================
@@ -507,10 +507,62 @@ static TemplateDeductionResult
DeduceNonTypeTemplateArgument(
S, TemplateParams, NTTP, DeducedTemplateArgument(New), T, Info, Deduced);
}
+static NamedDecl *DeduceTemplateArguments(Sema &S, NamedDecl *A,
+ TemplateArgument Default) {
+ switch (A->getKind()) {
+ case Decl::TemplateTypeParm: {
+ auto *T = cast<TemplateTypeParmDecl>(A);
+ // FIXME: DefaultArgument can't represent a pack.
+ if (T->isParameterPack())
----------------
mizvekov wrote:
The test `foo` in
`clang/test/CXX/temp/temp.decls/temp.fct/temp.func.order/p6.cpp` exercises
this, but it doesn't involve any default arguments, so any effects are
incidental / harmless.
I have actually just come up with a relevant test case that shows something
interesting that I wasn't aware before, this could possibly be new info for the
core issue.
https://godbolt.org/z/rnbzhnqG7
This shows a change of behavior introduced by P05522, only instead of
introducing ambiguity, it causes different candidates to be picked. Also
affects GCC.
---
Regarding the FIXME:
So the (Type | NonType | Template) template parameter declarations store the
default argument for themselves. They have a `setDefaultArgument` method for
setting that. While for template template parameter declarations the
`setDefaultArgument` takes a `TemplateArgument`, and that means it can use any
kind of TemplateArgument as a default argument, including packs.
The same is not true for (Type | NonType) template parameter declarations,
their `setDefaultArgument` only takes a (Type / Expression), which means a pack
can't be passed.
This is done presumably to save space in those declarations, while a Type and
an Expression are a single pointer, a `TemplateArgument` is considerably
larger, and you would need at least a pointer and an integer to represent a
pack.
To 'fix' this FIXME, we would have to change that, and allow those declarations
to be larger in at least some circumstances.
https://github.com/llvm/llvm-project/pull/89807
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