c++: cv-quals of dummy obj for non-dep memfn call [PR105637]

In non-dependent23.C below we expect the Base::foo calls to
resolve to the second, third and fourth overloads respectively in light
of the cv-qualifiers of 'this' in each case.  But ever since
r12-6075-g2decd2cabe5a4f, the calls incorrectly resolve to the first
overload at instantiation time.

This happens because the calls to Base::foo are all deemed
non-dependent (ever since r7-755-g23cb72663051cd made us ignore 'this'
dependence when considering the dependence of a non-static memfn call),
hence we end up checking the call ahead of time, using as the object
argument a dummy object of type Base.  Since this object argument is
cv-unqualified, the calls in turn resolve to the unqualified overload
of baseDevice.  Before r12-6075 this incorrect result would just get
silently discarded and we'd end up redoing OR at instantiation time
using 'this' as the object argument.  But after r12-6075 we now reuse
this incorrect result at instantiation time.

This patch fixes this by making maybe_dummy_object respect the cv-quals
of (the non-lambda) 'this' when returning a dummy object.  Thus, ahead
of time OR using a dummy object will give us the right answer that's
consistent with the instantiation time answer.

An earlier version of this patch didn't handle 'this'-capturing lambdas
correctly, which broke lambda-this22.C below.

	PR c++/105637

gcc/cp/ChangeLog:

	* tree.cc (maybe_dummy_object): When returning a dummy
	object, respect the cv-quals of 'this' if available.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/lambda/lambda-this22.C: New test.
	* g++.dg/template/non-dependent23.C: New test.

gcc/cp/tree.cc

@@ -4319,15 +4319,31 @@ maybe_dummy_object (tree type, tree* binfop)
   if (binfop)
     *binfop = binfo;
 
-  if (current_class_ref
-      /* current_class_ref might not correspond to current_class_type if
-	 we're in tsubst_default_argument or a lambda-declarator; in either
-	 case, we want to use current_class_ref if it matches CONTEXT.  */
-      && (same_type_ignoring_top_level_qualifiers_p
-	  (TREE_TYPE (current_class_ref), context)))
+  /* current_class_ref might not correspond to current_class_type if
+     we're in tsubst_default_argument or a lambda-declarator; in either
+     case, we want to use current_class_ref if it matches CONTEXT.  */
+  tree ctype = current_class_ref ? TREE_TYPE (current_class_ref) : NULL_TREE;
+  if (ctype
+      && same_type_ignoring_top_level_qualifiers_p (ctype, context))
     decl = current_class_ref;
   else
-    decl = build_dummy_object (context);
+    {
+      /* Return a dummy object whose cv-quals are consistent with (the
+	 non-lambda) 'this' if available.  */
+      if (ctype)
+	{
+	  int quals = TYPE_UNQUALIFIED;
+	  if (tree lambda = CLASSTYPE_LAMBDA_EXPR (ctype))
+	    {
+	      if (tree cap = lambda_expr_this_capture (lambda, false))
+		quals = cp_type_quals (TREE_TYPE (TREE_TYPE (cap)));
+	    }
+	  else
+	    quals = cp_type_quals (ctype);
+	  context = cp_build_qualified_type (context, quals);
+	}
+      decl = build_dummy_object (context);
+    }
 
   return decl;
 }

gcc/testsuite/g++.dg/cpp0x/lambda/lambda-this22.C

@@ -0,0 +1,20 @@
+// PR c++/105637
+// { dg-do compile { target c++11 } }
+
+struct Base {
+  void foo();                // #1
+  void foo() const = delete; // #2
+};
+
+template<class T>
+struct TopClass : T {
+  void failsToCompile() {
+    [this] { Base::foo(); }(); // should select #2, not #1
+  }
+
+  void failsToCompile() const {
+    [this] { Base::foo(); }(); // { dg-error "deleted" }
+  }
+};
+
+template struct TopClass<Base>;

gcc/testsuite/g++.dg/template/non-dependent23.C

@@ -0,0 +1,25 @@
+// PR c++/105637
+
+struct Base {
+  void foo();                // #1
+  void foo() const;          // #2
+  void foo() volatile;       // #3
+  void foo() const volatile; // #4
+};
+
+template<class T>
+struct TopClass : T {
+  void failsToCompile() const {
+    Base::foo(); // should select #2, not #1
+  }
+
+  void failsToCompile() volatile {
+    Base::foo();  // should select #3, not #1
+  }
+
+  void failsToCompile() const volatile {
+    Base::foo();  // should select #4, not #1
+  }
+};
+
+template struct TopClass<Base>;