Skip some is_constexpr_default_constructible tests under gcc7 (#1273)

Summary:
Pull Request resolved: https://github.com/facebook/folly/pull/1273

[Folly] Skip some `is_constexpr_default_constructible` tests under gcc7.

In gcc7 specifically, i.e. not gcc6 and not gcc8, some of the expectations fail. The compiler is wrong and this affects the outcome of `is_constexpr_default_constructible`, but this is mostly harmless as far as existing uses go. Fix the tests by skipping them when using gcc7.

Reviewed By: vitaut

Differential Revision: D18820312

fbshipit-source-id: a2d0d53ed2716fffba7840e2b167e9ad4ac012ff

folly/test/TraitsTest.cpp

@@ -403,6 +403,8 @@ TEST(Traits, is_instantiation_of) {
 }
 
 TEST(Traits, is_constexpr_default_constructible) {
+  constexpr auto const broken = kGnuc == 7 && !kIsClang;
+
   EXPECT_TRUE(is_constexpr_default_constructible_v<int>);
 
   struct Empty {};
@@ -411,7 +413,7 @@ TEST(Traits, is_constexpr_default_constructible) {
   struct NonTrivialDtor {
     ~NonTrivialDtor() {}
   };
-  EXPECT_FALSE(is_constexpr_default_constructible_v<NonTrivialDtor>);
+  EXPECT_FALSE(is_constexpr_default_constructible_v<NonTrivialDtor> && !broken);
 
   struct ConstexprCtor {
     int x, y;
@@ -423,7 +425,8 @@ TEST(Traits, is_constexpr_default_constructible) {
     int x, y;
     NonConstexprCtor() noexcept : x(7), y(11) {}
   };
-  EXPECT_FALSE(is_constexpr_default_constructible_v<NonConstexprCtor>);
+  EXPECT_FALSE(
+      is_constexpr_default_constructible_v<NonConstexprCtor> && !broken);
 
   struct NoDefaultCtor {
     constexpr NoDefaultCtor(int, int) noexcept {}