Update to C++17 and use std::optional for getSuperType (#4203)

This sets the C++ standard variable in the build to C++17, and makes use of std::optional (a C++17 library feature) in one place, to test that it's working.

10 files changed, 32 insertions, 37 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 336f54342..113aeb901 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -6,7 +6,7 @@ include(GNUInstallDirs)
 
 # The C++ standard whose features are required to build Binaryen.
 # Keep in sync with scripts/test/shared.py cxx_standard
-set(CXX_STANDARD 14)
+set(CXX_STANDARD 17)
 
 if(NOT CMAKE_BUILD_TYPE)
   message(STATUS "No build type selected, default to Release")
diff --git a/scripts/test/shared.py b/scripts/test/shared.py
index 82e4fe03e..d86d802d3 100644
--- a/scripts/test/shared.py
+++ b/scripts/test/shared.py
@@ -25,7 +25,7 @@ import sys
 
 # The C++ standard whose features are required to build Binaryen.
 # Keep in sync with CMakeLists.txt CXX_STANDARD
-cxx_standard = 14
+cxx_standard = 17
 
 
 def parse_args(args):
diff --git a/src/ir/module-utils.h b/src/ir/module-utils.h
index 13fa76fff..69c454410 100644
--- a/src/ir/module-utils.h
+++ b/src/ir/module-utils.h
@@ -597,15 +597,15 @@ inline void collectHeapTypes(Module& wasm,
         counts.note(child);
       }
     }
-    HeapType super;
-    if (ht.getSuperType(super)) {
-      if (!counts.count(super)) {
-        newTypes.insert(super);
+
+    if (auto super = ht.getSuperType()) {
+      if (!counts.count(*super)) {
+        newTypes.insert(*super);
         // We should unconditionally count supertypes, but while the type system
         // is in flux, skip counting them to keep the type orderings in nominal
         // test outputs more similar to the orderings in the equirecursive
         // outputs. FIXME
-        counts.note(super);
+        counts.note(*super);
       }
     }
   }
diff --git a/src/ir/struct-utils.h b/src/ir/struct-utils.h
index b052a15f7..442d990fb 100644
--- a/src/ir/struct-utils.h
+++ b/src/ir/struct-utils.h
@@ -222,13 +222,12 @@ private:
       auto& infos = combinedInfos[type];
 
       // Propagate shared fields to the supertype.
-      HeapType superType;
-      if (type.getSuperType(superType)) {
-        auto& superInfos = combinedInfos[superType];
-        auto& superFields = superType.getStruct().fields;
+      if (auto superType = type.getSuperType()) {
+        auto& superInfos = combinedInfos[*superType];
+        auto& superFields = superType->getStruct().fields;
         for (Index i = 0; i < superFields.size(); i++) {
           if (superInfos[i].combine(infos[i])) {
-            work.push(superType);
+            work.push(*superType);
           }
         }
       }
diff --git a/src/ir/subtypes.h b/src/ir/subtypes.h
index 661eaf119..64697e2a3 100644
--- a/src/ir/subtypes.h
+++ b/src/ir/subtypes.h
@@ -57,12 +57,12 @@ struct SubTypes {
   std::vector<HeapType> getAllSuperTypes(HeapType type) {
     std::vector<HeapType> ret;
     while (1) {
-      HeapType super;
-      if (!type.getSuperType(super)) {
+      auto super = type.getSuperType();
+      if (!super) {
         return ret;
       }
-      ret.push_back(super);
-      type = super;
+      ret.push_back(*super);
+      type = *super;
     }
   }
 
@@ -71,9 +71,8 @@ struct SubTypes {
 private:
   // Add a type to the graph.
   void note(HeapType type) {
-    HeapType super;
-    if (type.getSuperType(super)) {
-      typeSubTypes[super].push_back(type);
+    if (auto super = type.getSuperType()) {
+      typeSubTypes[*super].push_back(type);
     }
   }
 
diff --git a/src/ir/type-updating.cpp b/src/ir/type-updating.cpp
index c69cbe1be..d3ecf7ad6 100644
--- a/src/ir/type-updating.cpp
+++ b/src/ir/type-updating.cpp
@@ -65,9 +65,8 @@ void GlobalTypeRewriter::update() {
     }
 
     // Apply a super, if there is one
-    HeapType super;
-    if (type.getSuperType(super)) {
-      typeBuilder.setSubType(i, typeIndices[super]);
+    if (auto super = type.getSuperType()) {
+      typeBuilder.setSubType(i, typeIndices[*super]);
     }
   }
 
diff --git a/src/passes/Print.cpp b/src/passes/Print.cpp
index 96a1abe73..34fe92ff6 100644
--- a/src/passes/Print.cpp
+++ b/src/passes/Print.cpp
@@ -2515,9 +2515,8 @@ struct PrintSExpression : public UnifiedExpressionVisitor<PrintSExpression> {
   }
   // Module-level visitors
   void printSupertypeOr(HeapType curr, std::string noSuper) {
-    HeapType super;
-    if (curr.getSuperType(super)) {
-      TypeNamePrinter(o, currModule).print(super);
+    if (auto super = curr.getSuperType()) {
+      TypeNamePrinter(o, currModule).print(*super);
     } else {
       o << noSuper;
     }
diff --git a/src/wasm-type.h b/src/wasm-type.h
index a673b936f..4c401be6b 100644
--- a/src/wasm-type.h
+++ b/src/wasm-type.h
@@ -19,6 +19,7 @@
 
 #include "support/name.h"
 #include "wasm-features.h"
+#include <optional>
 #include <ostream>
 #include <vector>
 
@@ -382,12 +383,12 @@ public:
   const Struct& getStruct() const;
   Array getArray() const;
 
-  // Return whether there is a nontrivial (i.e. non-basic) nominal supertype and
-  // store it in `out` if there is. Nominal types (in the sense of isNominal,
+  // If there is a nontrivial (i.e. non-basic) nominal supertype, return it,
+  // else an empty optional. Nominal types (in the sense of isNominal,
   // i.e. Milestone 4 nominal types) may always have supertypes and other types
   // may have supertypes in `TypeSystem::Nominal` mode but not in
   // `TypeSystem::Equirecursive` mode.
-  bool getSuperType(HeapType& out) const;
+  std::optional<HeapType> getSuperType() const;
 
   // Whether this is a nominal type in the sense of being a GC Milestone 4
   // nominal type. Although all non-basic HeapTypes are nominal in
diff --git a/src/wasm/wasm-binary.cpp b/src/wasm/wasm-binary.cpp
index 9c8b83325..6ce2c7979 100644
--- a/src/wasm/wasm-binary.cpp
+++ b/src/wasm/wasm-binary.cpp
@@ -251,12 +251,11 @@ void WasmBinaryWriter::writeTypes() {
       WASM_UNREACHABLE("TODO GC type writing");
     }
     if (nominal) {
-      HeapType super;
-      bool hasSuper = type.getSuperType(super);
-      if (!hasSuper) {
+      auto super = type.getSuperType();
+      if (!super) {
         super = type.isFunction() ? HeapType::func : HeapType::data;
       }
-      writeHeapType(super);
+      writeHeapType(*super);
     }
   }
   finishSection(start);
diff --git a/src/wasm/wasm-type.cpp b/src/wasm/wasm-type.cpp
index 284a7e23b..94c6e96c6 100644
--- a/src/wasm/wasm-type.cpp
+++ b/src/wasm/wasm-type.cpp
@@ -1206,16 +1206,15 @@ Array HeapType::getArray() const {
   return getHeapTypeInfo(*this)->array;
 }
 
-bool HeapType::getSuperType(HeapType& out) const {
+std::optional<HeapType> HeapType::getSuperType() const {
   if (isBasic()) {
-    return false;
+    return {};
   }
   HeapTypeInfo* super = getHeapTypeInfo(*this)->supertype;
   if (super != nullptr) {
-    out = HeapType(uintptr_t(super));
-    return true;
+    return HeapType(uintptr_t(super));
   }
-  return false;
+  return {};
 }
 
 bool HeapType::isNominal() const {