[nullsafe] Add a FP test for null-checking assignment result

Summary:
Nullability of the assignment result is not refined in code snippets
like:
```
while ((a = foo.getA()) != null) {
    nonNullableVal = a;
}
```
Let's add a test for this.

Reviewed By: jvillard

Differential Revision: D22136218

fbshipit-source-id: 206c368d6

Author: artempyanykh

infer/tests/codetoanalyze/java/nullsafe/ReturnNotNullable.java

@@ -214,4 +214,16 @@ int returnsZero() {
     field = 0;
     return field;
   }
+
+  static class AssignmentResultCheck {
+    // T58407328
+    public Throwable nullCheckAssignmentResultAsNonnull_FP(Throwable error) {
+      Throwable cause;
+      while ((cause = error.getCause()) != null) {
+        error = cause;
+      }
+
+      return error;
+    }
+  }
 }

infer/tests/codetoanalyze/java/nullsafe/issues.exp

@@ -302,7 +302,9 @@ codetoanalyze/java/nullsafe/PropagatesNullable.java, codetoanalyze.java.nullsafe
 codetoanalyze/java/nullsafe/PropagatesNullable.java, codetoanalyze.java.nullsafe_default.TestPropagatesNullable$TestSecondParameter.test(java.lang.String,java.lang.String):void, 7, ERADICATE_NULLABLE_DEREFERENCE, no_bucket, WARNING, [`nullable(...)` is nullable and is not locally checked for null when calling `length()`.]
 codetoanalyze/java/nullsafe/PropagatesNullable.java, codetoanalyze.java.nullsafe_default.TestPropagatesNullable$TestSecondParameter.test(java.lang.String,java.lang.String):void, 11, ERADICATE_NULLABLE_DEREFERENCE, no_bucket, WARNING, [`nullable(...)` is nullable and is not locally checked for null when calling `length()`.]
 codetoanalyze/java/nullsafe/PropagatesNullable.java, codetoanalyze.java.nullsafe_default.TestPropagatesNullable$TestSecondParameter.test(java.lang.String,java.lang.String):void, 15, ERADICATE_NULLABLE_DEREFERENCE, no_bucket, WARNING, [`nullable(...)` is nullable and is not locally checked for null when calling `length()`.]
-codetoanalyze/java/nullsafe/ReturnNotNullable.java, Linters_dummy_method, 19, ERADICATE_META_CLASS_NEEDS_IMPROVEMENT, no_bucket, INFO, [], ReturnNotNullable, codetoanalyze.java.nullsafe_default, issues: 9, curr_mode: "Default"
+codetoanalyze/java/nullsafe/ReturnNotNullable.java, Linters_dummy_method, 19, ERADICATE_META_CLASS_NEEDS_IMPROVEMENT, no_bucket, INFO, [], ReturnNotNullable, codetoanalyze.java.nullsafe_default, issues: 11, curr_mode: "Default"
+codetoanalyze/java/nullsafe/ReturnNotNullable.java, codetoanalyze.java.nullsafe_default.ReturnNotNullable$AssignmentResultCheck.nullCheckAssignmentResultAsNonnull_FP(java.lang.Throwable):java.lang.Throwable, 0, ERADICATE_RETURN_NOT_NULLABLE, no_bucket, WARNING, [`nullCheckAssignmentResultAsNonnull_FP(...)`: return type is declared non-nullable but the method returns a nullable value: call to Throwable.getCause() at line 222 (declared nullable in nullsafe/third-party-signatures/java.sig at line 2).]
+codetoanalyze/java/nullsafe/ReturnNotNullable.java, codetoanalyze.java.nullsafe_default.ReturnNotNullable$AssignmentResultCheck.nullCheckAssignmentResultAsNonnull_FP(java.lang.Throwable):java.lang.Throwable, 2, ERADICATE_NULLABLE_DEREFERENCE, no_bucket, WARNING, [`error` is nullable and is not locally checked for null when calling `getCause()`: call to Throwable.getCause() at line 222 (declared nullable in nullsafe/third-party-signatures/java.sig at line 2).]
 codetoanalyze/java/nullsafe/ReturnNotNullable.java, codetoanalyze.java.nullsafe_default.ReturnNotNullable$ConditionalAssignment.test(boolean):java.lang.Object, 0, ERADICATE_RETURN_NOT_NULLABLE, no_bucket, WARNING, [`test(...)`: return type is declared non-nullable but the method returns a nullable value: field f1 at line 199.]
 codetoanalyze/java/nullsafe/ReturnNotNullable.java, codetoanalyze.java.nullsafe_default.ReturnNotNullable.constantToNullableIsOverannotated():java.lang.String, 0, ERADICATE_RETURN_OVER_ANNOTATED, no_bucket, ADVICE, [Method `constantToNullableIsOverannotated()` is annotated with `@Nullable` but never returns null.]
 codetoanalyze/java/nullsafe/ReturnNotNullable.java, codetoanalyze.java.nullsafe_default.ReturnNotNullable.getResourceNullable(java.lang.Class,java.lang.String):java.net.URL, 0, ERADICATE_RETURN_NOT_NULLABLE, no_bucket, WARNING, [`getResourceNullable(...)`: return type is declared non-nullable but the method returns a nullable value: call to Class.getResource(...) at line 177 (nullable according to nullsafe internal models).]

infer/tests/codetoanalyze/java/nullsafe/third-party-signatures/java.sig

@@ -1 +1,2 @@
-java.lang.String#concat(java.lang.String)
+java.lang.String#concat(java.lang.String)
+java.lang.Throwable#getCause() @Nullable