Summary
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Add a new lint option, `missing-explicit-ctor`, to `javac` to warn about reliance on a default constructor in a class that is part of a formal API.

Problem
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While convenient for informal classes, default constructors in formal API classes will not have javadoc and may be unintended and unwanted.

Solution
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Warn about reliance on a default constructor on classes where there is some likelihood of it being a problem, name public classes in a named package that has an unqualified export from its module. In addition, for nested classes, all the syntactically enclosing classes must be public too.

Specification
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New text displayed as a key for `javac`'s `-Xlint`:


         missing-explicit-ctor Warn about missing explicit constructors in public classes in exported packages.