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

[Orlando Cazalet-Hyams via cfe-commits]

================
@@ -0,0 +1,38 @@
+// 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 causes DW_TAG_template_alias emission for
+//// template aliases with default parameter values. See template-alias.cpp for
+////  more template alias tests.
+//// FIXME: We currently do not emit defaulted arguments.
+
+template<typename T>
+struct X {
+  char m;
+};
+
+template<typename T>
+struct Y {
+  char n;
+};
+
+template <typename NonDefault, template <typename C> class T = Y, int I = 5, typename... Ts>
+using A = X<NonDefault>;
+
+//// We should be able to emit type alias metadata which describes all the
+//// values, including the defaulted parameters and empty parameter pack.
+A<int> a;
+
+// CHECK: !DIDerivedType(tag: DW_TAG_template_alias, name: "A", file: ![[#]], line: [[#]], baseType: ![[baseType:[0-9]+]], extraData: ![[extraData:[0-9]+]])
+// CHECK: ![[baseType]] = distinct !DICompositeType(tag: DW_TAG_structure_type, name: "X",
+// CHECK: ![[int:[0-9]+]] = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+// CHECK: ![[extraData]] = !{![[NonDefault:[0-9]+]]}
+// CHECK: ![[NonDefault]] = !DITemplateTypeParameter(name: "NonDefault", type: ![[int]])
+
+//// FIXME: Ideally, we would describe the deafulted args, like this:
+// : ![[extraData]] = !{![[NonDefault:[0-9]+]], ![[T:[0-9]+]], ![[I:[0-9]+]], ![[Ts:[0-9]+]]}
----------------
OCHyams wrote:

I think the test will already start failing if this gets fixed because the current `extraData` metadata tuple contents check will fail (it'll have more than one element). I'm still happy to add CHECK-NOTs if you'd like, just thought I'd raise this in case it changes your stance.

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