c++: Make convert_like complain about bad ck_ref_bind again [PR95789]

convert_like issues errors about bad_p conversions at the beginning
of the function, but in the ck_ref_bind case, it only issues them
after we've called convert_like on the next conversion.

This doesn't work as expected since r10-7096 because when we see
a conversion from/to class type in a template, we return early, thereby
missing the error, and a bad_p conversion goes by undetected.  That
made the attached test to compile even though it should not.

I had thought that I could just move the ck_ref_bind/bad_p errors
above to the rest of them, but that regressed diagnostics because
expr then wasn't converted yet by the nested convert_like_real call.

So, for bad_p conversions, do the normal processing, but still return
the IMPLICIT_CONV_EXPR to avoid introducing trees that the template
processing can't handle well.  This I achieved by adding a wrapper
function.

gcc/cp/ChangeLog:

	PR c++/95789
	PR c++/96104
	PR c++/96179
	* call.c (convert_like_real_1): Renamed from convert_like_real.
	(convert_like_real): New wrapper for convert_like_real_1.

gcc/testsuite/ChangeLog:

	PR c++/95789
	PR c++/96104
	PR c++/96179
	* g++.dg/conversion/ref4.C: New test.
	* g++.dg/conversion/ref5.C: New test.
	* g++.dg/conversion/ref6.C: New test.

(cherry picked from commit 8e64d182850560dbedfabb88aac90d4fc6155067)

gcc/cp/call.c

@@ -171,6 +171,8 @@ static tree build_over_call (struct z_candidate *, int, tsubst_flags_t);
 		     /*c_cast_p=*/false, (COMPLAIN))
 static tree convert_like_real (conversion *, tree, tree, int, bool,
 			       bool, tsubst_flags_t);
+static tree convert_like_real_1 (conversion *, tree, tree, int, bool,
+				 bool, tsubst_flags_t);
 static void op_error (const op_location_t &, enum tree_code, enum tree_code,
 		      tree, tree, tree, bool);
 static struct z_candidate *build_user_type_conversion_1 (tree, tree, int,
@@ -7283,6 +7285,39 @@ maybe_warn_array_conv (location_t loc, conversion *c, tree expr)
 	     "are only available with %<-std=c++2a%> or %<-std=gnu++2a%>");
 }
 
+/* Wrapper for convert_like_real_1 that handles creating IMPLICIT_CONV_EXPR.  */
+
+static tree
+convert_like_real (conversion *convs, tree expr, tree fn, int argnum,
+		   bool issue_conversion_warnings,
+		   bool c_cast_p, tsubst_flags_t complain)
+{
+  /* Creating &TARGET_EXPR<> in a template breaks when substituting,
+     and creating a CALL_EXPR in a template breaks in finish_call_expr
+     so use an IMPLICIT_CONV_EXPR for this conversion.  We would have
+     created such codes e.g. when calling a user-defined conversion
+     function.  */
+  tree conv_expr = NULL_TREE;
+  if (processing_template_decl
+      && convs->kind != ck_identity
+      && (CLASS_TYPE_P (convs->type) || CLASS_TYPE_P (TREE_TYPE (expr))))
+    {
+      conv_expr = build1 (IMPLICIT_CONV_EXPR, convs->type, expr);
+      if (convs->kind != ck_ref_bind)
+	conv_expr = convert_from_reference (conv_expr);
+      if (!convs->bad_p)
+	return conv_expr;
+      /* Do the normal processing to give the bad_p errors.  But we still
+	 need to return the IMPLICIT_CONV_EXPR, unless we're returning
+	 error_mark_node.  */
+    }
+  expr = convert_like_real_1 (convs, expr, fn, argnum,
+			      issue_conversion_warnings, c_cast_p, complain);
+  if (expr == error_mark_node)
+    return error_mark_node;
+  return conv_expr ? conv_expr : expr;
+}
+
 /* Perform the conversions in CONVS on the expression EXPR.  FN and
    ARGNUM are used for diagnostics.  ARGNUM is zero based, -1
    indicates the `this' argument of a method.  INNER is nonzero when
@@ -7294,7 +7329,7 @@ maybe_warn_array_conv (location_t loc, conversion *c, tree expr)
    conversions to inaccessible bases are permitted.  */
 
 static tree
-convert_like_real (conversion *convs, tree expr, tree fn, int argnum,
+convert_like_real_1 (conversion *convs, tree expr, tree fn, int argnum,
 		     bool issue_conversion_warnings,
 		     bool c_cast_p, tsubst_flags_t complain)
 {
@@ -7397,19 +7432,6 @@ convert_like_real (conversion *convs, tree expr, tree fn, int argnum,
   if (issue_conversion_warnings && (complain & tf_warning))
     conversion_null_warnings (totype, expr, fn, argnum);
 
-  /* Creating &TARGET_EXPR<> in a template breaks when substituting,
-     and creating a CALL_EXPR in a template breaks in finish_call_expr
-     so use an IMPLICIT_CONV_EXPR for this conversion.  We would have
-     created such codes e.g. when calling a user-defined conversion
-     function.  */
-  if (processing_template_decl
-      && convs->kind != ck_identity
-      && (CLASS_TYPE_P (totype) || CLASS_TYPE_P (TREE_TYPE (expr))))
-    {
-      expr = build1 (IMPLICIT_CONV_EXPR, totype, expr);
-      return convs->kind == ck_ref_bind ? expr : convert_from_reference (expr);
-    }
-
   switch (convs->kind)
     {
     case ck_user:

gcc/testsuite/g++.dg/conversion/ref4.C

+// PR c++/95789
+// { dg-do compile { target c++11 } }
+
+struct B {
+    int n;
+};
+
+template <typename T>
+struct A {
+    B& get() const { return f; } // { dg-error "binding reference" }
+
+    B f;
+};
+
+int main() {
+    A<int> a;
+    a.f = {};
+
+    a.get().n = 10;
+    if (a.f.n != 0)
+      __builtin_abort();
+}

gcc/testsuite/g++.dg/conversion/ref5.C

+// PR c++/96104
+
+template <typename T> void fn(T &);
+class E {};
+struct F {
+  template <typename T> void mfn(T t) { t, fn(E()); } // { dg-error "cannot bind non-const lvalue reference" }
+};
+int
+main()
+{
+  E e;
+  F f;
+  f.mfn(e);
+}

gcc/testsuite/g++.dg/conversion/ref6.C

+// PR c++/96179
+// { dg-do compile { target c++11 } }
+
+template<typename T> struct vector
+{
+  void push_back(T) { }
+};
+
+struct dummy{
+        int a;
+};
+
+void Modify_Dummy(dummy &d){
+        d.a=1;
+}
+
+template <bool bla=true> void Templated_Function(){
+        vector<dummy> A;
+        A.push_back(Modify_Dummy(dummy{0})); // { dg-error "cannot bind non-const lvalue reference" }
+}
+
+int main(){
+        Templated_Function();
+}