[clang] [clang][DebugInfo] Revert to printing canonical typenames for template aliases (PR #110767)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Michael Buch (Michael137)

<details>
<summary>Changes</summary>

This was originally added in https://reviews.llvm.org/D142268 have LLDB display 
variable typenames that benefit from suppressing defaulted template arguments.

We currently represent template aliases as `DW_AT_typedef`s instead of 
`DW_TAG_template_alias`. This means for types like:
```
template &lt;class _Tp&gt;
using __remove_cv_t = __remove_cv(_Tp);

template &lt;class _Tp&gt;
using remove_cv_t = __remove_cv_t&lt;_Tp&gt;;

template&lt;typename T&gt;
class optional {
  using value_type = T;
  remove_cv_t&lt;value_type&gt; __val_;
}
```
we would generate DWARF like:
```
0x0000274f:       DW_TAG_typedef
                    DW_AT_type  (0x0000000000002758 
"__remove_cv_t&lt;value_type&gt;")
                    DW_AT_name  ("remove_cv_t&lt;value_type&gt;")

```

This is an actual libc++ type layout introduced in 
https://github.com/llvm/llvm-project/pull/110355, which uncovered a shortcoming 
of LLDB's data-formatter infrastructure, which caches formatters on the 
contents of `DW_AT_name`.

To unblock the libc++ change, I think we can revert this without much fallout.

Then we have two options for follow-up (or do both):
1. reland this but adjust the LLDB formatter cache so it doesn't cache 
formatters for template specializations
2. implement support for `DW_TAG_template_alias` in LLDB (and make Clang 
generate them by default).

---
Full diff: https://github.com/llvm/llvm-project/pull/110767.diff


2 Files Affected:

- (modified) clang/lib/CodeGen/CGDebugInfo.cpp (-9) 
- (modified) clang/test/CodeGenCXX/debug-info-alias.cpp (+1-1) 


``````````diff
diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp 
b/clang/lib/CodeGen/CGDebugInfo.cpp
index 8887c4de7c4c81..609957b75d6e7e 100644
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -1472,15 +1472,6 @@ llvm::DIType *CGDebugInfo::CreateType(const 
TemplateSpecializationType *Ty,
     return AliasTy;
   }
 
-  // Disable PrintCanonicalTypes here because we want
-  // the DW_AT_name to benefit from the TypePrinter's ability
-  // to skip defaulted template arguments.
-  //
-  // FIXME: Once -gsimple-template-names is enabled by default
-  // and we attach template parameters to alias template DIEs
-  // we don't need to worry about customizing the PrintingPolicy
-  // here anymore.
-  PP.PrintCanonicalTypes = false;
   printTemplateArgumentList(OS, Ty->template_arguments(), PP,
                             TD->getTemplateParameters());
   return DBuilder.createTypedef(Src, OS.str(), getOrCreateFile(Loc),
diff --git a/clang/test/CodeGenCXX/debug-info-alias.cpp 
b/clang/test/CodeGenCXX/debug-info-alias.cpp
index bf2dbee4659592..3cd1446d25270e 100644
--- a/clang/test/CodeGenCXX/debug-info-alias.cpp
+++ b/clang/test/CodeGenCXX/debug-info-alias.cpp
@@ -24,7 +24,7 @@ x::bar<int> bi;
 // CHECK: [[BFLOAT]] = !DIDerivedType(tag: DW_TAG_typedef, name: "bar<float>"
 x::bar<float> bf;
 // CHECK: !DIGlobalVariable(name: "bz",{{.*}} type: [[BBAZ:![0-9]+]]
-// CHECK: [[BBAZ]] = !DIDerivedType(tag: DW_TAG_typedef, name: "bar<baz<int> >"
+// CHECK: [[BBAZ]] = !DIDerivedType(tag: DW_TAG_typedef, name: "bar<baz<int, 
int> >"
 x::bar<baz<int>> bz;
 
 using

``````````

</details>


https://github.com/llvm/llvm-project/pull/110767
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