Re: [PATCH] Fix PR c++/60463, PR c++/60755 Incorrect discard of const qualifier

[Momchil Velikov <momchil dot velikov at gmail dot com>]

Ping

> > On 14.04.2014 23:53, Momchil Velikov wrote:
> > Hello,
> >
> > During overload resolution of function calls in the body of a lambda it
> > is possible to use an implicit/dummy 'this', which differs in const-ness
> > from the actual 'this' that would be captured, resulting in choosing a
> > non-const member function even when the captured 'this' is const. This
> > patch makes sure that the captured 'this' is the same as the one used in
> > the overload resolution.

Bootstrapped/regtested for C/C++ on x86_64-unknown-linux-gnu against
xg++ (GCC) 4.10.0 20140508 (experimental) [master revision ed50168:49aa3a5:e79f58c7b12f37014efb7425399c93814cddb4c4]


~chill

2014-05-08  Momchil Velikov  <momchil.velikov@gmail.com>

	PR c++/60463
	PR c++/60755
	* lambda.c (lambda_expr_this_capture): Add new parameter
	add_capture_p controlling whether the functions will try to
	capture 'this' via the default capture.
	(maybe_resolve_dummy): Likewise.
	* cp-tree.h: Adjust prototypes.
	* call.c, semantics.c: Change callers of these functions.
	* call.c (build_new_method_call_1): Use the actual 'this' that
	would be potentially captured for the overload resolution, instead
	of the dummy object.

2014-05-08  Momchil Velikov  <momchil.velikov@gmail.com>

	PR c++/60463
	PR c++/60755
	* g++.dg/cpp0x/lambda/lambda-const-this.C: New testcase.




diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 857df57..bfedfef 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -7726,7 +7726,11 @@ build_new_method_call_1 (tree instance, tree fns, vec<tree, va_gc> **args,
   if (DECL_DESTRUCTOR_P (fn))
     name = complete_dtor_identifier;
 
-  first_mem_arg = instance;
+  /* For the overload resolution we need to find the actual `this`
+     that would be captured if the call turns out to be to a
+     non-static member function.  Do not actually capture it at this
+     point.  */
+  first_mem_arg = maybe_resolve_dummy (instance, false);
 
   /* Get the high-water mark for the CONVERSION_OBSTACK.  */
   p = conversion_obstack_alloc (0);
@@ -7864,7 +7868,7 @@ build_new_method_call_1 (tree instance, tree fns, vec<tree, va_gc> **args,
 	      && !DECL_CONSTRUCTOR_P (fn)
 	      && is_dummy_object (instance))
 	    {
-	      instance = maybe_resolve_dummy (instance);
+	      instance = maybe_resolve_dummy (instance, true);
 	      if (instance == error_mark_node)
 		call = error_mark_node;
 	      else if (!is_dummy_object (instance))
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 34d3d20..ed8d099 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -5890,8 +5890,8 @@ extern void insert_pending_capture_proxies	(void);
 extern bool is_capture_proxy			(tree);
 extern bool is_normal_capture_proxy             (tree);
 extern void register_capture_members		(tree);
-extern tree lambda_expr_this_capture            (tree);
-extern tree maybe_resolve_dummy			(tree);
+extern tree lambda_expr_this_capture            (tree, bool);
+extern tree maybe_resolve_dummy			(tree, bool);
 extern tree nonlambda_method_basetype		(void);
 extern void maybe_add_lambda_conv_op            (tree);
 extern bool is_lambda_ignored_entity            (tree);
diff --git a/gcc/cp/lambda.c b/gcc/cp/lambda.c
index 5ba6f14..3ce9ebb 100644
--- a/gcc/cp/lambda.c
+++ b/gcc/cp/lambda.c
@@ -624,11 +624,12 @@ add_default_capture (tree lambda_stack, tree id, tree initializer)
   return var;
 }
 
-/* Return the capture pertaining to a use of 'this' in LAMBDA, in the form of an
-   INDIRECT_REF, possibly adding it through default capturing.  */
+/* Return the capture pertaining to a use of 'this' in LAMBDA, in the
+   form of an INDIRECT_REF, possibly adding it through default
+   capturing, if ADD_CAPTURE_P is false.  */
 
 tree
-lambda_expr_this_capture (tree lambda)
+lambda_expr_this_capture (tree lambda, bool add_capture_p)
 {
   tree result;
 
@@ -648,7 +649,8 @@ lambda_expr_this_capture (tree lambda)
 
   /* Try to default capture 'this' if we can.  */
   if (!this_capture
-      && LAMBDA_EXPR_DEFAULT_CAPTURE_MODE (lambda) != CPLD_NONE)
+      && (!add_capture_p
+          || LAMBDA_EXPR_DEFAULT_CAPTURE_MODE (lambda) != CPLD_NONE))
     {
       tree lambda_stack = NULL_TREE;
       tree init = NULL_TREE;
@@ -708,9 +710,14 @@ lambda_expr_this_capture (tree lambda)
 	}
 
       if (init)
-	this_capture = add_default_capture (lambda_stack,
-					    /*id=*/this_identifier,
-					    init);
+        {
+          if (add_capture_p)
+	    this_capture = add_default_capture (lambda_stack,
+					        /*id=*/this_identifier,
+					        init);
+          else
+	    this_capture = init;
+        }
     }
 
   if (!this_capture)
@@ -742,7 +749,7 @@ lambda_expr_this_capture (tree lambda)
    'this' capture.  */
 
 tree
-maybe_resolve_dummy (tree object)
+maybe_resolve_dummy (tree object, bool add_capture_p)
 {
   if (!is_dummy_object (object))
     return object;
@@ -758,7 +765,7 @@ maybe_resolve_dummy (tree object)
     {
       /* In a lambda, need to go through 'this' capture.  */
       tree lam = CLASSTYPE_LAMBDA_EXPR (current_class_type);
-      tree cap = lambda_expr_this_capture (lam);
+      tree cap = lambda_expr_this_capture (lam, add_capture_p);
       object = build_x_indirect_ref (EXPR_LOCATION (object), cap,
 				     RO_NULL, tf_warning_or_error);
     }
diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index 4afb821..d925f5c 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -1675,7 +1675,7 @@ finish_non_static_data_member (tree decl, tree object, tree qualifying_scope)
       object = maybe_dummy_object (scope, NULL);
     }
 
-  object = maybe_resolve_dummy (object);
+  object = maybe_resolve_dummy (object, true);
   if (object == error_mark_node)
     return error_mark_node;
 
@@ -2434,7 +2434,7 @@ finish_this_expr (void)
 
       /* In a lambda expression, 'this' refers to the captured 'this'.  */
       if (LAMBDA_TYPE_P (type))
-        result = lambda_expr_this_capture (CLASSTYPE_LAMBDA_EXPR (type));
+        result = lambda_expr_this_capture (CLASSTYPE_LAMBDA_EXPR (type), true);
       else
         result = current_class_ptr;
     }
diff --git a/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const-this.C b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const-this.C
new file mode 100644
index 0000000..2de00d3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const-this.C
@@ -0,0 +1,9 @@
+// PR c++/60463
+// PR c++/60755
+// { dg-do compile { target c++11 } }
+struct S {
+  void f(); // { dg-message "no known conversion for implicit 'this' parameter from 'const S\\*' to 'S\\*'" }
+  void g() const {
+    [=] { f(); } (); // { dg-error "no matching function for call to 'S::f\\(\\)'" }
+  }
+};