[gcc r16-506] c++: recursive instantiation diagnostic [PR120204]

[Jason Merrill via Gcc-cvs]

https://gcc.gnu.org/g:b5f61a09c1ab3a45398333b50963e533a980b62d

commit r16-506-gb5f61a09c1ab3a45398333b50963e533a980b62d
Author: Jason Merrill <ja...@redhat.com>
Date:   Fri May 9 18:08:43 2025 -0400

    c++: recursive instantiation diagnostic [PR120204]
    
    Here tsubst_baselink was returning error_mark_node silently despite
    tf_error; we need to actually give an error.
    
            PR c++/120204
    
    gcc/cp/ChangeLog:
    
            * pt.cc (tsubst_baselink): Always error if lookup fails.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp1y/constexpr-recursion3.C: New test.

Diff:
---
 gcc/cp/pt.cc                                      | 16 ++++++++++++----
 gcc/testsuite/g++.dg/cpp1y/constexpr-recursion3.C | 15 +++++++++++++++
 2 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 09f74a2814b6..0d64a1cfb128 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -17477,10 +17477,18 @@ tsubst_baselink (tree baselink, tree object_type,
 
       if (!baselink)
        {
-         if ((complain & tf_error)
-             && constructor_name_p (name, qualifying_scope))
-           error ("cannot call constructor %<%T::%D%> directly",
-                  qualifying_scope, name);
+         if (complain & tf_error)
+           {
+             if (constructor_name_p (name, qualifying_scope))
+               error ("cannot call constructor %<%T::%D%> directly",
+                      qualifying_scope, name);
+             else
+               /* Lookup succeeded at parse time, but failed during
+                  instantiation; must be because we're trying to refer to it
+                  while forming its declaration (c++/120204).  */
+               error ("declaration of %<%T::%D%> depends on itself",
+                      qualifying_scope, name);
+           }
          return error_mark_node;
        }
 
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-recursion3.C 
b/gcc/testsuite/g++.dg/cpp1y/constexpr-recursion3.C
new file mode 100644
index 000000000000..cadbdf08c975
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-recursion3.C
@@ -0,0 +1,15 @@
+// PR c++/120204
+// { dg-do compile { target c++14 } }
+
+template<class T, int t>
+struct array{};
+
+template <typename... TArgs> struct ILEArglist {
+  using Sizes = array<int, sizeof...(TArgs)>;
+  static constexpr int size() {        // { dg-bogus "not usable" }
+    Sizes &offsets_c = offsets;        // { dg-error "depends on itself" }
+    return 0;
+  }
+  array<char, size()> offsets(); // { dg-error "constant expression" }
+};
+auto arglist = ILEArglist<>();