add JSType.restrictByPossiblyFalsy()

I intend to use this method in an upcoming change to add code
that creates new AST nodes from existing ones that already have type
information. It'll be used to calculate the right types for new
AND and OR nodes, for example.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=210971649

src/com/google/javascript/rhino/jstype/EnumElementType.java

@@ -255,4 +255,9 @@ JSType resolveInternal(ErrorReporter reporter) {
     primitiveObjectType = ObjectType.cast(primitiveType);
     return this;
   }
+
+  @Override
+  public JSType restrictByPossiblyFalsy() {
+    return primitiveType.restrictByPossiblyFalsy();
+  }
 }

src/com/google/javascript/rhino/jstype/JSType.java

@@ -40,6 +40,7 @@
 package com.google.javascript.rhino.jstype;
 
 import static com.google.common.base.Preconditions.checkNotNull;
+import static com.google.javascript.rhino.jstype.JSTypeNative.NO_TYPE;
 
 import com.google.common.base.Predicate;
 import com.google.common.collect.HashBasedTable;
@@ -1433,6 +1434,15 @@ public JSType restrictByNotUndefined() {
     return this;
   }
 
+  /**
+   * If this is a union type, returns a union type that only includes members that may be falsy. For
+   * example, {@code null|number|Object} would restrict to {@code null|number}, since null and
+   * number may both be falsy, while {@code Object} is never falsy.
+   */
+  public JSType restrictByPossiblyFalsy() {
+    return getNativeType(NO_TYPE);
+  }
+
   /**
    * the logic of this method is similar to isSubtype,
    * except that it does not perform structural interface matching

src/com/google/javascript/rhino/jstype/ProxyObjectType.java

@@ -417,4 +417,9 @@ public boolean hasAnyTemplateTypesInternal() {
   public TemplateTypeMap getTemplateTypeMap() {
     return referencedType.getTemplateTypeMap();
   }
+
+  @Override
+  public JSType restrictByPossiblyFalsy() {
+    return referencedType.restrictByPossiblyFalsy();
+  }
 }

src/com/google/javascript/rhino/jstype/SymbolType.java

@@ -39,12 +39,12 @@
 
 package com.google.javascript.rhino.jstype;
 
+import static com.google.javascript.rhino.jstype.JSTypeNative.NO_TYPE;
 import static com.google.javascript.rhino.jstype.JSTypeNative.SYMBOL_OBJECT_TYPE;
 import static com.google.javascript.rhino.jstype.JSTypeNative.SYMBOL_VALUE_OR_OBJECT_TYPE;
 import static com.google.javascript.rhino.jstype.TernaryValue.FALSE;
 import static com.google.javascript.rhino.jstype.TernaryValue.UNKNOWN;
 
-
 /**
  * Symbol type.
  * @author johnlenz@google.com (John Lenz)
@@ -118,4 +118,9 @@ public BooleanLiteralSet getPossibleToBooleanOutcomes() {
   public <T> T visit(Visitor<T> visitor) {
     return visitor.caseSymbolType();
   }
+
+  @Override
+  public JSType restrictByPossiblyFalsy() {
+    return registry.getNativeType(NO_TYPE);
+  }
 }

src/com/google/javascript/rhino/jstype/UnionType.java

@@ -268,6 +268,15 @@ public JSType restrictByNotUndefined() {
     return restricted.build();
   }
 
+  @Override
+  public JSType restrictByPossiblyFalsy() {
+    UnionTypeBuilder restricted = UnionTypeBuilder.create(registry);
+    for (JSType t : alternatesRetainingStructuralSubtypes) {
+      restricted.addAlternate(t.restrictByPossiblyFalsy());
+    }
+    return restricted.build();
+  }
+
   @Override
   public TernaryValue testForEquality(JSType that) {
     TernaryValue result = null;

src/com/google/javascript/rhino/jstype/UnknownType.java

@@ -179,4 +179,9 @@ JSType resolveInternal(ErrorReporter reporter) {
   int recursionUnsafeHashCode() {
     return System.identityHashCode(this);
   }
+
+  @Override
+  public JSType restrictByPossiblyFalsy() {
+    return this;
+  }
 }

src/com/google/javascript/rhino/jstype/ValueType.java

@@ -81,4 +81,9 @@ final int recursionUnsafeHashCode() {
     // Subclasses of this type are unique within a JSTypeRegisty.
     return System.identityHashCode(this);
   }
+
+  @Override
+  public JSType restrictByPossiblyFalsy() {
+    return this;
+  }
 }

src/com/google/javascript/rhino/testing/TypeSubject.java

@@ -116,6 +116,10 @@ public void isNotUnknown() {
     check("isUnknownType()").that(actualNonNull().isUnknownType()).isFalse();
   }
 
+  public void isEmpty() {
+    check("isEmptyType()").that(actualNonNull().isEmptyType()).isTrue();
+  }
+
   public void isNotEmpty() {
     check("isEmptyType()").that(actualNonNull().isEmptyType()).isFalse();
   }

test/com/google/javascript/rhino/jstype/UnionTypeTest.java

@@ -136,6 +136,26 @@ public void testUnionType() throws Exception {
     Asserts.assertResolvesToSame(nullOrString);
   }
 
+  public void testRestrictByNotUndefined() throws Exception {
+    UnionType nullOrString = (UnionType) createUnionType(NULL_TYPE, STRING_OBJECT_TYPE);
+    UnionType nullOrStringOrUndefined =
+        (UnionType) createUnionType(NULL_TYPE, STRING_OBJECT_TYPE, VOID_TYPE);
+
+    assertType(nullOrStringOrUndefined.restrictByNotUndefined())
+        .isStructurallyEqualTo(nullOrString);
+  }
+
+  public void testRestrictByPossiblyFalsy() throws Exception {
+    UnionType allPossiblyFalsy =
+        (UnionType) createUnionType(NULL_TYPE, VOID_TYPE, STRING_TYPE, NUMBER_TYPE, BOOLEAN_TYPE);
+    UnionType nonePossiblyFalsy = (UnionType) createUnionType(ARRAY_TYPE, STRING_OBJECT_TYPE);
+    UnionType mixture = (UnionType) createUnionType(allPossiblyFalsy, nonePossiblyFalsy);
+
+    assertType(allPossiblyFalsy.restrictByPossiblyFalsy()).isStructurallyEqualTo(allPossiblyFalsy);
+    assertType(mixture.restrictByPossiblyFalsy()).isStructurallyEqualTo(allPossiblyFalsy);
+    assertType(nonePossiblyFalsy.restrictByPossiblyFalsy()).isEmpty();
+  }
+
   /**
    * Tests {@link JSType#getGreatestSubtype(JSType)} on union types.
    */