[clang][Parse] Fix crash when emitting template diagnostic

[tbaeder]

Summary

The integer value of the enum here is 6, which wasn't covered by the diagnostic.

This fixes https://github.com/llvm/llvm-project/issues/57415

[tbaeder]

clang/test/Parser/cxx-concept-declaration.cpp

3–4

I guess we can drop these two lines now.

[cjdb]

clang/include/clang/Basic/DiagnosticParseKinds.td

779

Is "concept template" a term of art? I have never heard of that before.

[tbaeder]

clang/include/clang/Basic/DiagnosticParseKinds.td

779

The enum member is called clang::TNK_Concept_template, so I made up "concept template" from that. Not sure what to use instead, would just "refers to a concept" be better?

[aaron.ballman]

Please be sure to add a release note for the fix as well.

clang/include/clang/Basic/DiagnosticParseKinds.td

776–780

I think the issue is that TNK_Undeclared_template was added but this diagnostic was not updated accordingly. As best I can tell, ParseDeclCXX.cpp:1715-1725 makes that kind unused in this diagnostic as well.

clang/test/Parser/cxx-concept-declaration.cpp

3–4

Agreed (feel free to land as an NFC commit)

6–10

[cjdb]

Thanks for looking into this so quickly. LGTM with Aaron's suggested changes.

[aaron.ballman]

LGTM presuming precommit CI comes back green, but please don't forget the release note.

clang/docs/ReleaseNotes.rst

- ``-Wtautological-compare`` missed warnings for tautological comparisons
   involving a negative integer literal. This fixes
   `Issue 42918 <https://github.com/llvm/llvm-project/issues/42918>`_.
 - Fix a crash when generating code coverage information for an
   ``if consteval`` statement. This fixes
   `Issue 57377 <https://github.com/llvm/llvm-project/issues/57377>`_.
 - Fix assert that triggers a crash during template name lookup when a type was
   incomplete but was not also a TagType. This fixes
   `Issue 57387 <https://github.com/llvm/llvm-project/issues/57387>`_.
+- Fix a crash when emitting a concept-related diagnostic. This fixes
+  `Issue 57415 <https://github.com/llvm/llvm-project/issues/57415>`_.
 
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Clang will now correctly diagnose as ill-formed a constant expression where an
   enum without a fixed underlying type is set to a value outside the range of
   the enumeration's values. Due to the extended period of time this bug was
   present in major C++ implementations (including Clang), this error has the

clang/include/clang/Basic/DiagnosticParseKinds.td

 def ext_template_template_param_typename : ExtWarn<
   "template template parameter using 'typename' is a C++17 extension">,
   InGroup<CXX17>;
 def warn_cxx14_compat_template_template_param_typename : Warning<
   "template template parameter using 'typename' is "
   "incompatible with C++ standards before C++17">,
   InGroup<CXXPre17Compat>, DefaultIgnore;
 def err_template_spec_syntax_non_template : Error<
   "identifier followed by '<' indicates a class template specialization but "
   "%0 %select{does not refer to a template|refers to a function template|"
-  "<unused>|refers to a variable template|<unused>|refers to a concept}1">;
+  "<unused>|refers to a variable template|<unused>|<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_unexpected_template_in_unqualified_id : Error<
   "'template' keyword not permitted here">;
 def err_unexpected_template_in_destructor_name : Error<
   "'template' keyword not permitted in destructor name">;
 def err_unexpected_template_after_using : Error<
   "'template' keyword not permitted after 'using' keyword">;
 def err_two_right_angle_brackets_need_space : Error<

clang/test/Parser/cxx-concept-declaration.cpp

 
 // Support parsing of concepts
 // Disabled for now.
-// expected-no-diagnostics
 
-// RUN:  %clang_cc1 -std=c++14 -x c++ -verify %s
-// template<typename T> concept C1 = true;
+// RUN:  %clang_cc1 -std=c++20 -x c++ -verify %s
+template<typename T> concept C1 = true;
+
+template<class T>
+concept C = true;
+
+template<class T>
+class C<int> {}; //expected-error{{identifier followed by '<' indicates a class template specialization but 'C' refers to a concept}}