[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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@360308 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 11 insertions, 0 deletions

diff --git a/lib/Serialization/ASTReader.cpp b/lib/Serialization/ASTReader.cpp
index 64af0fa8a1..a09507f6da 100644
--- a/lib/Serialization/ASTReader.cpp
+++ b/lib/Serialization/ASTReader.cpp
@@ -8745,6 +8745,11 @@ ASTReader::ReadTemplateName(ModuleFile &F, const RecordData &Record,
     return Context.getOverloadedTemplateName(Decls.begin(), Decls.end());
   }
 
+  case TemplateName::AssumedTemplate: {
+    DeclarationName Name = ReadDeclarationName(F, Record, Idx);
+    return Context.getAssumedTemplateName(Name);
+  }
+
   case TemplateName::QualifiedTemplate: {
     NestedNameSpecifier *NNS = ReadNestedNameSpecifier(F, Record, Idx);
     bool hasTemplKeyword = Record[Idx++];
diff --git a/lib/Serialization/ASTWriter.cpp b/lib/Serialization/ASTWriter.cpp
index accd8ccdfa..5012febd74 100644
--- a/lib/Serialization/ASTWriter.cpp
+++ b/lib/Serialization/ASTWriter.cpp
@@ -5877,6 +5877,12 @@ void ASTRecordWriter::AddTemplateName(TemplateName Name) {
     break;
   }
 
+  case TemplateName::AssumedTemplate: {
+    AssumedTemplateStorage *ADLT = Name.getAsAssumedTemplateName();
+    AddDeclarationName(ADLT->getDeclName());
+    break;
+  }
+
   case TemplateName::QualifiedTemplate: {
     QualifiedTemplateName *QualT = Name.getAsQualifiedTemplateName();
     AddNestedNameSpecifier(QualT->getQualifier());