Compiler complains about “missing return statement” even though it is impossible to reach condition where return statement would be missing

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JLS 14.21, Unreachable Statements is the section that deals with this:

The if statement, whether or not it has an else part, is handled in an unusual manner. For this reason, it is discussed separately at the end of this section.

Ultimately it has to do with how conditional compilation is handled. Consider this method:

public int foo() {
    if (DEBUG) {
        return 5;
    }
}

If DEBUG is static final boolean true; you might think the compiler should be smart enough to realize the method will always return 5. But if it’s changed to false, the code is no longer valid.

The method must be valid for all paths through the method without a source code change, allowing optimizing compilers to omit bytecode without source modifications regardless of the flag’s value.

The very end of the linked JLS section goes in to significant detail.

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