[clang] [llvm] [Clang] Emit DW_TAG_template_alias for template aliases (PR #87623)

[Michael Buch via llvm-commits]

================
@@ -1332,6 +1332,54 @@ llvm::DIType *CGDebugInfo::CreateType(const TemplateSpecializationType *Ty,
   auto PP = getPrintingPolicy();
   Ty->getTemplateName().print(OS, PP, TemplateName::Qualified::None);
 
+  SourceLocation Loc = AliasDecl->getLocation();
+
+  if (CGM.getCodeGenOpts().DebugTemplateAlias) {
+    // TemplateSpecializationType doesn't know if its template args are
+    // being substituted into a parameter pack. We can find out if that's
+    // the case now by inspecting the TypeAliasTemplateDecl template
+    // parameters. Insert Ty's template args into SpecArgs, bundling args
+    // passed to a parameter pack into a TemplateArgument::Pack.
+    SmallVector<TemplateArgument> SpecArgs;
+    {
+      ArrayRef SubstArgs = Ty->template_arguments();
+      for (const NamedDecl *P : TD->getTemplateParameters()->asArray()) {
+        if (P->isParameterPack()) {
+          SpecArgs.push_back(TemplateArgument(SubstArgs));
+          break;
+        }
+        // Skip defaulted args.
+        if (SubstArgs.empty()) {
+          // If SubstArgs is now empty (we're taking from it each iteration) and
+          // this template parameter isn't a pack, then that should mean we're
+          // using default values for the remaining template parameters.
+          break;
----------------
Michael137 wrote:

Ah apologies! I must've misremembered, I could've sworn there were cases where we don't handle the defaulted-ness.

> Is your suggestion to emit something like this for the template alias case (from my previous comment)?
Yup, would default values ever matter for name reconstruction? The only thing I know of that uses `DW_AT_default_value` in LLDB is when formatting the typename of a variable when printing it (we omit defaulted template parameters). 

https://github.com/llvm/llvm-project/pull/87623