[Clang] Improve diagnostics when using a concept as template argument

When using the name of a template variable or concept in places
where an expression was expected, Clang would drop the cxxscope token
preceeding it, if any.

This leads to subpar diagnostics - complaining about the
identifier being undeclared as clang would not know to look into a
non-global scope.

We make sure the scope is preserved.

When encountering `ns::Concept foo x;`, Clang would also fail
to provide the same quality as it does at global scope.

Reviewed By: aaron.ballman, erichkeane

Differential Revision: https://reviews.llvm.org/D146719

1 files changed, 44 insertions, 0 deletions

diff --git a/clang/test/Parser/cxx-template-template-recovery.cpp b/clang/test/Parser/cxx-template-template-recovery.cpp
new file mode 100644
index 0000000..1230c86
--- /dev/null
+++ b/clang/test/Parser/cxx-template-template-recovery.cpp
@@ -0,0 +1,44 @@
+// RUN: %clang_cc1 -std=c++20 -verify -fsyntax-only %s
+
+namespace a {
+  template <typename T>
+  concept C1 = true; // #C1
+
+  template <typename T>
+  auto V1 = true; // #V1
+
+  namespace b {
+    template <typename T>
+    concept C2 = true; // #C2
+    template <typename T>
+    auto V2 = true; // #V2
+  }
+}
+
+template <typename T>
+concept C3 = true; // #C3
+template <typename T>
+auto V3 = true; // #V3
+template <template <typename T> typename C>
+constexpr bool test = true;
+
+static_assert(test<a::C1>); // expected-error {{too few template arguments for concept 'C1'}} \
+                            // expected-note@#C1 {{here}}
+static_assert(test<a::b::C2>); // expected-error {{too few template arguments for concept 'C2'}} \
+                               // expected-note@#C2 {{here}}
+static_assert(test<C3>); // expected-error {{too few template arguments for concept 'C3'}} \
+                         // expected-note@#C3 {{here}}
+
+static_assert(test<a::V1>); // expected-error {{use of variable template 'V1' requires template arguments}} \
+                            // expected-note@#V1 {{here}}
+static_assert(test<a::b::V2>); // expected-error {{use of variable template 'V2' requires template arguments}} \
+                            // expected-note@#V2 {{here}}
+static_assert(test<V3>); // expected-error {{use of variable template 'V3' requires template arguments}} \
+                         // expected-note@#V3 {{here}}
+
+
+void f() {
+    C3 t1 = 0;  // expected-error {{expected 'auto' or 'decltype(auto)' after concept name}}
+    a::C1 t2 = 0; // expected-error {{expected 'auto' or 'decltype(auto)' after concept name}}
+    a::b::C2 t3 = 0; // expected-error {{expected 'auto' or 'decltype(auto)' after concept name}}
+}