Support nullability inference for generic constructors in diamond calls

[coderabbitai]

Summary

Add support for nullability inference of type variables declared by generic constructors when a constructor call uses the diamond operator.

Rationale

GenericsChecks.generateConstraintsForCall currently registers type variables returned by getCallTypeParameters(callTree). For a NewClassTree, these are class type variables. A constructor can also declare its own type variables. The constraint solver treats unregistered type variables as fixed types, so constructor type variables in formal parameter types do not participate in nullability inference.

Affected area

• nullaway/src/main/java/com/uber/nullaway/generics/GenericsChecks.java
• Generic nullability inference for NewClassTree diamond calls

Required changes

• For NewClassTree calls, register the constructor type variables from getMethodSymbolForCall(callTree).getTypeParameters() with ConstraintSolver.
• Keep registration of class type variables. These variables remain necessary for constructed-result substitution.
• Add regression coverage for a diamond constructor call where the constructor declares its own type variables and nullability inference requires those variables.

Acceptance criteria

• Constructor-declared type variables participate in constraint generation and solving for diamond constructor calls.
• Existing class type-variable inference for diamond constructor calls remains unchanged.
• A regression test demonstrates the expected result for a generic constructor with constructor-declared type variables.

Backlinks

• PR Do not treat type variables from caller as inference variables #1824: Do not treat type variables from caller as inference variables #1824
• Review comment: Do not treat type variables from caller as inference variables #1824 (comment)

Requested by: @msridhar

[msridhar]

It seems we do have tests for generic constructors with an explicit type argument passing the type variable:

NullAway/nullaway/src/test/java/com/uber/nullaway/jspecify/GenericsTests.java

Line 2998 in b51e010

public void newNullableWithArgAndConstructorType() {

But, we do not have tests for generic constructors that require the type argument nullability to be inferred, or that use the diamond operator. We should test combinations of these when fixing this issue.