostream>

struct A
{
    void foo() {std::cout << "1\n";}
    
    template <typename T = int>
    void foo() {std::cout << "2\n";}
};

int main()
{
    A x;
    x.template foo();
}

Clang prints 2. But the code shouldn't be accepted at all, since [temp.names]/5 says that the name after .template/->template must be a template-id, aka must have explicitly specified template arguments. Here's a stackoverflow thread.

Note: this doesn't apply to foo::template MyClassTemplate, which is legal, but seems to be made optional (and deprecated at the same time) in C++2b.

[MarcusBoay]

Hi, I'm interested in fixing this bug. I am quite new to LLVM even though I have experience building toy compilers from my undergraduate compilers course. Is there more information for a beginner to fix this bug?

[languagelawyer]

diff --git a/clang/include/clang/Basic/DiagnosticParseKinds.td b/clang/include/clang/Basic/DiagnosticParseKinds.td
index f21e841bc..552067f84 100644
--- a/clang/include/clang/Basic/DiagnosticParseKinds.td
+++ b/clang/include/clang/Basic/DiagnosticParseKinds.td
@@ -773,6 +773,10 @@ def err_template_spec_syntax_non_template : Error<
   "<unused>|refers to a variable template|<unused>|refers to a concept}1">;
 def err_id_after_template_in_nested_name_spec : Error<
   "expected template name after 'template' keyword in nested name specifier">;
+def err_template_kw_no_args_not_type_template : Error<
+  "A name prefixed by 'template' "
+  "shall be followed by a template argument list "
+  "or refer to a class template or an alias template">;
 def err_unexpected_template_in_unqualified_id : Error<
   "'template' keyword not permitted here">;
 def err_unexpected_template_in_destructor_name : Error<
diff --git a/clang/lib/Parse/ParseExprCXX.cpp b/clang/lib/Parse/ParseExprCXX.cpp
index 2d38891c7..41a06e835 100644
--- a/clang/lib/Parse/ParseExprCXX.cpp
+++ b/clang/lib/Parse/ParseExprCXX.cpp
@@ -2852,8 +2852,10 @@ bool Parser::ParseUnqualifiedId(CXXScopeSpec &SS, ParsedType ObjectType,
              Actions.ActOnTemplateName(
                  getCurScope(), SS, *TemplateKWLoc, Result, ObjectType,
                  EnteringContext, Template,
-                 /*AllowInjectedClassName*/ true) == TNK_Non_template)
+                 /*AllowInjectedClassName*/ true) != TNK_Type_template) {
+      Diag(IdLoc, diag::err_template_kw_no_args_not_type_template);
       return true;
+    }
 
     return false;
   }

14:13: error: A name prefixed by 'template' shall be followed by a template argument list or refer to a class template or an alias template
        x.template foo();
                   ^
1 error generated.

Done
(Haven't run any Clang tests 🤡)

UPD: 6 are failing, but looks like they do it «in a good way». Either the code relied on that .template foo() is allowed, esp.

llvm-project/clang/test/CXX/drs/dr0xx.cpp

Lines 1122 to 1136 in 4cb24ef

	namespace dr96 { // dr96: no
	struct A {
	void f(int);
	template<typename T> int f(T);
	template<typename T> struct S {};
	} a;
	template<template<typename> class X> struct B {};
	template<typename T>
	void test() {
	int k1 = a.template f<int>(0);
	// FIXME: This is ill-formed, because 'f' is not a template-id and does not
	// name a class template.
	// FIXME: What about alias templates?
	int k2 = a.template f(1);

or expected to fail later with slightly different error message e.g.

llvm-project/clang/test/SemaCXX/cxx1y-variable-templates_in_class.cpp

Line 387 in 632ea69

int &r = A::template n; // expected-error {{use of variable template 'n' requires template arguments}}

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. Assign the issue to you.
2. Fix the issue locally.
3. Run the test suite locally.
   3.1) Remember that the subdirectories under test/ create fine-grained testing targets, so you can
   e.g. use make check-clang-ast to only run Clang's AST tests.
4. Create a git commit
5. Run git clang-format HEAD~1 to format your changes.
6. Submit the patch to Phabricator.
   6.1) Detailed instructions can be found here

For more instructions on how to submit a patch to LLVM, see our documentation.

If you have any further questions about this issue, don't hesitate to ask via a comment on this Github issue.

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