Fix implementation of [temp.param]p14's first sentence. (#83487)

The first sentence says:

If a template-parameter of a class template, variable template, or alias
template has a default template-argument, each subsequent
template-parameter shall either have a default template-argument
supplied or be a template parameter pack.

However, we were only testing for "not a function function template",
and referring to an older version of the standard. As far as I can tell,
CWG2032 added the variable-template, and the alias-template pre-dates
the standard on github.

This patch started as a bug fix for #83461 , but ended up fixing a
number of similar cases, so those are validated as well.

Author: erichkeane

clang/docs/ReleaseNotes.rst

@@ -301,6 +301,12 @@ Bug Fixes to C++ Support
 - Clang now properly reports supported C++11 attributes when using
   ``__has_cpp_attribute`` and parses attributes with arguments in C++03
   (`#82995 <https://github.com/llvm/llvm-project/issues/82995>`_)
+- Clang now properly diagnoses missing 'default' template arguments on a variety
+  of templates. Previously we were diagnosing on any non-function template
+  instead of only on class, alias, and variable templates, as last updated by
+  CWG2032.
+  Fixes (`#83461 <https://github.com/llvm/llvm-project/issues/83461>`_)
+
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaTemplate.cpp

@@ -3141,12 +3141,14 @@ bool Sema::CheckTemplateParameterList(TemplateParameterList *NewParams,
            diag::note_template_param_prev_default_arg_in_other_module)
           << PrevModuleName;
       Invalid = true;
-    } else if (MissingDefaultArg && TPC != TPC_FunctionTemplate) {
-      // C++ [temp.param]p11:
-      //   If a template-parameter of a class template has a default
-      //   template-argument, each subsequent template-parameter shall either
-      //   have a default template-argument supplied or be a template parameter
-      //   pack.
+    } else if (MissingDefaultArg &&
+               (TPC == TPC_ClassTemplate || TPC == TPC_FriendClassTemplate ||
+                TPC == TPC_VarTemplate || TPC == TPC_TypeAliasTemplate)) {
+      // C++ 23[temp.param]p14:
+      // If a template-parameter of a class template, variable template, or
+      // alias template has a default template argument, each subsequent
+      // template-parameter shall either have a default template argument
+      // supplied or be a template parameter pack.
       Diag((*NewParam)->getLocation(),
            diag::err_template_param_default_arg_missing);
       Diag(PreviousDefaultArgLoc, diag::note_template_param_prev_default_arg);

clang/test/SemaCXX/GH83461.cpp

@@ -0,0 +1,60 @@
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+struct S {
+  template<typename Ty = int>
+  friend void foo(auto){}
+
+  template<typename Ty = int, typename Tz>
+  friend void foo2(){}
+};
+
+template<typename T>
+struct TemplS {
+  template<typename Ty = int>
+  friend void foo3(auto){}
+
+  template<typename Ty = int, typename Tz>
+  friend void foo4(){}
+};
+
+void Inst() {
+  TemplS<int>();
+}
+// expected-error@+2{{template parameter missing a default argument}}
+// expected-note@+1{{previous default template argument defined here}}
+template<typename T = int, typename U>
+struct ClassTempl{};
+
+struct HasFriendClassTempl {
+  // expected-error@+1{{default template argument not permitted on a friend template}}
+  template<typename T = int, typename U>
+  friend struct Friend;
+
+  // expected-error@+3{{cannot define a type in a friend declaration}}
+  // expected-error@+1{{default template argument not permitted on a friend template}}
+  template<typename T = int, typename U>
+  friend struct Friend2{};
+};
+
+template<typename Ty>
+struct HasFriendClassTempl2 {
+  // expected-error@+3{{template parameter missing a default argument}}
+  // expected-note@+2{{previous default template argument defined here}}
+  // expected-note@#INST2{{in instantiation of template class}}
+  template<typename T = int, typename U>
+  friend struct Friend;
+};
+
+void Inst2() {
+  HasFriendClassTempl2<int>(); // #INST2
+}
+
+// expected-error@+2{{template parameter missing a default argument}}
+// expected-note@+1{{previous default template argument defined here}}
+template<typename T = int, typename U>
+static constexpr U VarTempl;
+
+// expected-error@+2{{template parameter missing a default argument}}
+// expected-note@+1{{previous default template argument defined here}}
+template<typename T = int, typename U>
+using TypeAlias = U;