Re: [C++ Patch] PR 92804 [10 Regression] ICE trying to use concept as a nested-name-specifier

[Paolo Carlini]

Hi,

On 22/01/20 17:27, Jason Merrill wrote:
On 1/22/20 10:22 AM, Paolo Carlini wrote:
Hi,
in this simple issue we either wrongly talked about variable 
template-id in c++17 mode or ICEd in c++2a. I think we simply want to 
handle concept-ids first, both as represented in c++17 mode and as 
represented in c++2a mode. Tested x86_64-linux.
What happens if you try to use a function template/function concept name?

AFAICS no ICEs, no regressions but indeed we can do better, tell 
concepts from function templates. The below does that and passes testing 
but I'm not sure if the wording is optimal, whether we always want to 
talk about concept-id.

Thanks! Paolo.

///////////////////////


diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index caafbefda8e..85a4ea5be82 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -6467,16 +6467,27 @@ cp_parser_nested_name_specifier_opt (cp_parser *parser,
                      tree fns = get_fns (tid);
                      if (OVL_SINGLE_P (fns))
                        tmpl = OVL_FIRST (fns);
-                     error_at (token->location, "function template-id %qD "
-                               "in nested-name-specifier", tid);
+                     if (function_concept_p (fns))
+                       error_at (token->location, "concept-id %qD "
+                                 "in nested-name-specifier", tid);
+                     else
+                       error_at (token->location, "function template-id "
+                                 "%qD in nested-name-specifier", tid);
                    }
                  else
                    {
-                     /* Variable template.  */
                      tmpl = TREE_OPERAND (tid, 0);
-                     gcc_assert (variable_template_p (tmpl));
-                     error_at (token->location, "variable template-id %qD "
-                               "in nested-name-specifier", tid);
+                     if (variable_concept_p (tmpl)
+                         || standard_concept_p (tmpl))
+                       error_at (token->location, "concept-id %qD "
+                                 "in nested-name-specifier", tid);
+                     else
+                       {
+                         /* Variable template.  */
+                         gcc_assert (variable_template_p (tmpl));
+                         error_at (token->location, "variable template-id "
+                                   "%qD in nested-name-specifier", tid);
+                       }
                    }
                  if (tmpl)
                    inform (DECL_SOURCE_LOCATION (tmpl),
diff --git a/gcc/testsuite/g++.dg/concepts/pr92804-1.C 
b/gcc/testsuite/g++.dg/concepts/pr92804-1.C
new file mode 100644
index 00000000000..cc21426bb9e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/concepts/pr92804-1.C
@@ -0,0 +1,19 @@
+// { dg-do compile { target c++17 } }
+// { dg-options "-fconcepts" }
+
+template<typename T>
+concept foo = true;  // { dg-message "declared here" }
+
+template<typename T>
+void bar(T t)
+{
+  if constexpr (foo<T>::value)  // { dg-error "17:concept-id .foo<T>. in 
nested-name-specifier" }
+  // { dg-error "expected|value" "" { target c++17 } .-1 }
+  {
+  }
+}
+
+int main()
+{
+  bar(1);
+}
diff --git a/gcc/testsuite/g++.dg/concepts/pr92804-2.C 
b/gcc/testsuite/g++.dg/concepts/pr92804-2.C
new file mode 100644
index 00000000000..98ec4ae598e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/concepts/pr92804-2.C
@@ -0,0 +1,19 @@
+// { dg-do compile { target c++17 } }
+// { dg-options "-w -fconcepts" }
+
+template<typename T>
+concept bool foo() { return true; };  // { dg-message "declared here" }
+
+template<typename T>
+void bar(T t)
+{
+  if constexpr (foo<T>::value)  // { dg-error "17:concept-id .foo<T>. in 
nested-name-specifier" }
+  // { dg-error "expected|value" "" { target *-*-* } .-1 }
+  {
+  }
+}
+
+int main()
+{
+  bar(1);
+}