Same type checking and array types and containsTypeEquivalent question

[Laird Nelson]

I had a question about a particular branch of "same type" checking in the
compiler.

In TypeEqualityVisitor, when it is testing for "same typeness" with regard
to array types
<https://github.com/openjdk/jdk/blob/jdk-27%2B3/src/jdk.compiler/share/classes/com/sun/tools/javac/code/Types.java#L1430-L1440>,
it does the following:

return s.hasTag(ARRAY)
                    && containsTypeEquivalent(t.elemtype, elemtype(s));

So if (t is an array type and) s is an array type and t's element type
contains a type that is equivalent to s's element type, then t and s are in
this case deemed to be the same.

(containsTypeEquivalent tests to see if t and s are the same type or
contain each other
<https://github.com/openjdk/jdk/blob/jdk-27%2B3/src/jdk.compiler/share/classes/com/sun/tools/javac/code/Types.java#L4623-L4626>
.)

I'm surprised by the use of containsTypeEquivalent here, since it seems to
me that the only way in which it could do anything interesting in this
usage beyond applying a "same type" test is if it were possible for an
array type's element type to be a wildcard. But this is impossible, so
containsTypeEquivalent, when called from this location with these
arguments, would seem to always devolve to a simple "same type" check,
since containsType devolves to a "same type" test
<https://github.com/openjdk/jdk/blob/jdk-27%2B3/src/jdk.compiler/share/classes/com/sun/tools/javac/code/Types.java#L1575>
in the presence of non-wildcard arguments.

After a brief conversation, Dan Smith suggested I write here and see if
TypeEqualityVisitor's visitArrayType method needs to be refactored, or if
there's some edge case I'm not seeing (entirely possible), perhaps around
error cases and such, that requires this usage of containsTypeEquivalent in
the array type code path.

Happy new year,
Laird
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