  We introduce a novel approach for testing static typing implementations based
on the concept of API-driven program synthesis. The idea is to synthesize
type-intensive but small and well-typed programs by leveraging and combining
application programming interfaces (APIs) derived from existing software
libraries. Our primary insight is backed up by real-world evidence: a
significant number of compiler typing bugs are caused by small test cases that
employ APIs from the standard library of the language under test. This is
attributed to the inherent complexity of the majority of these APIs, which
often exercise a wide range of sophisticated type-related features. The main
contribution of our approach is the ability to produce small client programs
with increased feature coverage, without bearing the burden of generating the
corresponding well-formed API definitions from scratch. To validate diverse
aspects of static typing procedures (i.e., soundness, precision of type
inference), we also enrich our API-driven approach with fault-injection and
semantics-preserving modes, along with their corresponding test oracles.
  We evaluate our implemented tool, Thalia on testing the static typing
implementations of the compilers for three popular languages, namely, Scala,
Kotlin, and Groovy. Thalia has uncovered 84 typing bugs (77 confirmed and 22
fixed), most of which are triggered by test cases featuring APIs that rely on
parametric polymorphism, overloading, and higher-order functions. Our
comparison with state-of-the-art shows that Thalia yields test programs with
distinct characteristics, offering additional and complementary benefits.
