[PATCH] c++: Fix ICE during mangling of a conversion operator function template [PR126093]

Hi!

We ICE when trying to mangle the TEMPLATE_DECL of a conversion operator,
because FNDECL_USED_AUTO macro requires FUNCTION_DECL, but here we
have a TEMPLATE_DECL instead.
In this case, DECL_CONV_FN_TYPE contains the right type, so this patch
just guards the FNDECL_USED_AUTO macro use on FUNCTION_DECLs.

So far tested on
GXX_TESTSUITE_STDS=98,11,14,17,20,23,26,29 make check-g++ -j32 -k RUNTESTFLAGS="dg.exp='reflect/*'"
Ok for trunk if it passes full bootstrap/regtest?

2026-08-07  Jakub Jelinek  <[email protected]>

	PR c++/126093
	* mangle.cc (write_unqualified_name): Only check FNDECL_USED_AUTO
	on FUNCTION_DECLs.

	* g++.dg/reflect/mangle1.C: Add 2 new tests for reflections of
	conversion templates.

--- gcc/cp/mangle.cc.jj	2026-08-06 13:24:02.000000000 +0200
+++ gcc/cp/mangle.cc	2026-08-07 14:06:15.092252321 +0200
@@ -1567,7 +1567,8 @@ write_unqualified_name (tree decl)
 	      fn_type = get_mostly_instantiated_function_type (decl);
 	      type = TREE_TYPE (fn_type);
 	    }
-	  else if (FNDECL_USED_AUTO (decl))
+	  else if (TREE_CODE (decl) == FUNCTION_DECL
+		   && FNDECL_USED_AUTO (decl))
 	    type = DECL_SAVED_AUTO_RETURN_TYPE (decl);
 	  else
 	    type = DECL_CONV_FN_TYPE (decl);
--- gcc/testsuite/g++.dg/reflect/mangle1.C.jj	2026-08-06 13:24:02.480925614 +0200
+++ gcc/testsuite/g++.dg/reflect/mangle1.C	2026-08-07 14:30:30.275043881 +0200
@@ -15,6 +15,10 @@ struct S : B {
   int : 0;
   static int var;
 };
+struct Q {
+  template <typename T>
+  operator T () { return T (); }
+};
 struct W { union {}; union {}; union {}; union {}; };
 template <auto> struct TCls {};
 template <auto> void TFn ();
@@ -65,6 +69,10 @@ namespace NS2 {
   struct Z {
   };
   struct AA { int a, b; };
+  struct Q {
+    template <int N>
+    operator int () { return N; }
+  };
 }
 
 constexpr auto ctx = std::meta::access_context::current ();
@@ -147,6 +155,8 @@ baz (int x)
   bar <241, ^^NS2::TCls> (); // class template
   bar <250, ^^TFn> (); // function template
   bar <251, ^^NS2::TFn> (); // function template
+  bar <252, members_of (^^Q, ctx)[0]> (); // function template
+  bar <253, members_of (^^NS2::Q, ctx)[0]> (); // function template
   bar <260, ^^TVar> (); // variable template
   bar <261, ^^NS2::TVar> (); // variable template
   bar <270, ^^TAlias> (); // alias template
@@ -239,6 +249,8 @@ baz (int x)
 // { dg-final { scan-assembler "_Z3barILi241ELDmct3NS24TClsEEvv" } }
 // { dg-final { scan-assembler "_Z3barILi250ELDmft3TFnEEvv" } }
 // { dg-final { scan-assembler "_Z3barILi251ELDmft3NS23TFnEEvv" } }
+// { dg-final { scan-assembler "_Z3barILi252ELDmft1QcvT_EEvv" } }
+// { dg-final { scan-assembler "_Z3barILi253ELDmft3NS21QcviEEvv" } }
 // { dg-final { scan-assembler "_Z3barILi260ELDmvt4TVarEEvv" } }
 // { dg-final { scan-assembler "_Z3barILi261ELDmvt3NS24TVarEEvv" } }
 // { dg-final { scan-assembler "_Z3barILi270ELDmat6TAliasEEvv" } }

	Jakub