[PATCH] c++, v3: Fix ICE with parameter uses in expansion stmts [PR121575]

[Jakub Jelinek]

On Wed, Aug 27, 2025 at 01:45:38PM +0200, Jason Merrill wrote:
> > would be wrong.  Guess
> >           if (DECL_CONTEXT (t)
> >               && !uses_template_parms (DECL_CONTEXT (t)))
> >             RETURN (t);
> > would fix these ICEs, shall I go with that
> 
> Sounds good.

The following passed bootstrap/regtest on x86_64-linux and i686-linux, ok?

2025-08-27  Jakub Jelinek  <ja...@redhat.com>
            Jason Merrill  <ja...@redhat.com>

        PR c++/121575
        * pt.cc (tsubst_expr) <case PARM_DECL>: If DECL_CONTEXT (t) isn't a
        template return t for PARM_DECLs without local specialization.

        * g++.dg/cpp26/expansion-stmt20.C: New test.

--- gcc/cp/pt.cc.jj     2025-08-25 16:29:07.122358976 +0200
+++ gcc/cp/pt.cc        2025-08-25 16:45:15.210314704 +0200
@@ -22321,6 +22321,11 @@ tsubst_expr (tree t, tree args, tsubst_f
              if (DECL_NAME (t) == this_identifier && current_class_ptr)
                RETURN (current_class_ptr);
 
+             /* Parameters of non-templates map to themselves (e.g. in
+                expansion statement body).  */
+             if (DECL_CONTEXT (t) && !uses_template_parms (DECL_CONTEXT (t)))
+               RETURN (t);
+
              /* This can happen for a parameter name used later in a function
                 declaration (such as in a late-specified return type).  Just
                 make a dummy decl, since it's only used for its type.  */
--- gcc/testsuite/g++.dg/cpp26/expansion-stmt20.C.jj    2025-08-25 
16:43:07.510020392 +0200
+++ gcc/testsuite/g++.dg/cpp26/expansion-stmt20.C       2025-08-25 
16:43:07.510020392 +0200
@@ -0,0 +1,59 @@
+// PR c++/121575
+// { dg-do run { target c++11 } }
+// { dg-options "" }
+
+struct A { int x, y; };
+int c;
+
+void
+qux (A p)
+{
+  if (p.x != 1 || p.y != 3)
+    __builtin_abort ();
+  ++c;
+}
+
+void
+foo ()
+{
+  A p { 1, 3 };
+  template for (auto _ : {})                   // { dg-warning "'template for' 
only available with" "" { target c++23_down } }
+    qux (p);
+  template for (auto _ : { 0 })                        // { dg-warning 
"'template for' only available with" "" { target c++23_down } }
+    qux (p);
+}
+
+void
+bar (A p)
+{
+  template for (auto _ : {})                   // { dg-warning "'template for' 
only available with" "" { target c++23_down } }
+    qux (p);
+  template for (auto _ : { 0, 1 })             // { dg-warning "'template for' 
only available with" "" { target c++23_down } }
+    qux (p);
+}
+
+A
+baz ()
+{
+  A p { 1, 3 };
+  template for (auto _ : {})                   // { dg-warning "'template for' 
only available with" "" { target c++23_down } }
+    qux (p);
+  template for (auto _ : { 0, 1, 2 })          // { dg-warning "'template for' 
only available with" "" { target c++23_down } }
+    qux (p);
+  return p;
+}
+
+int
+main ()
+{
+  foo ();
+  if (c != 1)
+    __builtin_abort ();
+  bar ({ 1, 3 });
+  if (c != 3)
+    __builtin_abort ();
+  if (baz ().x != 1 || baz ().y != 3)
+    __builtin_abort ();
+  if (c != 9)
+    __builtin_abort ();
+}


        Jakub