[c++20] Implement P0846R0: allow (ADL-only) calls to template-ids whose

template name is not visible to unqualified lookup.

In order to support this without a severe degradation in our ability to
diagnose typos in template names, this change significantly restructures
the way we handle template-id-shaped syntax for which lookup of the
template name finds nothing.

Instead of eagerly diagnosing an undeclared template name, we now form a
placeholder template-name representing a name that is known to not find
any templates. When the parser sees such a name, it attempts to
disambiguate whether we have a less-than comparison or a template-id.
Any diagnostics or typo-correction for the name are delayed until its
point of use.

The upshot should be a small improvement of our diagostic quality
overall: we now take more syntactic context into account when trying to
resolve an undeclared identifier on the left hand side of a '<'. In
fact, this works well enough that the backwards-compatible portion (for
an undeclared identifier rather than a lookup that finds functions but
no function templates) is enabled in all language modes.

llvm-svn: 360308

6 files changed, 46 insertions, 2 deletions

diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index 63488853759..c144e01626e 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -5226,6 +5226,11 @@ ASTContext::getNameForTemplate(TemplateName Name,
     return DeclarationNameInfo((*Storage->begin())->getDeclName(), NameLoc);
   }
 
+  case TemplateName::AssumedTemplate: {
+    AssumedTemplateStorage *Storage = Name.getAsAssumedTemplateName();
+    return DeclarationNameInfo(Storage->getDeclName(), NameLoc);
+  }
+
   case TemplateName::DependentTemplate: {
     DependentTemplateName *DTN = Name.getAsDependentTemplateName();
     DeclarationName DName;
@@ -5273,7 +5278,8 @@ TemplateName ASTContext::getCanonicalTemplateName(TemplateName Name) const {
   }
 
   case TemplateName::OverloadedTemplate:
-    llvm_unreachable("cannot canonicalize overloaded template");
+  case TemplateName::AssumedTemplate:
+    llvm_unreachable("cannot canonicalize unresolved template");
 
   case TemplateName::DependentTemplate: {
     DependentTemplateName *DTN = Name.getAsDependentTemplateName();
@@ -7620,6 +7626,13 @@ ASTContext::getOverloadedTemplateName(UnresolvedSetIterator Begin,
   return TemplateName(OT);
 }
 
+/// Retrieve a template name representing an unqualified-id that has been
+/// assumed to name a template for ADL purposes.
+TemplateName ASTContext::getAssumedTemplateName(DeclarationName Name) const {
+  auto *OT = new (*this) AssumedTemplateStorage(Name);
+  return TemplateName(OT);
+}
+
 /// Retrieve the template name that represents a qualified
 /// template name such as \c std::vector.
 TemplateName
diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
index 7ce2abee6ce..d10db32e872 100644
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -8126,6 +8126,14 @@ Expected<TemplateName> ASTImporter::Import_New(TemplateName From) {
                                                ToTemplates.end());
   }
 
+  case TemplateName::AssumedTemplate: {
+    AssumedTemplateStorage *FromStorage = From.getAsAssumedTemplateName();
+    auto DeclNameOrErr = Import_New(FromStorage->getDeclName());
+    if (!DeclNameOrErr)
+      return DeclNameOrErr.takeError();
+    return ToContext.getAssumedTemplateName(*DeclNameOrErr);
+  }
+
   case TemplateName::QualifiedTemplate: {
     QualifiedTemplateName *QTN = From.getAsQualifiedTemplateName();
     auto QualifierOrErr = Import_New(QTN->getQualifier());
diff --git a/clang/lib/AST/ASTStructuralEquivalence.cpp b/clang/lib/AST/ASTStructuralEquivalence.cpp
index bf084e325cc..3eafa3d9508 100644
--- a/clang/lib/AST/ASTStructuralEquivalence.cpp
+++ b/clang/lib/AST/ASTStructuralEquivalence.cpp
@@ -180,6 +180,12 @@ static bool IsStructurallyEquivalent(StructuralEquivalenceContext &Context,
     return I1 == E1 && I2 == E2;
   }
 
+  case TemplateName::AssumedTemplate: {
+    AssumedTemplateStorage *TN1 = N1.getAsAssumedTemplateName(),
+                           *TN2 = N1.getAsAssumedTemplateName();
+    return TN1->getDeclName() == TN2->getDeclName();
+  }
+
   case TemplateName::QualifiedTemplate: {
     QualifiedTemplateName *QN1 = N1.getAsQualifiedTemplateName(),
                           *QN2 = N2.getAsQualifiedTemplateName();
diff --git a/clang/lib/AST/ItaniumMangle.cpp b/clang/lib/AST/ItaniumMangle.cpp
index 1dba7ccc90a..8bc19a34e6f 100644
--- a/clang/lib/AST/ItaniumMangle.cpp
+++ b/clang/lib/AST/ItaniumMangle.cpp
@@ -1891,6 +1891,7 @@ void CXXNameMangler::mangleType(TemplateName TN) {
     break;
 
   case TemplateName::OverloadedTemplate:
+  case TemplateName::AssumedTemplate:
     llvm_unreachable("can't mangle an overloaded template name as a <type>");
 
   case TemplateName::DependentTemplate: {
@@ -2030,6 +2031,7 @@ bool CXXNameMangler::mangleUnresolvedTypeOrSimpleId(QualType Ty,
     }
 
     case TemplateName::OverloadedTemplate:
+    case TemplateName::AssumedTemplate:
     case TemplateName::DependentTemplate:
       llvm_unreachable("invalid base for a template specialization type");
 
diff --git a/clang/lib/AST/ODRHash.cpp b/clang/lib/AST/ODRHash.cpp
index 9d484bd5de5..ba3b2ee49c5 100644
--- a/clang/lib/AST/ODRHash.cpp
+++ b/clang/lib/AST/ODRHash.cpp
@@ -140,6 +140,7 @@ void ODRHash::AddTemplateName(TemplateName Name) {
     break;
   // TODO: Support these cases.
   case TemplateName::OverloadedTemplate:
+  case TemplateName::AssumedTemplate:
   case TemplateName::QualifiedTemplate:
   case TemplateName::DependentTemplate:
   case TemplateName::SubstTemplateTemplateParm:
diff --git a/clang/lib/AST/TemplateName.cpp b/clang/lib/AST/TemplateName.cpp
index 2474d6278b2..06e1dcec744 100644
--- a/clang/lib/AST/TemplateName.cpp
+++ b/clang/lib/AST/TemplateName.cpp
@@ -66,6 +66,8 @@ TemplateName::TemplateName(void *Ptr) {
 TemplateName::TemplateName(TemplateDecl *Template) : Storage(Template) {}
 TemplateName::TemplateName(OverloadedTemplateStorage *Storage)
     : Storage(Storage) {}
+TemplateName::TemplateName(AssumedTemplateStorage *Storage)
+    : Storage(Storage) {}
 TemplateName::TemplateName(SubstTemplateTemplateParmStorage *Storage)
     : Storage(Storage) {}
 TemplateName::TemplateName(SubstTemplateTemplateParmPackStorage *Storage)
@@ -87,6 +89,8 @@ TemplateName::NameKind TemplateName::getKind() const {
     = Storage.get<UncommonTemplateNameStorage*>();
   if (uncommon->getAsOverloadedStorage())
     return OverloadedTemplate;
+  if (uncommon->getAsAssumedTemplateName())
+    return AssumedTemplate;
   if (uncommon->getAsSubstTemplateTemplateParm())
     return SubstTemplateTemplateParm;
   return SubstTemplateTemplateParmPack;
@@ -113,6 +117,14 @@ OverloadedTemplateStorage *TemplateName::getAsOverloadedTemplate() const {
   return nullptr;
 }
 
+AssumedTemplateStorage *TemplateName::getAsAssumedTemplateName() const {
+  if (UncommonTemplateNameStorage *Uncommon =
+          Storage.dyn_cast<UncommonTemplateNameStorage *>())
+    return Uncommon->getAsAssumedTemplateName();
+
+  return nullptr;
+}
+
 SubstTemplateTemplateParmStorage *
 TemplateName::getAsSubstTemplateTemplateParm() const {
   if (UncommonTemplateNameStorage *uncommon =
@@ -230,7 +242,9 @@ TemplateName::print(raw_ostream &OS, const PrintingPolicy &Policy,
   } else if (SubstTemplateTemplateParmPackStorage *SubstPack
                                         = getAsSubstTemplateTemplateParmPack())
     OS << *SubstPack->getParameterPack();
-  else {
+  else if (AssumedTemplateStorage *Assumed = getAsAssumedTemplateName()) {
+    Assumed->getDeclName().print(OS, Policy);
+  } else {
     OverloadedTemplateStorage *OTS = getAsOverloadedTemplate();
     (*OTS->begin())->printName(OS);
   }