Fix MSVC Demangling with auto NTTP mangled names for function pointer, pointer to data and integral types (#96590)

As cited here, https://github.com/llvm/llvm-project/pull/92477, undname
needs updating to support the new auto NTTP name mangling.

In short the deduced type of the auto NTTP parameter is mangled as `$M
<type> <nttp-param>`. However the deduced type is not printed for the
undecorated name so the `$M <type>` is parsed but simply ignored when
stringifying the generated AST.

1 files changed, 32 insertions, 6 deletions

diff --git a/llvm/lib/Demangle/MicrosoftDemangle.cpp b/llvm/lib/Demangle/MicrosoftDemangle.cpp
index cd7ff40..e18de9a 100644
--- a/llvm/lib/Demangle/MicrosoftDemangle.cpp
+++ b/llvm/lib/Demangle/MicrosoftDemangle.cpp
@@ -53,6 +53,18 @@ static bool consumeFront(std::string_view &S, std::string_view C) {
   return true;
 }
 
+static bool consumeFront(std::string_view &S, std::string_view PrefixA,
+                         std::string_view PrefixB, bool A) {
+  const std::string_view &Prefix = A ? PrefixA : PrefixB;
+  return consumeFront(S, Prefix);
+}
+
+static bool startsWith(std::string_view S, std::string_view PrefixA,
+                       std::string_view PrefixB, bool A) {
+  const std::string_view &Prefix = A ? PrefixA : PrefixB;
+  return llvm::itanium_demangle::starts_with(S, Prefix);
+}
+
 static bool isMemberPointer(std::string_view MangledName, bool &Error) {
   Error = false;
   const char F = MangledName.front();
@@ -2256,6 +2268,18 @@ Demangler::demangleTemplateParameterList(std::string_view &MangledName) {
 
     NodeList &TP = **Current;
 
+    // <auto-nttp> ::= $ M <type> <nttp>
+    const bool IsAutoNTTP = consumeFront(MangledName, "$M");
+    if (IsAutoNTTP) {
+      // The deduced type of the auto NTTP parameter isn't printed so
+      // we want to ignore the AST created from demangling the type.
+      //
+      // TODO: Avoid the extra allocations to the bump allocator in this case.
+      (void)demangleType(MangledName, QualifierMangleMode::Drop);
+      if (Error)
+        return nullptr;
+    }
+
     TemplateParameterReferenceNode *TPRN = nullptr;
     if (consumeFront(MangledName, "$$Y")) {
       // Template alias
@@ -2266,15 +2290,17 @@ Demangler::demangleTemplateParameterList(std::string_view &MangledName) {
     } else if (consumeFront(MangledName, "$$C")) {
       // Type has qualifiers.
       TP.N = demangleType(MangledName, QualifierMangleMode::Mangle);
-    } else if (llvm::itanium_demangle::starts_with(MangledName, "$1") ||
-               llvm::itanium_demangle::starts_with(MangledName, "$H") ||
-               llvm::itanium_demangle::starts_with(MangledName, "$I") ||
-               llvm::itanium_demangle::starts_with(MangledName, "$J")) {
+    } else if (startsWith(MangledName, "$1", "1", !IsAutoNTTP) ||
+               startsWith(MangledName, "$H", "H", !IsAutoNTTP) ||
+               startsWith(MangledName, "$I", "I", !IsAutoNTTP) ||
+               startsWith(MangledName, "$J", "J", !IsAutoNTTP)) {
       // Pointer to member
       TP.N = TPRN = Arena.alloc<TemplateParameterReferenceNode>();
       TPRN->IsMemberPointer = true;
 
-      MangledName.remove_prefix(1);
+      if (!IsAutoNTTP)
+        MangledName.remove_prefix(1); // Remove leading '$'
+
       // 1 - single inheritance       <name>
       // H - multiple inheritance     <name> <number>
       // I - virtual inheritance      <name> <number> <number>
@@ -2342,7 +2368,7 @@ Demangler::demangleTemplateParameterList(std::string_view &MangledName) {
       }
       TPRN->IsMemberPointer = true;
 
-    } else if (consumeFront(MangledName, "$0")) {
+    } else if (consumeFront(MangledName, "$0", "0", !IsAutoNTTP)) {
       // Integral non-type template parameter
       bool IsNegative = false;
       uint64_t Value = 0;