[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

1 files changed, 4 insertions, 1 deletions

diff --git a/test/CXX/temp/temp.spec/temp.explicit/p5.cpp b/test/CXX/temp/temp.spec/temp.explicit/p5.cpp
index ca1f9a391e..5f0c4547cf 100644
--- a/test/CXX/temp/temp.spec/temp.explicit/p5.cpp
+++ b/test/CXX/temp/temp.spec/temp.explicit/p5.cpp
@@ -8,7 +8,10 @@ namespace N {
   };
 }
 
-template class Z<int>; // expected-error{{explicit instantiation of non-template class 'Z'}}
+template class Z<int>; // expected-error{{explicit instantiation of undeclared template class 'Z'}}
+
+struct Q;
+template class Q<int>; // expected-error{{explicit instantiation of non-template class 'Q'}}
 
 // FIXME: This example from the standard is wrong; note posted to CWG reflector
 // on 10/27/2009