[Clang] Fall back to DW_TAG_typedef for instantiation dependent template aliases (#90032)

Workaround for issue #89774 until it can be properly fixed.

When `-gtemplate-alias` is specified Clang emits a DW_TAG_template_alias
for template aliases. This patch avoids an assertion failure by falling
back to the `-gno-template-alias` (default) behaviour, emitting a
DW_TAG_typedef, if the alias is instantiation dependent.

Author: OCHyams

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -1372,7 +1372,26 @@ llvm::DIType *CGDebugInfo::CreateType(const TemplateSpecializationType *Ty,
 
   SourceLocation Loc = AliasDecl->getLocation();
 
-  if (CGM.getCodeGenOpts().DebugTemplateAlias) {
+  if (CGM.getCodeGenOpts().DebugTemplateAlias &&
+      // The TemplateSpecializationType doesn't contain any instantiation
+      // information; dependent template arguments can't be resolved. For now,
+      // fall back to DW_TAG_typedefs for template aliases that are
+      // instantiation dependent, e.g.:
+      // ```
+      // template <int>
+      // using A = int;
+      //
+      // template<int I>
+      // struct S {
+      //   using AA = A<I>; // Instantiation dependent.
+      //   AA aa;
+      // };
+      //
+      // S<0> s;
+      // ```
+      // S::AA's underlying type A<I> is dependent on I so will be emitted as a
+      // DW_TAG_typedef.
+      !Ty->isInstantiationDependentType()) {
     auto ArgVector = ::GetTemplateArgs(TD, Ty);
     TemplateArgs Args = {TD->getTemplateParameters(), ArgVector};
 

clang/test/CodeGenCXX/dependent-template-alias.cpp

@@ -0,0 +1,21 @@
+// RUN: %clang_cc1 -triple x86_64-unk-unk -o - -emit-llvm -debug-info-kind=standalone -gtemplate-alias %s -gsimple-template-names=simple \
+// RUN: | FileCheck %s
+
+//// Check that -gtemplate-alias falls back to DW_TAG_typedef emission
+//// for instantiation dependent type aliases.
+
+template <int>
+using A = int;
+
+template<int I>
+struct S {
+  using AA = A<I>;
+  AA aa;
+};
+
+S<0> s;
+
+// CHECK: !DIDerivedType(tag: DW_TAG_member, name: "aa", scope: ![[#]], file: ![[#]], line: [[#]], baseType: ![[AA:[0-9]+]], size: 32)
+// CHECK: [[AA]] = !DIDerivedType(tag: DW_TAG_typedef, name: "AA", file: ![[#]], line: [[#]], baseType: ![[A:[0-9]+]])
+// CHECK: [[A]] = !DIDerivedType(tag: DW_TAG_typedef, name: "A<I>", file: ![[#]], line: [[#]], baseType: ![[int:[0-9]+]])
+// CHECK: [[int]] = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)