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>

3 files changed, 4 insertions, 8 deletions

diff --git a/src/corelib/kernel/qmetatype.cpp b/src/corelib/kernel/qmetatype.cpp
index 5ad7967605..1047c8c0aa 100644
--- a/src/corelib/kernel/qmetatype.cpp
+++ b/src/corelib/kernel/qmetatype.cpp
@@ -2006,8 +2006,7 @@ static bool convertToEnum(QMetaType fromType, const void *from, QMetaType toType
         *static_cast<qint64 *>(to) = value;
         return true;
     default:
-        Q_UNREACHABLE();
-        return false;
+        Q_UNREACHABLE_RETURN(false);
     }
 }
 
diff --git a/src/corelib/kernel/qpropertyprivate.h b/src/corelib/kernel/qpropertyprivate.h
index 966821fa67..4387f0fbeb 100644
--- a/src/corelib/kernel/qpropertyprivate.h
+++ b/src/corelib/kernel/qpropertyprivate.h
@@ -194,8 +194,7 @@ struct BindingFunctionVTable
                     return true;
                 } else {
                     // Our code will never instantiate this
-                    Q_UNREACHABLE();
-                    return false;
+                    Q_UNREACHABLE_RETURN(false);
                 }
             },
             /*destroy*/[](void *f){ static_cast<Callable *>(f)->~Callable(); },
diff --git a/src/corelib/kernel/qvariant.cpp b/src/corelib/kernel/qvariant.cpp
index 5e949487db..3fb80f203e 100644
--- a/src/corelib/kernel/qvariant.cpp
+++ b/src/corelib/kernel/qvariant.cpp
@@ -2261,8 +2261,7 @@ static bool integralEquals(uint promotedType, const QVariant::Private *d1, const
     if (promotedType == QMetaType::ULongLong)
         return qulonglong(l1) == qulonglong(l2);
 
-    Q_UNREACHABLE();
-    return 0;
+    Q_UNREACHABLE_RETURN(0);
 }
 
 namespace {
@@ -2306,8 +2305,7 @@ static std::optional<int> integralCompare(uint promotedType, const QVariant::Pri
     if (promotedType == QMetaType::ULongLong)
         return spaceShip<qulonglong>(l1, l2);
 
-    Q_UNREACHABLE();
-    return 0;
+    Q_UNREACHABLE_RETURN(0);
 }
 
 static std::optional<int> numericCompare(const QVariant::Private *d1, const QVariant::Private *d2)