Depend on operator overload synthesis for three-way and equality comparisons. (flutter#174892)

This depends on the implicitly generated `operator!=` from `operator==`
in a few cases (see the exceptions below) and add `operator<=>` in some
of the more obvious instances.

I've tried to be extremely conservative in this first pass. There are a
few more candidates that were not converted because:

* `operator!=` was different from `!operator=.` For instance, if it had
early returns. Not sure how the compiler generated those and didn't want
the behavior or performance characteristics to change.
* They were virtual.
* `operator==` didn't actually compare all struct members. Dubious but I
didn't want to change existing behavior.
* There were template parameters.

Author: chinmaygarde

engine/src/flutter/display_list/display_list.h

@@ -243,9 +243,6 @@ class SaveLayerOptions {
   bool operator==(const SaveLayerOptions& other) const {
     return flags_ == other.flags_;
   }
-  bool operator!=(const SaveLayerOptions& other) const {
-    return flags_ != other.flags_;
-  }
 
  private:
   union {

engine/src/flutter/display_list/dl_color.h

@@ -181,9 +181,6 @@ struct DlColor {
            green_ == other.green_ && blue_ == other.blue_ &&
            color_space_ == other.color_space_;
   }
-  bool operator!=(DlColor const& other) const {
-    return !this->operator==(other);
-  }
 
  private:
   DlScalar alpha_;

engine/src/flutter/display_list/dl_paint.h

@@ -203,7 +203,6 @@ class DlPaint {
   }
 
   bool operator==(DlPaint const& other) const;
-  bool operator!=(DlPaint const& other) const { return !(*this == other); }
 
  private:
 #define ASSERT_ENUM_FITS(last_enum, num_bits)                    \

engine/src/flutter/display_list/dl_vertices.h

@@ -231,8 +231,6 @@ class DlVertices {
 
   bool operator==(DlVertices const& other) const;
 
-  bool operator!=(DlVertices const& other) const { return !(*this == other); }
-
  private:
   // Constructors are designed to encapsulate arrays sequentially in memory
   // which means they can only be called by intantiations that use the

engine/src/flutter/display_list/geometry/dl_path.h

@@ -92,7 +92,6 @@ class DlPath : public impeller::PathSource {
   DlPathFillType GetFillType() const override;
 
   bool operator==(const DlPath& other) const;
-  bool operator!=(const DlPath& other) const { return !(*this == other); }
 
   bool IsVolatile() const;
   bool IsConvex() const override;

engine/src/flutter/flow/embedded_views.h

@@ -56,10 +56,6 @@ class ImageFilterMutation {
     return *filter_ == *other.filter_ && filter_rect_ == other.filter_rect_;
   }
 
-  bool operator!=(const ImageFilterMutation& other) const {
-    return !operator==(other);
-  }
-
  private:
   std::shared_ptr<DlImageFilter> filter_;
   const DlRect filter_rect_;
@@ -107,8 +103,6 @@ class Mutator {
 
   bool operator==(const Mutator& other) const { return data_ == other.data_; }
 
-  bool operator!=(const Mutator& other) const { return !operator==(other); }
-
   bool IsClipType() {
     switch (GetType()) {
       case MutatorType::kClipRect:

engine/src/flutter/flow/raster_cache_key.h

@@ -64,10 +64,6 @@ class RasterCacheKeyID {
            GetHash() == other.GetHash() && child_ids_ == other.child_ids_;
   }
 
-  bool operator!=(const RasterCacheKeyID& other) const {
-    return !operator==(other);
-  }
-
  private:
   const uint64_t unique_id_;
   const RasterCacheKeyType type_;

engine/src/flutter/fml/command_line.h

@@ -66,7 +66,6 @@ class CommandLine final {
     bool operator==(const Option& other) const {
       return name == other.name && value == other.value;
     }
-    bool operator!=(const Option& other) const { return !operator==(other); }
 
     std::string name;
     std::string value;
@@ -103,7 +102,6 @@ class CommandLine final {
            options_ == other.options_ &&
            positional_args_ == other.positional_args_;
   }
-  bool operator!=(const CommandLine& other) const { return !operator==(other); }
 
   // Returns true if this command line has the option |name| (and if |index| is
   // non-null, sets |*index| to the index of the *last* occurrence of the given

engine/src/flutter/fml/time/time_delta.h

@@ -96,12 +96,7 @@ class TimeDelta {
     return TimeDelta::FromNanoseconds(delta_ % other.delta_);
   }
 
-  bool operator==(TimeDelta other) const { return delta_ == other.delta_; }
-  bool operator!=(TimeDelta other) const { return delta_ != other.delta_; }
-  bool operator<(TimeDelta other) const { return delta_ < other.delta_; }
-  bool operator<=(TimeDelta other) const { return delta_ <= other.delta_; }
-  bool operator>(TimeDelta other) const { return delta_ > other.delta_; }
-  bool operator>=(TimeDelta other) const { return delta_ >= other.delta_; }
+  constexpr auto operator<=>(const TimeDelta& other) const = default;
 
   static constexpr TimeDelta FromTimespec(struct timespec ts) {
     return TimeDelta::FromSeconds(ts.tv_sec) +

engine/src/flutter/fml/time/time_point.h

@@ -49,26 +49,23 @@ class TimePoint {
     return TimePoint(ticks);
   }
 
-  TimeDelta ToEpochDelta() const { return TimeDelta::FromNanoseconds(ticks_); }
+  constexpr TimeDelta ToEpochDelta() const {
+    return TimeDelta::FromNanoseconds(ticks_);
+  }
 
   // Compute the difference between two time points.
-  TimeDelta operator-(TimePoint other) const {
+  constexpr TimeDelta operator-(TimePoint other) const {
     return TimeDelta::FromNanoseconds(ticks_ - other.ticks_);
   }
 
-  TimePoint operator+(TimeDelta duration) const {
+  constexpr TimePoint operator+(TimeDelta duration) const {
     return TimePoint(ticks_ + duration.ToNanoseconds());
   }
-  TimePoint operator-(TimeDelta duration) const {
+  constexpr TimePoint operator-(TimeDelta duration) const {
     return TimePoint(ticks_ - duration.ToNanoseconds());
   }
 
-  bool operator==(TimePoint other) const { return ticks_ == other.ticks_; }
-  bool operator!=(TimePoint other) const { return ticks_ != other.ticks_; }
-  bool operator<(TimePoint other) const { return ticks_ < other.ticks_; }
-  bool operator<=(TimePoint other) const { return ticks_ <= other.ticks_; }
-  bool operator>(TimePoint other) const { return ticks_ > other.ticks_; }
-  bool operator>=(TimePoint other) const { return ticks_ >= other.ticks_; }
+  constexpr auto operator<=>(const TimePoint& other) const = default;
 
  private:
   explicit constexpr TimePoint(int64_t ticks) : ticks_(ticks) {}