================
@@ -1836,7 +1836,19 @@ static TemplateParameterList
*GetTemplateParameterList(TemplateDecl *TD) {
// Make sure we get the template parameter list from the most
// recent declaration, since that is the only one that is guaranteed to
// have all the default template argument information.
- return cast<TemplateDecl>(TD->getMostRecentDecl())->getTemplateParameters();
+ Decl *ND = TD->getMostRecentDecl();
+ // Skip past friend Decls because they are not supposed to contain default
+ // template arguments. Moreover, these declarations may introduce template
+ // parameters living in different template depths than the corresponding
+ // template parameters in TD, causing unmatched constraint substitution.
+ //
+ // C++23 N4950 [temp.param]p12
----------------
hokein wrote:
nit: this is not a new thing in C++23, I also find it in the C++20 spec N4860.
I'd remove the `C++23 N4950`.
And I think this is also a FIXME, clang should diagnose this error case, maybe
add the `FIXME` in the comment.
I think this function is not only used for class templates, but also for
function templates. For function templates, there is an exception per the
standard (temp.param 12): `If a friend function template declaration specifies
a default template-argument, that declaration shall be a definition and shall
be the only declaration of the function template in the translation unit.`
https://github.com/llvm/llvm-project/pull/86914
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