[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, 1 insertions, 3 deletions

diff --git a/clang/test/Modules/module-private.cpp b/clang/test/Modules/module-private.cpp
index 42ab185760b..30957865d1c 100644
--- a/clang/test/Modules/module-private.cpp
+++ b/clang/test/Modules/module-private.cpp
@@ -18,9 +18,7 @@ int test_broken() {
   int *ip = 0;
   f1(ip); // expected-error{{use of undeclared identifier 'f1'}}
 
-  vector<int> vec; // expected-error{{use of undeclared identifier 'vector'}} \
-  // expected-error{{expected '(' for function-style cast or type construction}} \
-  // expected-error{{use of undeclared identifier 'vec'}}
+  vector<int> vec; // expected-error{{no template named 'vector'}}
 
   VisibleStruct vs;
   vs.field = 0; // expected-error{{no member named 'field' in 'VisibleStruct'}}