C++ PATCH to constexpr handling of lambda this capture

[Jason Merrill]

The constexpr code rejects uses of 'this' in a constant-expression,
but that shouldn't apply to a lambda 'this' capture proxy, which can
have a constant value.

Tested x86_64-pc-linux-gnu, applying to trunk and 7.

commit 1f69899dda31109c95ac4636eb66788449d6382e
Author: Jason Merrill <ja...@redhat.com>
Date:   Wed May 31 14:45:04 2017 -0400

    * constexpr.c (potential_constant_expression_1): Allow 'this' capture.

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 157b20d..8bbe950 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -5296,7 +5296,7 @@ potential_constant_expression_1 (tree t, bool want_rval, 
bool strict,
       {
         tree x = TREE_OPERAND (t, 0);
         STRIP_NOPS (x);
-        if (is_this_parameter (x))
+        if (is_this_parameter (x) && !is_capture_proxy (x))
          {
            if (DECL_CONTEXT (x)
                && !DECL_DECLARED_CONSTEXPR_P (DECL_CONTEXT (x)))
diff --git a/gcc/testsuite/g++.dg/cpp1z/lambda-this3.C 
b/gcc/testsuite/g++.dg/cpp1z/lambda-this3.C
new file mode 100644
index 0000000..b5a1fc6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/lambda-this3.C
@@ -0,0 +1,10 @@
+// { dg-options -std=c++1z }
+
+struct S {
+  int i;
+  constexpr S() : i(5) { 
+    ([*this] () { return i + 10; }());
+  }
+  constexpr operator int() const { return i; }
+};
+constexpr int x = S();