[clang][DebugInfo] Simplify logic to determine DW_AT_default_value fo…

…r template parameters

DWARFv5 added support for labelling template parameters with
DW_AT_default_value to indicate whether the particular instantiation
defaulted parameter. The current implementation only supports a limited
set of possible cases. Namely for non-value-dependent integral template
parameters and simple type template parameters.

Useful cases that don't work are:
1. Type template parameters with defaults that are
   themselves templates. E.g.,
```
template<typename T1, typename T2 = Foo<T1>> class C1;
template<typename T = Foo<int>> class C2;
etc.
```
2. Template template parameters

`clang::isSubstitutedDefaultArgument` already implement the required logic
to determine whether a template argument is defaulted. This patch re-uses
this logic for DWARF CodeGen.

Differential Revision: https://reviews.llvm.org/D139988

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -1999,35 +1999,23 @@ CGDebugInfo::CollectTemplateParams(Optional<TemplateArgs> OArgs,
     const TemplateArgument &TA = Args.Args[i];
     StringRef Name;
     bool defaultParameter = false;
-    if (Args.TList)
+    if (Args.TList) {
       Name = Args.TList->getParam(i)->getName();
+
+      NamedDecl const *ND = Args.TList->getParam(i);
+      defaultParameter = clang::isSubstitutedDefaultArgument(
+          CGM.getContext(), TA, ND, Args.Args, Args.TList->getDepth());
+    }
+
     switch (TA.getKind()) {
     case TemplateArgument::Type: {
       llvm::DIType *TTy = getOrCreateType(TA.getAsType(), Unit);
-
-      if (Args.TList)
-        if (auto *templateType =
-                dyn_cast_or_null<TemplateTypeParmDecl>(Args.TList->getParam(i)))
-          if (templateType->hasDefaultArgument())
-            defaultParameter =
-                templateType->getDefaultArgument() == TA.getAsType();
-
       TemplateParams.push_back(DBuilder.createTemplateTypeParameter(
           TheCU, Name, TTy, defaultParameter));
 
     } break;
     case TemplateArgument::Integral: {
       llvm::DIType *TTy = getOrCreateType(TA.getIntegralType(), Unit);
-      if (Args.TList)
-        if (auto *templateType = dyn_cast_or_null<NonTypeTemplateParmDecl>(
-                Args.TList->getParam(i)))
-          if (templateType->hasDefaultArgument() &&
-              !templateType->getDefaultArgument()->isValueDependent())
-            defaultParameter = llvm::APSInt::isSameValue(
-                templateType->getDefaultArgument()->EvaluateKnownConstInt(
-                    CGM.getContext()),
-                TA.getAsIntegral());
-
       TemplateParams.push_back(DBuilder.createTemplateValueParameter(
           TheCU, Name, TTy, defaultParameter,
           llvm::ConstantInt::get(CGM.getLLVMContext(), TA.getAsIntegral())));

clang/test/CodeGenCXX/debug-info-template-parameter.cpp

@@ -7,26 +7,33 @@
 
 // CHECK: DILocalVariable(name: "f1", {{.*}}, type: ![[TEMPLATE_TYPE:[0-9]+]]
 // CHECK: [[TEMPLATE_TYPE]] = {{.*}}!DICompositeType({{.*}}, templateParams: ![[F1_TYPE:[0-9]+]]
-// CHECK: [[F1_TYPE]] = !{![[FIRST:[0-9]+]], ![[SECOND:[0-9]+]], ![[THIRD:[0-9]+]], ![[FORTH:[0-9]+]]}
+// CHECK: [[F1_TYPE]] = !{![[FIRST:[0-9]+]], ![[SECOND:[0-9]+]], ![[THIRD:[0-9]+]], ![[FORTH:[0-9]+]], ![[FIFTH:[0-9]+]]}
 // CHECK: [[FIRST]] = !DITemplateTypeParameter(name: "T", type: !{{[0-9]*}})
 // CHECK: [[SECOND]] = !DITemplateValueParameter(name: "i", type: !{{[0-9]*}}, value: i32 6)
 // PRE17: [[THIRD]] = !DITemplateValueParameter(name: "b", type: !{{[0-9]*}}, value: i8 0)
 // CXX17: [[THIRD]] = !DITemplateValueParameter(name: "b", type: !{{[0-9]*}}, value: i1 false)
+// CHECK: [[FIFTH]] = !DITemplateTypeParameter(name: "d", type: !{{[0-9]*}})
 
 // CHECK: DILocalVariable(name: "f2", {{.*}}, type: ![[TEMPLATE_TYPE:[0-9]+]]
 // CHECK: [[TEMPLATE_TYPE]] = {{.*}}!DICompositeType({{.*}}, templateParams: ![[F2_TYPE:[0-9]+]]
-// CHECK: [[F2_TYPE]] = !{![[FIRST:[0-9]+]], ![[SECOND:[0-9]+]], ![[THIRD:[0-9]+]], ![[FORTH:[0-9]+]]}
+// CHECK: [[F2_TYPE]] = !{![[FIRST:[0-9]+]], ![[SECOND:[0-9]+]], ![[THIRD:[0-9]+]], ![[FORTH:[0-9]+]], ![[FIFTH:[0-9]+]]}
 // CHECK: [[FIRST]] = !DITemplateTypeParameter(name: "T", type: !{{[0-9]*}}, defaulted: true)
 // CHECK: [[SECOND]] = !DITemplateValueParameter(name: "i", type: !{{[0-9]*}}, defaulted: true, value: i32 3)
 // PRE17: [[THIRD]] = !DITemplateValueParameter(name: "b", type: !{{[0-9]*}}, defaulted: true, value: i8 1)
 // CXX17: [[THIRD]] = !DITemplateValueParameter(name: "b", type: !{{[0-9]*}}, defaulted: true, value: i1 true)
+// CHECK: [[FIFTH]] = !DITemplateTypeParameter(name: "d", type: !{{[0-9]*}}, defaulted: true)
 
-template <typename T = char, int i = 3, bool b = true, int x = sizeof(T)>
+template <typename T>
+class bar {
+};
+
+template <typename T = char, int i = 3, bool b = true, int x = sizeof(T),
+          typename d = bar<T>>
 class foo {
 };
 
 int main() {
-  foo<int, 6, false, 3> f1;
+  foo<int, 6, false, 3, double> f1;
   foo<> f2;
   return 0;
 }