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

[Michael Buch via cfe-commits]

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@@ -1313,6 +1313,61 @@ llvm::DIType *CGDebugInfo::CreateType(const BlockPointerType *Ty,
   return DBuilder.createPointerType(EltTy, Size);
 }
 
+static llvm::SmallVector<TemplateArgument>
+GetTemplateArgs(const TemplateDecl *TD, const TemplateSpecializationType *Ty) {
+  assert(Ty->isTypeAlias());
+  // 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. It also
+  // doesn't know the value of any defaulted args, so collect those now
+  // too.
+  SmallVector<TemplateArgument> SpecArgs;
+  ArrayRef SubstArgs = Ty->template_arguments();
+  for (const NamedDecl *Param : TD->getTemplateParameters()->asArray()) {
+    // If Param is a parameter pack, pack the remaining arguments.
+    if (Param->isParameterPack()) {
+      SpecArgs.push_back(TemplateArgument(SubstArgs));
+      break;
+    }
+
+    // Take the next argument.
+    if (!SubstArgs.empty()) {
+      SpecArgs.push_back(SubstArgs.front());
+      SubstArgs = SubstArgs.drop_front();
+      continue;
+    }
+
+    // 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. Push the
+    // default value for each parameter.
+    if (auto *P = dyn_cast<TemplateTypeParmDecl>(Param)) {
+      assert(P->hasDefaultArgument() &&
+             "expected defaulted template type parameter");
+      SpecArgs.push_back(TemplateArgument(P->getDefaultArgument(),
+                                          /*IsNullPtr=*/false,
+                                          /*IsDefaulted=*/true));
+    } else if (auto *P = dyn_cast<NonTypeTemplateParmDecl>(Param)) {
+      assert(P->hasDefaultArgument() &&
+             "expected defaulted template non-type parameter");
+      SpecArgs.push_back(TemplateArgument(P->getDefaultArgument(),
+                                          /*IsDefaulted=*/true));
+    } else if (auto *P = dyn_cast<TemplateTemplateParmDecl>(Param)) {
+      assert(P->hasDefaultArgument() &&
+             "expected defaulted template template parameter");
+      SpecArgs.push_back(TemplateArgument(
+          P->getDefaultArgument().getArgument().getAsTemplate(),
+          /*IsDefaulted=*/true));
----------------
Michael137 wrote:

Could we get away with just doing `SpecArgs.push_back(P->getDefaultArgument())`? Or were there some lifetime concerns?

Otherwise LGTM

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