[libcxxabi] [llvm] [ItaniumDemangle] Unconditionally parse substitution template arguments (PR #131970)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/131970

With the current mangled name parser, when we try to parse a type name inside the template argument of an operator, we end up not parsing any template arguments of substitutions, which ends up causing things to fail in certain cases. This is compliant with the Itanium ABI documentation as far as I can tell.

I tried to look through the git history to find out why the limitation on parsing template arguments for substitutions inside of operators existed, but I only ended up at the import commit, so I suspect the original motivation is lost to time.

A regression test from LLVM has been added.

>From b5ca4bc286493d6b059172f57c1307aaa621325a Mon Sep 17 00:00:00 2001
From: Aiden Grossman <aidengrossman at google.com>
Date: Wed, 19 Mar 2025 03:56:05 +0000
Subject: [PATCH] [ItaniumDemangle] Unconditionally parse substitution template
 arguments

With the current mangled name parser, when we try to parse a type name
inside the template argument of an operator, we end up not parsing any
template arguments of substitutions, which ends up causing things to
fail in certain cases. This is compliant with the Itanium ABI
documentation as far as I can tell.

I tried to look through the git history to find out why the limitation
on parsing template arguments for substitutions inside of operators
existed, but I only ended up at the import commit, so I suspect the
original motivation is lost to time.

A regression test from LLVM has been added.
---
 libcxxabi/src/demangle/ItaniumDemangle.h     | 6 +++---
 libcxxabi/test/test_demangle.pass.cpp        | 1 +
 llvm/include/llvm/Demangle/ItaniumDemangle.h | 6 +++---
 3 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/libcxxabi/src/demangle/ItaniumDemangle.h b/libcxxabi/src/demangle/ItaniumDemangle.h
index 3df41b5f4d7d0..45ec8d41fee41 100644
--- a/libcxxabi/src/demangle/ItaniumDemangle.h
+++ b/libcxxabi/src/demangle/ItaniumDemangle.h
@@ -4626,10 +4626,10 @@ Node *AbstractManglingParser<Derived, Alloc>::parseType() {
       //   <template-template-param> ::= <template-param>
       //                             ::= <substitution>
       //
-      // If this is followed by some <template-args>, and we're permitted to
-      // parse them, take the second production.
+      // If this is followed by some <template-args>, take the second
+      // production.
 
-      if (look() == 'I' && (!IsSubst || TryToParseTemplateArgs)) {
+      if (look() == 'I') {
         if (!IsSubst)
           Subs.push_back(Result);
         Node *TA = getDerived().parseTemplateArgs();
diff --git a/libcxxabi/test/test_demangle.pass.cpp b/libcxxabi/test/test_demangle.pass.cpp
index e9c74f70a094b..fad6c101f742d 100644
--- a/libcxxabi/test/test_demangle.pass.cpp
+++ b/libcxxabi/test/test_demangle.pass.cpp
@@ -30247,6 +30247,7 @@ const char* cases[][2] = {
     {"_Z1fDSDRm", "f(_Sat unsigned long _Fract)"},
 
     {"_Z11bfloat16addDF16bDF16b", "bfloat16add(std::bfloat16_t, std::bfloat16_t)"},
+    {"_ZNK4llvm11SmallStringILj16EEcvNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEEEv", "llvm::SmallString<16u>::operator std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>() const"},
     // clang-format on
 };
 
diff --git a/llvm/include/llvm/Demangle/ItaniumDemangle.h b/llvm/include/llvm/Demangle/ItaniumDemangle.h
index b0363c1a7a786..15400694b3c70 100644
--- a/llvm/include/llvm/Demangle/ItaniumDemangle.h
+++ b/llvm/include/llvm/Demangle/ItaniumDemangle.h
@@ -4626,10 +4626,10 @@ Node *AbstractManglingParser<Derived, Alloc>::parseType() {
       //   <template-template-param> ::= <template-param>
       //                             ::= <substitution>
       //
-      // If this is followed by some <template-args>, and we're permitted to
-      // parse them, take the second production.
+      // If this is followed by some <template-args>, take the second
+      // production.
 
-      if (look() == 'I' && (!IsSubst || TryToParseTemplateArgs)) {
+      if (look() == 'I') {
         if (!IsSubst)
           Subs.push_back(Result);
         Node *TA = getDerived().parseTemplateArgs();