ignore defaulted args -- see inline discussion

Author: OCHyams

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -1332,38 +1332,21 @@ GetTemplateArgs(const TemplateDecl *TD, const TemplateSpecializationType *Ty) {
       break;
     }
 
-    // Take the next argument.
-    if (!SubstArgs.empty()) {
-      SpecArgs.push_back(SubstArgs.front());
-      SubstArgs = SubstArgs.drop_front();
-      continue;
-    }
-
-    // If SubstArgs is now empty (we're taking from it each iteration) and
-    // this template parameter isn't a pack, then that should mean we're
-    // using default values for the remaining template parameters. Push the
-    // default value for each parameter.
-    if (auto *P = dyn_cast<TemplateTypeParmDecl>(Param)) {
-      assert(P->hasDefaultArgument() &&
-             "expected defaulted template type parameter");
-      SpecArgs.emplace_back(P->getDefaultArgument(),
-                            /*IsNullPtr=*/false,
-                            /*IsDefaulted=*/true);
-    } else if (auto *P = dyn_cast<NonTypeTemplateParmDecl>(Param)) {
-      assert(P->hasDefaultArgument() &&
-             "expected defaulted template non-type parameter");
-      SpecArgs.emplace_back(P->getDefaultArgument(),
-                            /*IsDefaulted=*/true);
-    } else if (auto *P = dyn_cast<TemplateTemplateParmDecl>(Param)) {
-      assert(P->hasDefaultArgument() &&
-             "expected defaulted template template parameter");
-      SpecArgs.emplace_back(
-          P->getDefaultArgument().getArgument().getAsTemplate(),
-          /*IsDefaulted=*/true);
-    } else {
-      llvm_unreachable("Unexpected template parameter kind");
+    // Skip defaulted args.
+    // FIXME: Ideally, we wouldn't do this. We can read the default values
+    // for each parameter. However, defaulted arguments which are dependent
+    // values or dependent types can't (easily?) be resolved here.
+    if (SubstArgs.empty()) {
+      // If SubstArgs is now empty (we're taking from it each iteration) and
+      // this template parameter isn't a pack, then that should mean we're
+      // using default values for the remaining template parameters (after
+      // which there may be an empty pack too which we will ignore).
       break;
     }
+
+    // Take the next argument.
+    SpecArgs.push_back(SubstArgs.front());
+    SubstArgs = SubstArgs.drop_front();
   }
   return SpecArgs;
 }

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

@@ -4,6 +4,7 @@
 //// 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 {
@@ -25,9 +26,13 @@ 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: ![[extraData]] = !{![[NonDefault:[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]] = !{}
+
+//// FIXME: Ideally, we would describe the deafulted args, like this:
+// : ![[extraData]] = !{![[NonDefault:[0-9]+]], ![[T:[0-9]+]], ![[I:[0-9]+]], ![[Ts:[0-9]+]]}
+// : ![[NonDefault]] = !DITemplateTypeParameter(name: "NonDefault", type: ![[int]])
+// : ![[T]] = !DITemplateValueParameter(tag: DW_TAG_GNU_template_template_param, name: "T", defaulted: true, value: !"Y")
+// : ![[I]] = !DITemplateValueParameter(name: "I", type: ![[int]], defaulted: true, value: i32 5)
+// : ![[Ts]] = !DITemplateValueParameter(tag: DW_TAG_GNU_template_parameter_pack, name: "Ts", value: ![[types:[0-9]+]])
+// : ![[types]] = !{}