Infer @Nullable type arguments for type variables from unmarked code (#1181)

Fixes #1178

Author: msridhar

nullaway/src/main/java/com/uber/nullaway/generics/GenericsChecks.java

@@ -453,8 +453,12 @@ public void checkTypeParameterNullnessForAssignability(
           && methodInvocationTree.getTypeArguments().isEmpty()) {
         // generic method call with no explicit generic arguments
         // update inferred type arguments based on the assignment context
-        InferSubstitutionViaAssignmentContextVisitor inferVisitor =
-            new InferSubstitutionViaAssignmentContextVisitor(state, config);
+        boolean invokedMethodIsNullUnmarked =
+            CodeAnnotationInfo.instance(state.context)
+                .isSymbolUnannotated(methodSymbol, config, analysis.getHandler());
+        InferGenericMethodSubstitutionViaAssignmentContextVisitor inferVisitor =
+            new InferGenericMethodSubstitutionViaAssignmentContextVisitor(
+                state, config, invokedMethodIsNullUnmarked);
         Type returnType = methodSymbol.getReturnType();
         returnType.accept(inferVisitor, lhsType);
 

nullaway/src/main/java/com/uber/nullaway/generics/InferSubstitutionViaAssignmentContextVisitor.java renamed to nullaway/src/main/java/com/uber/nullaway/generics/InferGenericMethodSubstitutionViaAssignmentContextVisitor.java

@@ -14,16 +14,19 @@
  * Visitor that infers a substitution for type variables via types appearing at the same position in
  * a type provided via the assignment context.
  */
-public class InferSubstitutionViaAssignmentContextVisitor
+public class InferGenericMethodSubstitutionViaAssignmentContextVisitor
     extends Types.DefaultTypeVisitor<Void, Type> {
 
   private final VisitorState state;
   private final Config config;
+  private final boolean invokedMethodIsNullUnmarked;
   private final Map<TypeVariable, Type> inferredSubstitution = new LinkedHashMap<>();
 
-  InferSubstitutionViaAssignmentContextVisitor(VisitorState state, Config config) {
+  InferGenericMethodSubstitutionViaAssignmentContextVisitor(
+      VisitorState state, Config config, boolean invokedMethodIsNullUnmarked) {
     this.state = state;
     this.config = config;
+    this.invokedMethodIsNullUnmarked = invokedMethodIsNullUnmarked;
   }
 
   @Override
@@ -64,9 +67,12 @@ public Void visitTypeVar(Type.TypeVar typeVar, Type lhsType) {
     Type upperBound = typeVar.getUpperBound();
     boolean typeVarHasNullableUpperBound =
         Nullness.hasNullableAnnotation(upperBound.getAnnotationMirrors().stream(), config);
-    if (typeVarHasNullableUpperBound) { // can just use the lhs type nullability
+    if (typeVarHasNullableUpperBound
+        || invokedMethodIsNullUnmarked) { // can just use the lhs type nullability
       inferredSubstitution.put(typeVar, lhsType);
     } else { // rhs can't be nullable.  use lhsType but strip @Nullable annotation
+      // TODO we should just strip out the top-level @Nullable annotation;
+      //  stripMetadata() also removes nested @Nullable annotations
       inferredSubstitution.put(typeVar, lhsType.stripMetadata());
     }
     return null;

nullaway/src/test/java/com/uber/nullaway/jspecify/GenericMethodTests.java

@@ -506,6 +506,26 @@ public void issue1176() {
         .doTest();
   }
 
+  @Test
+  public void issue1178() {
+    makeHelper()
+        .addSourceLines(
+            "SampleNullUnmarkedCall.java",
+            "import org.jspecify.annotations.*;",
+            "@NullMarked",
+            "class SampleNullUnmarkedCall {",
+            "  @NullUnmarked",
+            "  static class Foo<T> {",
+            "    Foo(T t) {}",
+            "    static <U> Foo<U> id(U u) { return new Foo<>(u); }",
+            "  }",
+            "  static void test() {",
+            "    Foo<@Nullable Object> x = Foo.id(null);",
+            "  }",
+            "}")
+        .doTest();
+  }
+
   private CompilationTestHelper makeHelper() {
     return makeTestHelperWithArgs(
         Arrays.asList(