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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Erich Keane (erichkeane)

<details>
<summary>Changes</summary>

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.

---
Full diff: https://github.com/llvm/llvm-project/pull/83487.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaTemplate.cpp (+8-6) 
- (added) clang/test/SemaCXX/GH83461.cpp (+60) 


``````````diff
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index e91033dd886891..a7910bda874c8d 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/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);
diff --git a/clang/test/SemaCXX/GH83461.cpp b/clang/test/SemaCXX/GH83461.cpp
new file mode 100644
index 00000000000000..509535e883e6d9
--- /dev/null
+++ b/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 at +2{{template parameter missing a default argument}}
+// expected-note at +1{{previous default template argument defined here}}
+template<typename T = int, typename U>
+struct ClassTempl{};
+
+struct HasFriendClassTempl {
+  // expected-error at +1{{default template argument not permitted on a friend template}}
+  template<typename T = int, typename U>
+  friend struct Friend;
+
+  // expected-error at +3{{cannot define a type in a friend declaration}}
+  // expected-error at +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 at +3{{template parameter missing a default argument}}
+  // expected-note at +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 at +2{{template parameter missing a default argument}}
+// expected-note at +1{{previous default template argument defined here}}
+template<typename T = int, typename U>
+static constexpr U VarTempl;
+
+// expected-error at +2{{template parameter missing a default argument}}
+// expected-note at +1{{previous default template argument defined here}}
+template<typename T = int, typename U>
+using TypeAlias = U;

``````````

</details>


https://github.com/llvm/llvm-project/pull/83487