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

[Orlando Cazalet-Hyams via llvm-commits]

================
@@ -0,0 +1,33 @@
+// 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.
+
+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]+]], ![[T:[0-9]+]], ![[I:[0-9]+]], ![[Ts:[0-9]+]]}
+// CHECK: ![[NonDefault]] = !DITemplateTypeParameter(name: "NonDefault", type: ![[int]])
+// CHECK: ![[T]] = !DITemplateValueParameter(tag: DW_TAG_GNU_template_template_param, name: "T", defaulted: true, value: !"Y")
+// CHECK: ![[I]] = !DITemplateValueParameter(name: "I", type: ![[int]], defaulted: true, value: i32 5)
+// CHECK: ![[Ts]] = !DITemplateValueParameter(tag: DW_TAG_GNU_template_parameter_pack, name: "Ts", value: ![[types:[0-9]+]])
+// CHECK: ![[types]] = !{}
----------------
OCHyams wrote:

For anyone interested, the dwarf for this test case looks like this:
```
$ clang -gsimple-template-names test9.cpp -g -c -o - -gtemplate-alias | llvm-dwarfdump - --name A --show-children
-:      file format elf64-x86-64

0x00000029: DW_TAG_template_alias
              DW_AT_type        (0x00000044 "X<int>")
              DW_AT_name        ("A")
              DW_AT_decl_file   ("/home/och/scratch/test9.cpp")
              DW_AT_decl_line   (13)

0x00000031:   DW_TAG_template_type_parameter
                DW_AT_type      (0x0000005a "int")
                DW_AT_name      ("NonDefault")

0x00000037:   DW_TAG_GNU_template_template_param
                DW_AT_name      ("T")
                DW_AT_default_value     (true)
                DW_AT_GNU_template_name ("Y")

0x0000003a:   DW_TAG_template_value_parameter
                DW_AT_type      (0x0000005a "int")
                DW_AT_name      ("I")
                DW_AT_default_value     (true)
                DW_AT_const_value       (5)

0x00000041:   DW_TAG_GNU_template_parameter_pack
                DW_AT_name      ("Ts")

0x00000043:   NULL
```

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