[SILGen] InitAccessors: Start emitting init property initialization expressions

Similar to regular stored properties emit initialization expressions
for properties with init accessors at constructor's prolog.

lib/SILGen/SILGenConstructor.cpp

@@ -1459,6 +1459,49 @@ emitAndStoreInitialValueInto(SILGenFunction &SGF,
     std::move(result).forwardInto(SGF, loc, init);
 }
 
+void SILGenFunction::emitMemberInitializationViaInitAccessor(
+    DeclContext *dc, VarDecl *selfDecl, PatternBindingDecl *member,
+    SubstitutionMap subs) {
+  auto *var = member->getSingleVar();
+  assert(var->hasInitAccessor());
+
+  auto init = member->getExecutableInit(0);
+  if (!init)
+    return;
+
+  auto *varPattern = member->getPattern(0);
+
+  // Cleanup after this initialization.
+  FullExpr scope(Cleanups, varPattern);
+
+  auto resultType =
+      getInitializationTypeInContext(member->getDeclContext(), dc, varPattern);
+
+  RValue initResult = emitApplyOfStoredPropertyInitializer(
+    init, var, subs, resultType.second, resultType.first, SGFContext());
+
+  SILLocation loc(init);
+  loc.markAutoGenerated();
+
+  auto selfValue = emitSelfForMemberInit(*this, varPattern, selfDecl);
+
+  ManagedValue selfRef = selfValue;
+  if (selfValue.isLValue()) {
+    auto accessToSelf =
+      B.createBeginAccess(loc, selfValue.getValue(), SILAccessKind::Modify,
+                          SILAccessEnforcement::Unknown,
+                          /*noNestedConflict=*/false,
+                          /*fromBuiltin=*/false);
+    selfRef = ManagedValue::forUnmanaged(accessToSelf);
+  }
+
+  emitAssignOrInit(loc, selfRef, var,
+                   std::move(initResult).getAsSingleValue(*this, loc), subs);
+
+  if (selfValue.isLValue())
+    B.createEndAccess(loc, selfRef.getValue(), /*aborted=*/false);
+}
+
 void SILGenFunction::emitMemberInitializers(DeclContext *dc,
                                             VarDecl *selfDecl,
                                             NominalTypeDecl *nominal) {
@@ -1469,11 +1512,12 @@ void SILGenFunction::emitMemberInitializers(DeclContext *dc,
     if (auto pbd = dyn_cast<PatternBindingDecl>(member)) {
       if (pbd->isStatic()) continue;
 
-      // Skip properties with init accessors, they could only be used
-      // explicitly and in memberwise initializers.
+      // Emit default initialization for an init accessor property.
       if (auto *var = pbd->getSingleVar()) {
-        if (var->hasInitAccessor())
+        if (var->hasInitAccessor()) {
+          emitMemberInitializationViaInitAccessor(dc, selfDecl, pbd, subs);
           continue;
+        }
       }
 
       for (auto i : range(pbd->getNumPatternEntries())) {