[gcc r15-9362] c++: lambda in constraint of lambda [PR119175]

[Jason Merrill via Gcc-cvs]

https://gcc.gnu.org/g:39892d9618ee0f06dd09271589878b0df7b1e75d

commit r15-9362-g39892d9618ee0f06dd09271589878b0df7b1e75d
Author: Jason Merrill <ja...@redhat.com>
Date:   Wed Apr 9 13:22:56 2025 -0400

    c++: lambda in constraint of lambda [PR119175]
    
    Here when we went to mangle the constraints of from<0>, the outer lambda has
    no mangling scope, but the inner one was treated as having the outer one as
    its scope.  And mangling the outer one means mangling its constraints, which
    include the inner one.  So infinite recursion.
    
    But a lambda closure type isn't a scope that anything should have for
    mangling, the inner lambda should also have no mangling scope.
    
            PR c++/119175
    
    gcc/cp/ChangeLog:
    
            * mangle.cc (decl_mangling_context): Look through lambda type.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp2a/concepts-lambda23.C: New test.

Diff:
---
 gcc/cp/mangle.cc                               |  6 ++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-lambda23.C | 12 ++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/gcc/cp/mangle.cc b/gcc/cp/mangle.cc
index 02129c6b89a1..3d5e96b2c944 100644
--- a/gcc/cp/mangle.cc
+++ b/gcc/cp/mangle.cc
@@ -1048,6 +1048,12 @@ decl_mangling_context (tree decl)
       tree extra = LAMBDA_TYPE_EXTRA_SCOPE (TREE_TYPE (decl));
       if (extra)
        return extra;
+      tcontext = CP_DECL_CONTEXT (decl);
+      if (LAMBDA_TYPE_P (tcontext))
+       /* Lambda type context means this lambda appears between the
+          lambda-introducer and the open brace of another lambda (c++/119175).
+          That isn't a real scope; look further into the enclosing scope.  */
+       return decl_mangling_context (TYPE_NAME (tcontext));
     }
   else if (template_type_parameter_p (decl))
      /* template type parms have no mangling context.  */
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-lambda23.C 
b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda23.C
new file mode 100644
index 000000000000..f44212031dd7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda23.C
@@ -0,0 +1,12 @@
+// PR c++/119175
+// { dg-do compile { target c++20 } }
+
+template<int = 0>
+static void from() requires requires {
+  []<int> requires requires { [] {}; } {};
+}
+{}
+
+int main() {
+  from();
+}