Export of internal Abseil changes

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f2ffea8ae1c1b152b63bf561375cfb6eb6b9dbe5 by Derek Mauro <dmauro@google.com>:

Internal change

PiperOrigin-RevId: 432180490
Change-Id: I5e318e1d06fe26eee08920fe39f8a6285eb8e217

--
e5d0de4b3293833eab3f77252a091ac4ed74b210 by Derek Mauro <dmauro@google.com>:

absl::optional - Add a workaround for an internal compiler error in GCC5 though GCC10

PiperOrigin-RevId: 432047437
Change-Id: I524a9098dadc116d8a423dd66f7ec5ee894f2874
GitOrigin-RevId: f2ffea8ae1c1b152b63bf561375cfb6eb6b9dbe5

absl/types/optional.h

@@ -282,15 +282,16 @@ class optional : private optional_internal::optional_data<T>,
   optional& operator=(optional&& src) = default;
 
   // Value assignment operators
-  template <
-      typename U = T,
-      typename = typename std::enable_if<absl::conjunction<
-          absl::negation<
-              std::is_same<optional<T>, typename std::decay<U>::type>>,
-          absl::negation<
-              absl::conjunction<std::is_scalar<T>,
-                                std::is_same<T, typename std::decay<U>::type>>>,
-          std::is_constructible<T, U>, std::is_assignable<T&, U>>::value>::type>
+  template <typename U = T,
+            int&...,  // Workaround an internal compiler error in GCC 5 to 10.
+            typename = typename std::enable_if<absl::conjunction<
+                absl::negation<
+                    std::is_same<optional<T>, typename std::decay<U>::type> >,
+                absl::negation<absl::conjunction<
+                    std::is_scalar<T>,
+                    std::is_same<T, typename std::decay<U>::type> > >,
+                std::is_constructible<T, U>,
+                std::is_assignable<T&, U> >::value>::type>
   optional& operator=(U&& v) {
     this->assign(std::forward<U>(v));
     return *this;
@@ -298,13 +299,14 @@ class optional : private optional_internal::optional_data<T>,
 
   template <
       typename U,
+      int&...,  // Workaround an internal compiler error in GCC 5 to 10.
       typename = typename std::enable_if<absl::conjunction<
-          absl::negation<std::is_same<T, U>>,
+          absl::negation<std::is_same<T, U> >,
           std::is_constructible<T, const U&>, std::is_assignable<T&, const U&>,
           absl::negation<
               optional_internal::
                   is_constructible_convertible_assignable_from_optional<
-                      T, U>>>::value>::type>
+                      T, U> > >::value>::type>
   optional& operator=(const optional<U>& rhs) {
     if (rhs) {
       this->assign(*rhs);
@@ -315,13 +317,14 @@ class optional : private optional_internal::optional_data<T>,
   }
 
   template <typename U,
+            int&...,  // Workaround an internal compiler error in GCC 5 to 10.
             typename = typename std::enable_if<absl::conjunction<
-                absl::negation<std::is_same<T, U>>, std::is_constructible<T, U>,
-                std::is_assignable<T&, U>,
+                absl::negation<std::is_same<T, U> >,
+                std::is_constructible<T, U>, std::is_assignable<T&, U>,
                 absl::negation<
                     optional_internal::
                         is_constructible_convertible_assignable_from_optional<
-                            T, U>>>::value>::type>
+                            T, U> > >::value>::type>
   optional& operator=(optional<U>&& rhs) {
     if (rhs) {
       this->assign(std::move(*rhs));

absl/types/optional_test.cc

@@ -27,6 +27,32 @@
 #include "absl/meta/type_traits.h"
 #include "absl/strings/string_view.h"
 
+// The following types help test an internal compiler error in GCC5 though
+// GCC10. The case OptionalTest.InternalCompilerErrorInGcc5ToGcc10 crashes the
+// compiler without a workaround. This test case should remain at the beginning
+// of the file as the internal compiler error is sensitive to other constructs
+// in this file.
+template <class T, class...>
+using GccIceHelper1 = T;
+template <typename T>
+struct GccIceHelper2 {};
+template <typename T>
+class GccIce {
+  template <typename U,
+            typename SecondTemplateArgHasToExistForSomeReason = void,
+            typename DependentType = void,
+            typename = std::is_assignable<GccIceHelper1<T, DependentType>&, U>>
+  GccIce& operator=(GccIceHelper2<U> const&) {}
+};
+
+TEST(OptionalTest, InternalCompilerErrorInGcc5ToGcc10) {
+  GccIce<int> instantiate_ice_with_same_type_as_optional;
+  static_cast<void>(instantiate_ice_with_same_type_as_optional);
+  absl::optional<int> val1;
+  absl::optional<int> val2;
+  val1 = val2;
+}
+
 struct Hashable {};
 
 namespace std {