[gcc r17-3301] c++: ICE w/ maybe_warn_nodiscard on non-dep call [PR123495, PR126860]

https://gcc.gnu.org/g:0e74517b70d06105bd40c8caa6dc192b4af3ccff

commit r17-3301-g0e74517b70d06105bd40c8caa6dc192b4af3ccff
Author: Patrick Palka <[email protected]>
Date:   Fri Aug 14 14:33:46 2026 -0400

    c++: ICE w/ maybe_warn_nodiscard on non-dep call [PR123495, PR126860]
    
    Another latent issue uncovered by the NON_DEPENDENT_EXPR removal.
    cp_get_fndecl_from_callee, called from maybe_warn_nodiscard, tries to
    fold the templated callee via constant evaluation, but constant
    evaluation expects non-templated trees.  So use fold_non_dependent_expr
    which instantiates templated trees beforehand.  We could use f_n_d_init
    also but there should be no difference for a result that doesn't end up
    as NULL_TREE.
    
            PR c++/123495
            PR c++/126860
    
    gcc/cp/ChangeLog:
    
            * cvt.cc (cp_get_fndecl_from_callee): Use fold_non_dependent_expr
            instead of maybe_constant_init.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/template/non-dependent36.C: New test.
            * g++.dg/template/non-dependent37.C: New test.
    
    Reviewed-by: Jason Merrill <[email protected]>

Diff:
---
 gcc/cp/cvt.cc                                   | 2 +-
 gcc/testsuite/g++.dg/template/non-dependent36.C | 4 ++++
 gcc/testsuite/g++.dg/template/non-dependent37.C | 9 +++++++++
 3 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/cvt.cc b/gcc/cp/cvt.cc
index a378b51a2bed..33d528b3c324 100644
--- a/gcc/cp/cvt.cc
+++ b/gcc/cp/cvt.cc
@@ -1069,7 +1069,7 @@ cp_get_fndecl_from_callee (tree fn, bool fold /* = true */)
   if (type == NULL_TREE || !INDIRECT_TYPE_P (type))
     return NULL_TREE;
   if (fold)
-    fn = maybe_constant_init (fn);
+    fn = fold_non_dependent_expr (fn);
   STRIP_NOPS (fn);
   if (TREE_CODE (fn) == ADDR_EXPR
       || TREE_CODE (fn) == FDESC_EXPR)
diff --git a/gcc/testsuite/g++.dg/template/non-dependent36.C b/gcc/testsuite/g++.dg/template/non-dependent36.C
new file mode 100644
index 000000000000..a4bf06974028
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/non-dependent36.C
@@ -0,0 +1,4 @@
+// PR c++/123495
+
+template <int N>
+void foo () { ((int (*) ()) 0) (); }
diff --git a/gcc/testsuite/g++.dg/template/non-dependent37.C b/gcc/testsuite/g++.dg/template/non-dependent37.C
new file mode 100644
index 000000000000..ad58cf6f10ce
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/non-dependent37.C
@@ -0,0 +1,9 @@
+// PR c++/126860
+
+struct A {
+  int (*foo)();
+};
+
+template <typename T> struct S : public A {
+  void bar() { A::foo(); }
+};