[clang][ExtractAPI] improve template argument name deduction (llvm#77716)

The names of template arguments in partial specializations or parameters
used as types might be mangled according to index and depth. Instead of
looping through parameter lists to find matches like we do now, they can
be deduced via their QualTypes or as written from the AST.

Author: evelez7

clang/include/clang/ExtractAPI/DeclarationFragments.h

@@ -27,8 +27,6 @@
 #include "clang/AST/TypeLoc.h"
 #include "clang/Basic/Specifiers.h"
 #include "clang/Lex/MacroInfo.h"
-#include "llvm/ADT/SmallVector.h"
-#include "llvm/ADT/StringRef.h"
 #include <vector>
 
 namespace clang {
@@ -315,13 +313,9 @@ class DeclarationFragmentsBuilder {
   static DeclarationFragments
       getFragmentsForTemplateParameters(ArrayRef<NamedDecl *>);
 
-  static std::string
-  getNameForTemplateArgument(const ArrayRef<NamedDecl *>, std::string);
-
-  static DeclarationFragments
-  getFragmentsForTemplateArguments(const ArrayRef<TemplateArgument>,
-                                   ASTContext &,
-                                   const std::optional<ArrayRef<NamedDecl *>>);
+  static DeclarationFragments getFragmentsForTemplateArguments(
+      const ArrayRef<TemplateArgument>, ASTContext &,
+      const std::optional<ArrayRef<TemplateArgumentLoc>>);
 
   static DeclarationFragments getFragmentsForConcept(const ConceptDecl *);
 
@@ -430,12 +424,7 @@ DeclarationFragmentsBuilder::getFunctionSignature(const FunctionT *Function) {
   if (isa<FunctionDecl>(Function) &&
       dyn_cast<FunctionDecl>(Function)->getDescribedFunctionTemplate() &&
       StringRef(ReturnType.begin()->Spelling).starts_with("type-parameter")) {
-    std::string ProperArgName =
-        getNameForTemplateArgument(dyn_cast<FunctionDecl>(Function)
-                                       ->getDescribedFunctionTemplate()
-                                       ->getTemplateParameters()
-                                       ->asArray(),
-                                   ReturnType.begin()->Spelling);
+    std::string ProperArgName = Function->getReturnType().getAsString();
     ReturnType.begin()->Spelling.swap(ProperArgName);
   }
   ReturnType.append(std::move(After));

clang/lib/ExtractAPI/DeclarationFragments.cpp

@@ -14,14 +14,11 @@
 #include "clang/ExtractAPI/DeclarationFragments.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclCXX.h"
-#include "clang/AST/QualTypeNames.h"
 #include "clang/AST/Type.h"
 #include "clang/AST/TypeLoc.h"
-#include "clang/Basic/OperatorKinds.h"
 #include "clang/ExtractAPI/TypedefUnderlyingTypeResolver.h"
 #include "clang/Index/USRGeneration.h"
 #include "llvm/ADT/StringSwitch.h"
-#include <typeinfo>
 
 using namespace clang::extractapi;
 using namespace llvm;
@@ -535,9 +532,7 @@ DeclarationFragmentsBuilder::getFragmentsForVarTemplate(const VarDecl *Var) {
       getFragmentsForType(T, Var->getASTContext(), After);
   if (StringRef(ArgumentFragment.begin()->Spelling)
           .starts_with("type-parameter")) {
-    std::string ProperArgName = getNameForTemplateArgument(
-        Var->getDescribedVarTemplate()->getTemplateParameters()->asArray(),
-        ArgumentFragment.begin()->Spelling);
+    std::string ProperArgName = T.getAsString();
     ArgumentFragment.begin()->Spelling.swap(ProperArgName);
   }
   Fragments.append(std::move(ArgumentFragment))
@@ -570,12 +565,7 @@ DeclarationFragmentsBuilder::getFragmentsForParam(const ParmVarDecl *Param) {
 
   if (StringRef(TypeFragments.begin()->Spelling)
           .starts_with("type-parameter")) {
-    std::string ProperArgName = getNameForTemplateArgument(
-        dyn_cast<FunctionDecl>(Param->getDeclContext())
-            ->getDescribedFunctionTemplate()
-            ->getTemplateParameters()
-            ->asArray(),
-        TypeFragments.begin()->Spelling);
+    std::string ProperArgName = Param->getOriginalType().getAsString();
     TypeFragments.begin()->Spelling.swap(ProperArgName);
   }
 
@@ -668,11 +658,7 @@ DeclarationFragmentsBuilder::getFragmentsForFunction(const FunctionDecl *Func) {
       getFragmentsForType(Func->getReturnType(), Func->getASTContext(), After);
   if (StringRef(ReturnValueFragment.begin()->Spelling)
           .starts_with("type-parameter")) {
-    std::string ProperArgName =
-        getNameForTemplateArgument(Func->getDescribedFunctionTemplate()
-                                       ->getTemplateParameters()
-                                       ->asArray(),
-                                   ReturnValueFragment.begin()->Spelling);
+    std::string ProperArgName = Func->getReturnType().getAsString();
     ReturnValueFragment.begin()->Spelling.swap(ProperArgName);
   }
 
@@ -961,25 +947,6 @@ DeclarationFragmentsBuilder::getFragmentsForTemplateParameters(
   return Fragments;
 }
 
-// Find the name of a template argument from the template's parameters.
-std::string DeclarationFragmentsBuilder::getNameForTemplateArgument(
-    const ArrayRef<NamedDecl *> TemplateParameters, std::string TypeParameter) {
-  // The arg is a generic parameter from a partial spec, e.g.
-  // T in template<typename T> Foo<T, int>.
-  //
-  // Those names appear as "type-parameter-<index>-<depth>", so we must find its
-  // name from the template's parameter list.
-  for (unsigned i = 0; i < TemplateParameters.size(); ++i) {
-    const auto *Parameter =
-        dyn_cast<TemplateTypeParmDecl>(TemplateParameters[i]);
-    if (TypeParameter.compare("type-parameter-" +
-                              std::to_string(Parameter->getDepth()) + "-" +
-                              std::to_string(Parameter->getIndex())) == 0)
-      return std::string(TemplateParameters[i]->getName());
-  }
-  llvm_unreachable("Could not find the name of a template argument.");
-}
-
 // Get fragments for template arguments, e.g. int in template<typename T>
 // Foo<int>;
 //
@@ -989,7 +956,7 @@ std::string DeclarationFragmentsBuilder::getNameForTemplateArgument(
 DeclarationFragments
 DeclarationFragmentsBuilder::getFragmentsForTemplateArguments(
     const ArrayRef<TemplateArgument> TemplateArguments, ASTContext &Context,
-    const std::optional<ArrayRef<NamedDecl *>> TemplateParameters) {
+    const std::optional<ArrayRef<TemplateArgumentLoc>> TemplateArgumentLocs) {
   DeclarationFragments Fragments;
   for (unsigned i = 0, end = TemplateArguments.size(); i != end; ++i) {
     if (i)
@@ -1003,8 +970,10 @@ DeclarationFragmentsBuilder::getFragmentsForTemplateArguments(
 
     if (StringRef(ArgumentFragment.begin()->Spelling)
             .starts_with("type-parameter")) {
-      std::string ProperArgName = getNameForTemplateArgument(
-          TemplateParameters.value(), ArgumentFragment.begin()->Spelling);
+      std::string ProperArgName = TemplateArgumentLocs.value()[i]
+                                      .getTypeSourceInfo()
+                                      ->getType()
+                                      .getAsString();
       ArgumentFragment.begin()->Spelling.swap(ProperArgName);
     }
     Fragments.append(std::move(ArgumentFragment));
@@ -1089,7 +1058,7 @@ DeclarationFragmentsBuilder::getFragmentsForClassTemplatePartialSpecialization(
       .append("<", DeclarationFragments::FragmentKind::Text)
       .append(getFragmentsForTemplateArguments(
           Decl->getTemplateArgs().asArray(), Decl->getASTContext(),
-          Decl->getTemplateParameters()->asArray()))
+          Decl->getTemplateArgsAsWritten()->arguments()))
       .append(">", DeclarationFragments::FragmentKind::Text)
       .append(";", DeclarationFragments::FragmentKind::Text);
 }
@@ -1130,7 +1099,7 @@ DeclarationFragmentsBuilder::getFragmentsForVarTemplatePartialSpecialization(
       .append("<", DeclarationFragments::FragmentKind::Text)
       .append(getFragmentsForTemplateArguments(
           Decl->getTemplateArgs().asArray(), Decl->getASTContext(),
-          Decl->getTemplateParameters()->asArray()))
+          Decl->getTemplateArgsAsWritten()->arguments()))
       .append(">", DeclarationFragments::FragmentKind::Text)
       .append(";", DeclarationFragments::FragmentKind::Text);
 }