Long live Q_UNREACHABLE_RETURN()!

This is a combination of Q_UNREACHABLE() with a return statement.

ATM, the return statement is unconditionally included. If we notice
that some compilers warn about return after __builtin_unreachable(),
then we can map Q_UNREACHABLE_RETURN(...) to Q_UNREACHABLE() without
having to touch all the code that uses explicit Q_UNREACHABLE() +
return.

The fact that Boost has BOOST_UNREACHABLE_RETURN() indicates that
there are compilers that complain about a lack of return after
Q_UNREACHABLE (we know that MSVC, ICC, and GHS are among them), as
well as compilers that complained about a return being present
(Coverity). Take this opportunity to properly adapt to Coverity, by
leaving out the return statement on this compiler.

Apply the macro around the code base, using a clang-tidy transformer
rule:

    const std::string unr = "unr", val = "val", ret = "ret";
    auto makeUnreachableReturn = cat("Q_UNREACHABLE_RETURN(",
                                    ifBound(val, cat(node(val)), cat("")),
                                    ")");
    auto ignoringSwitchCases = [](auto stmt) {
        return anyOf(stmt, switchCase(subStmt(stmt)));
    };

    makeRule(
       stmt(ignoringSwitchCases(stmt(isExpandedFromMacro("Q_UNREACHABLE")).bind(unr)),
            nextStmt(returnStmt(optionally(hasReturnValue(expr().bind(val)))).bind(ret))),
       {changeTo(node(unr), cat(makeUnreachableReturn,
                                ";")),  // TODO: why is the ; lost w/o this?
        changeTo(node(ret), cat(""))},
       cat("use ", makeUnreachableReturn))
    );

where nextStmt() is copied from some upstream clang-tidy check's
private implementation and subStmt() is a private matcher that gives
access to SwitchCase's SubStmt.

A.k.a. qt-use-unreachable-return.

There were some false positives, suppressed them with NOLINTNEXTLINE.

They're not really false positiives, it's just that Clang sees the
world in one way and if conditonal compilation (#if) differs for other
compilers, Clang doesn't know better. This is an artifact of matching
two consecutive statements.

I haven't figured out how to remove the empty line left by the
deletion of the return statement, if it, indeed, was on a separate
line, so post-processed the patch to remove all the lines matching
^\+ *$ from the diff:

  git commit -am meep
  git reset --hard HEAD^
  git diff HEAD..HEAD@{1} | sed '/^\+ *$/d' | recountdiff - | patch -p1

[ChangeLog][QtCore][QtAssert] Added Q_UNREACHABLE_RETURN() macro.

Change-Id: I9782939f16091c964f25b7826e1c0dbd13a71305
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 1 insertions, 2 deletions

diff --git a/src/network/access/http2/hpacktable.cpp b/src/network/access/http2/hpacktable.cpp
index 0b69ee86a9..74a09a207f 100644
--- a/src/network/access/http2/hpacktable.cpp
+++ b/src/network/access/http2/hpacktable.cpp
@@ -346,8 +346,7 @@ quint32 FieldLookupTable::indexOfChunk(const Chunk *chunk) const
             return quint32(i);
     }
 
-    Q_UNREACHABLE();
-    return 0;
+    Q_UNREACHABLE_RETURN(0);
 }
 
 quint32 FieldLookupTable::keyToIndex(const SearchEntry &key) const