Improve handling of synthetic generated constructors in refaster

The return type is a non-null 'void' type in recent JDK versions.

#1106

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=313289453

Author: cushon

core/src/main/java/com/google/errorprone/refaster/UClassDecl.java

@@ -22,6 +22,7 @@
 import com.google.common.collect.DiscreteDomain;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Range;
+import com.google.errorprone.util.ASTHelpers;
 import com.sun.source.tree.ClassTree;
 import com.sun.source.tree.MethodTree;
 import com.sun.source.tree.ModifiersTree;
@@ -112,11 +113,12 @@ public Choice<Unifier> visitClass(ClassTree node, Unifier unifier) {
     Choice<UnifierWithRemainingMembers> path =
         Choice.of(UnifierWithRemainingMembers.create(unifier, getMembers()));
     for (Tree targetMember : node.getMembers()) {
-      if (!(targetMember instanceof MethodTree)
-          || ((MethodTree) targetMember).getReturnType() != null) {
+      if (targetMember instanceof MethodTree
+          && ASTHelpers.isGeneratedConstructor((MethodTree) targetMember)) {
         // skip synthetic constructors
-        path = path.thenChoose(match(targetMember));
+        continue;
       }
+      path = path.thenChoose(match(targetMember));
     }
     return path.condition(s -> s.remainingMembers().isEmpty())
         .transform(UnifierWithRemainingMembers::unifier);

core/src/main/java/com/google/errorprone/refaster/UTemplater.java

@@ -515,9 +515,11 @@ public UNewClass visitNewClass(NewClassTree tree, Void v) {
   public UClassDecl visitClass(ClassTree tree, Void v) {
     ImmutableList.Builder<UMethodDecl> decls = ImmutableList.builder();
     for (MethodTree decl : Iterables.filter(tree.getMembers(), MethodTree.class)) {
-      if (decl.getReturnType() != null) {
-        decls.add(visitMethod(decl, null));
+      if (ASTHelpers.isGeneratedConstructor(decl)) {
+        // skip synthetic constructors
+        continue;
       }
+      decls.add(visitMethod(decl, null));
     }
     return UClassDecl.create(decls.build());
   }