[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, 5 insertions, 2 deletions

diff --git a/clang/test/SemaTemplate/rdar9173693.cpp b/clang/test/SemaTemplate/rdar9173693.cpp
index 86b49545a30..ed09a64a0dd 100644
--- a/clang/test/SemaTemplate/rdar9173693.cpp
+++ b/clang/test/SemaTemplate/rdar9173693.cpp
@@ -2,5 +2,8 @@
 
 // <rdar://problem/9173693>
 template< bool C > struct assert { };
-template< bool > struct assert_arg_pred_impl { }; // expected-note 3 {{declared here}}
-template< typename Pred > assert<false> assert_not_arg( void (*)(Pred), typename assert_arg_pred<Pred>::type ); // expected-error 5 {{}}
+// FIXME: We diagnose the same problem multiple times here because we have no
+// way to indicate in the token stream that we already tried to annotate a
+// template-id and we failed.
+template< bool > struct assert_arg_pred_impl { }; // expected-note 4 {{declared here}}
+template< typename Pred > assert<false> assert_not_arg( void (*)(Pred), typename assert_arg_pred<Pred>::type ); // expected-error 6 {{}}