[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

1 files changed, 14 insertions, 1 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