[Clang] Fall back to DW_TAG_typedef for instantiation dependent template aliases

[OCHyams]

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.

[llvmbot]

@llvm/pr-subscribers-clang-codegen
@llvm/pr-subscribers-debuginfo

@llvm/pr-subscribers-clang

Author: Orlando Cazalet-Hyams (OCHyams)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/90032.diff

2 Files Affected:

• (modified) clang/lib/CodeGen/CGDebugInfo.cpp (+20-1)
• (added) clang/test/CodeGenCXX/dependent-template-alias.cpp (+21)

diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
index 539ded5cca5e1b..787db350487417 100644
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/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};
 
diff --git a/clang/test/CodeGenCXX/dependent-template-alias.cpp b/clang/test/CodeGenCXX/dependent-template-alias.cpp
new file mode 100644
index 00000000000000..deb243f9fc88d0
--- /dev/null
+++ b/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)

[dwblaikie]

Comment in the code should probably mention this as a FIXME and include a reference to the issue?

Also, there's another bug here - the DW_TAG_typedef is in the CU scope, instead of the struct scope. But if the struct is a non-template, the typedef is in the struct scope as it should be, not the CU scope...

[OCHyams]

Comment in the code should probably mention this as a FIXME and include a reference to the issue?

Sure, added in f78949a

Also, there's another bug here - the DW_TAG_typedef is in the CU scope, instead of the struct scope. But if the struct is a non-template, the typedef is in the struct scope as it should be, not the CU scope...

That does looks odd - I can reproduce it with normal (language-level) typedefs too (rather than with template aliases): https://godbolt.org/z/GsGKqhKzz. I can open a separate issue?

Now that I think about it, I recall @CarlosAlbertoEnciso running into something similar a while ago... I'm sure a bug was filed about something similar but I can't find it. Does this ring any bells @CarlosAlbertoEnciso?

[dwblaikie]

Comment in the code should probably mention this as a FIXME and include a reference to the issue?

Sure, added in f78949a

Thanks!

Also, there's another bug here - the DW_TAG_typedef is in the CU scope, instead of the struct scope. But if the struct is a non-template, the typedef is in the struct scope as it should be, not the CU scope...

That does looks odd - I can reproduce it with normal (language-level) typedefs too (rather than with template aliases): https://godbolt.org/z/GsGKqhKzz. I can open a separate issue?

Yep!

[OCHyams]

Here's that issue - #91451.

Carlos pointed me to bz44884 (#44229) which may be related