Prevent ConstructorLeaksThis false positive on constructor

reference

Fixes #789

RELNOTES: Fixed false positive in ConstructorLeaksThis

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=185219776

core/src/main/java/com/google/errorprone/bugpatterns/ConstructorLeaksThis.java

@@ -29,6 +29,7 @@
 import com.sun.source.tree.MemberSelectTree;
 import com.sun.source.tree.Tree;
 import com.sun.source.util.TreeScanner;
+import com.sun.tools.javac.code.Symbol;
 import com.sun.tools.javac.code.Symbol.ClassSymbol;
 import javax.lang.model.element.Name;
 
@@ -83,7 +84,11 @@ public Void visitAssignment(AssignmentTree node, Void unused) {
 
   private void checkForThis(
       ExpressionTree node, Name identifier, ClassSymbol thisClass, VisitorState state) {
-    if (identifier.contentEquals("this") && thisClass.equals(ASTHelpers.getSymbol(node).owner)) {
+    Symbol sym = ASTHelpers.getSymbol(node);
+    if (sym != null
+        && !sym.isConstructor()
+        && identifier.contentEquals("this")
+        && thisClass.equals(sym.owner)) {
       state.reportMatch(describeMatch(node));
     }
   }

core/src/test/java/com/google/errorprone/bugpatterns/testdata/ConstructorLeaksThisNegativeCases.java

@@ -97,4 +97,13 @@ static class Inner {
     @SuppressWarnings("ConstructorLeaksThis")
     final FixtureController that = new FixtureController(this);
   }
+
+  static final class WithTwoConstructors {
+    public WithTwoConstructors() {
+      // 'this' references another constructor, not an object
+      this(0);
+    }
+
+    public WithTwoConstructors(int i) {}
+  }
 }