Autogenerate default Java constructors

[owengray-google]

Describe the bug
You do not autogenerate the default Java constructor. We autogenerate it downstream, but that causes us issues with the explicitly-hidden-constructor case.

(the first set of assertions is the Documentable tree, the second set is the component tree we/dackka generate from it)

@Test
fun `Default constructors are autogenerated when no explicit constructor is present`() {
    val moduleJ = """
        public class Foo {}
    """.trimIndent().render(java = true)
    val classlikeJ = moduleJ.page("Foo").data.content
    val moduleK = """
        public class Foo {}
    """.trimIndent().render(java = false)
    val classlikeK = moduleK.page("Foo").data.content

    // test the Documentables tree. The correct behavior would be both isNotEmpty
    for (module in listOf(moduleJ, moduleK)) {
        val classlike = module.explicitClasslike("Foo")
        if (module == moduleK)
            assertThat((classlike as DClass).constructors).isEmpty()
        else
            assertThat((classlike as DClass).constructors).isNotEmpty()
    }

    // test the Documentables tree. The correct behavior is both isNotEmpty
    for (classlike in listOf(classlikeJ, classlikeK)) {
        assertThat(classlike.data.publicConstructorsDetails).isNotEmpty()
    }
}

@Test
fun `Default constructors are not autogenerated when private-constuctor pattern is used`() {
    val moduleJ = """
        public class Foo { private Foo() {} }
    """.trimIndent().render(java = true)
    val classlikeJ = moduleJ.page("Foo").data.content
    val moduleK = """
        public class Foo private constructor() {}
    """.trimIndent().render(java = false)
    val classlikeK = moduleK.page("Foo").data.content

    // test the Documentables tree. The correct behavior is both isEmpty
    for (module in listOf(moduleJ, moduleK)) {
        val classlike = module.explicitClasslike("Foo")
        assertThat((classlike as DClass).constructors).isEmpty()
    }

    // test our Components tree. The correct behavior is both isEmpty.
    // Our fix for the next test breaks us in this case. I think this is unavoidably upstream.
    for (classlike in listOf(classlikeJ, classlikeK)) {
        if (classlike == classlikeK)
            assertThat(classlike.data.publicConstructorsDetails).isNotEmpty()
        else
            assertThat(classlike.data.publicConstructorsDetails).isEmpty()        }
}

[IgnatBeresnev]

The problem seems to be that PsiClass#getConstructors returns an empty array for the following class:

/**
 * Java documentation
 */
public class JavaClass {
    
}

We create DClass from Java PSIs in DefaultPsiToDocumentableTranslator.

I don't see any methods that would return the default empty constructor, so we might need to hardcode it (i.e create a DFunction for the constructor on the fly). However, there might already exist a function among PSIs that can either return the default primary constructor or create a synthetic one - it needs to be researched first.

For anyone looking to contribute, Developer Guides might be a good place to start.