[PATCH v2] c++: Check invalid use of constrained auto with trailing return type [PR100589]

[Da Xie]

Add check for constrained auto type specifier in function declaration or
function type declaration with trailing return type. Issue error if such
usage is detected.

Test file renamed, and added a new test for type declaration.

Successfully bootstrapped and regretested on x86_64-pc-linux-gnu:
Added 6 passed and 4 unsupported tests.

        PR c++/100589

gcc/cp/ChangeLog:

        * decl.cc (grokdeclarator): Issue an error for a declarator with
        constrained auto type specifier and trailing return types. Include
        function names if available.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp2a/concepts-pr100589.C: New test.

Signed-off-by: Da Xie <xxie...@163.com>
---
 gcc/cp/decl.cc                                 | 13 +++++++++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-pr100589.C |  9 +++++++++
 2 files changed, 22 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-pr100589.C

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 9ca8c6c4481..337ee65752e 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -14037,6 +14037,19 @@ grokdeclarator (const cp_declarator *declarator,
                                "invalid use of %<decltype(auto)%>");
                    return error_mark_node;
                  }
+               else if (is_constrained_auto (type))
+                 {
+                   if (funcdecl_p)
+                     error_at (typespec_loc,
+                               "%qs function with trailing return type "
+                               "has constrained %<auto%> type specifier "
+                               "rather than plain %<auto%>",
+                               name);
+                   else
+                     error_at (typespec_loc,
+                               "invalid use of constrained %<auto%> type");
+                   return error_mark_node;
+                 }
                tree tmpl = CLASS_PLACEHOLDER_TEMPLATE (auto_node);
                if (!tmpl)
                  if (tree late_auto = type_uses_auto (late_return_type))
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-pr100589.C 
b/gcc/testsuite/g++.dg/cpp2a/concepts-pr100589.C
new file mode 100644
index 00000000000..0c60d31f29b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-pr100589.C
@@ -0,0 +1,9 @@
+// PR c++/100589
+// { dg-do compile { target c++20 } }
+
+template<class T>
+concept false_concept = false;
+
+false_concept auto f() -> int; // { dg-error "'f' .* trailing return type.* 
constrained 'auto'" }
+
+using type = false_concept auto() -> int; // { dg-error "invalid use of 
constrained 'auto' type"}
-- 
2.34.1