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

[Da Xie]

This bug comes from a missing check when a function declaration has a
late-specified return type and a constrained auto type specifier. By
adding such a check, we can catch such uses and diagnose them as errors.

Successfully bootstrapped and regretested on x86_64-pc-linux-gnu:
adds 6 new test results and 4 UNSUPPORTED results to g++.sum.  The 4
UNSUPPORTED results are due to the fact that concepts are not supported
before c++20, so tests cannot pass with c++17 and lower standards.

        PR c++/100589

gcc/cp/ChangeLog:

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

gcc/testsuite/ChangeLog:

        * g++.dg/other/pr100589.C: New test.

Signed-off-by: Da Xie <xxie...@163.com>
---
 gcc/cp/decl.cc                        | 7 +++++++
 gcc/testsuite/g++.dg/other/pr100589.C | 7 +++++++
 2 files changed, 14 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/other/pr100589.C

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 9ca8c6c4481..f283ed996a2 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -14037,6 +14037,13 @@ grokdeclarator (const cp_declarator *declarator,
                                "invalid use of %<decltype(auto)%>");
                    return error_mark_node;
                  }
+               else if (is_constrained_auto (type))
+                 {
+                   error_at (typespec_loc, "invalid use of "
+                             "constrained auto type specifier "
+                             "in function with trailing return 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/other/pr100589.C 
b/gcc/testsuite/g++.dg/other/pr100589.C
new file mode 100644
index 00000000000..3fa054ed34d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/other/pr100589.C
@@ -0,0 +1,7 @@
+// PR c++/100589
+// { dg-do compile { target c++20 } }
+
+template<class T>
+concept false_concept = false;
+
+false_concept auto f() -> int; // { dg-error "constrained auto type 
specifier.* trailing return type" }
\ No newline at end of file
-- 
2.34.1