From 223c5d18954c4fa90a339785c0492fede208f81d Mon Sep 17 00:00:00 2001
From: Viktoriia Bakalova <bakalova@google.com>
Date: Fri, 13 Sep 2024 11:19:00 +0000
Subject: [PATCH] [ItaniumDemangle] Add template name to the substituions list
 during demangling.

When demangling a template template parameter (`method<bool, Bar>(Bar<bool> b)`), the current demangler version first enters the template argument (`bool`) into the substitutions list, then the whole template specialization (`Bar<bool>`). The template name (`Bar`) never becomes a substitution candidate on its own.

This is different when mangling. Mangling `method<bool, Bar>(Bar<bool> b, Bar<int> i)` substitutes the `Bar` in the second parameter with the substitution for `TemplateTemplateParmDecl`.

This leads to a discrepancy between mangler and demangler,
see https://github.com/llvm/llvm-project/issues/108009.
---
 libcxxabi/src/demangle/ItaniumDemangle.h     | 1 +
 libcxxabi/test/test_demangle.pass.cpp        | 3 +++
 llvm/include/llvm/Demangle/ItaniumDemangle.h | 1 +
 3 files changed, 5 insertions(+)

diff --git a/libcxxabi/src/demangle/ItaniumDemangle.h b/libcxxabi/src/demangle/ItaniumDemangle.h
index 3b041efe3aac0..d1f8da9d6b57b 100644
--- a/libcxxabi/src/demangle/ItaniumDemangle.h
+++ b/libcxxabi/src/demangle/ItaniumDemangle.h
@@ -4336,6 +4336,7 @@ Node *AbstractManglingParser<Derived, Alloc>::parseType() {
     // parse them, take the second production.
 
     if (TryToParseTemplateArgs && look() == 'I') {
+      Subs.push_back(Result);
       Node *TA = getDerived().parseTemplateArgs();
       if (TA == nullptr)
         return nullptr;
diff --git a/libcxxabi/test/test_demangle.pass.cpp b/libcxxabi/test/test_demangle.pass.cpp
index 77f79e0d40e84..44af7e041cbcf 100644
--- a/libcxxabi/test/test_demangle.pass.cpp
+++ b/libcxxabi/test/test_demangle.pass.cpp
@@ -30024,6 +30024,9 @@ const char* cases[][2] =
     // See https://github.com/itanium-cxx-abi/cxx-abi/issues/165.
     {"_ZN1C1fIiEEvDTtlNS_UlT_TL0__E_EEE", "void C::f<int>(decltype(C::'lambda'(int, auto){}))"},
 
+    // See https://github.com/llvm/llvm-project/issues/108009.
+    {"_ZN3FooIiE6methodIb3BarEEvT0_IT_ES3_IiE", "void Foo<int>::method<bool, Bar>(Bar<bool>, Bar<int>)"},
+
     // C++20 class type non-type template parameters:
     {"_Z1fIXtl1BLPi0ELi1EEEEvv", "void f<B{(int*)0, 1}>()"},
     {"_Z1fIXtl1BLPi32EEEEvv", "void f<B{(int*)32}>()"},
diff --git a/llvm/include/llvm/Demangle/ItaniumDemangle.h b/llvm/include/llvm/Demangle/ItaniumDemangle.h
index 0af0224bc83fa..21144bd8c0706 100644
--- a/llvm/include/llvm/Demangle/ItaniumDemangle.h
+++ b/llvm/include/llvm/Demangle/ItaniumDemangle.h
@@ -4336,6 +4336,7 @@ Node *AbstractManglingParser<Derived, Alloc>::parseType() {
     // parse them, take the second production.
 
     if (TryToParseTemplateArgs && look() == 'I') {
+      Subs.push_back(Result);
       Node *TA = getDerived().parseTemplateArgs();
       if (TA == nullptr)
         return nullptr;