Enable -Wunreachable-code

...motivated by <https://gerrit.libreoffice.org/#/c/41565/2> adding dead code
at the end of a switch statement, after the last case's "break".

-Wunreachable-code appears to work well on Clang, while it appears to have no
effect on GCC.

Most of the affected places are apparently temporary/TODO/FIXME cases of
disabling code via "if (false)", which can be written with an extra set of
parentheses as "if ((false))" to silence -Wunreachable-code on Clang (which thus
needed loplugin:unnecessaryparen to be adapted accordingly).  In some cases,
the controlling expression was more complex than just "false" and needed to be
rewritten by taking it out of the if statement to silence Clang.

One noteworthy case where the nature of the disabled code wasn't immediately
apparent:

Sep 12 16:59:58 <sberg> quikee, is that "if (false)" in
ScExponentialSmoothingDialog::ApplyOutput
(sc/source/ui/StatisticsDialogs/ExponentialSmoothingDialog.cxx) some work-in-
progress or dead code?
Sep 12 17:02:03 <quikee> sberg: WIP, but you can remove it
Sep 12 17:04:47 <sberg> quikee, I'll wrap the false in an extra set of
parentheses for now, to silence -Wunreachable-code (I wouldn't want to
remove it, as I have no idea whether I should then also remove the "Initial
value" comment preceding it)
Sep 12 17:07:29 <quikee> sberg: both are different ways to calculate the
"intital value"... so no

Another case where the nature of the dead code, following while (true) loops
without breaks, is unclear is sd/source/ui/remotecontrol/BluetoothServer.cxx,
where I added TODO markers to the workarounds that silence the warnings for now.

basic/source/sbx/sbxvalue.cxx had a variable of type double, of automatic
storage duration, and without an initalizer at the top of a switch statement.
Clang warning about it is arguably a false positive.

Apart from that, this didn't find any cases of genuinely dead code in the
existing code base.

Change-Id: Ib00b822c8efec94278c048783d5997b8ba86a94c
Reviewed-on: https://gerrit.libreoffice.org/42217
Tested-by: Stephan Bergmann <sbergman@redhat.com>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

3 files changed, 13 insertions, 1 deletions

diff --git a/compilerplugins/clang/test/unnecessaryparen.cxx b/compilerplugins/clang/test/unnecessaryparen.cxx
index cb237c551889..51f792769bc2 100644
--- a/compilerplugins/clang/test/unnecessaryparen.cxx
+++ b/compilerplugins/clang/test/unnecessaryparen.cxx
@@ -34,6 +34,12 @@ int main()
 
     int v1 = (static_cast<short>(1)) + 1; // expected-error {{unnecessary parentheses around cast [loplugin:unnecessaryparen]}}
     (void)v1;
+
+    // No warnings, used to silence -Wunreachable-code:
+    if ((false)) {
+        return 0;
+    }
+    x = (true) ? 0 : 1;
 };
 
 /* vim:set shiftwidth=4 softtabstop=4 expandtab cinoptions=b1,g0,N-s cinkeys+=0=break: */
diff --git a/compilerplugins/clang/test/vclwidgets.cxx b/compilerplugins/clang/test/vclwidgets.cxx
index 9ead1c905289..c470f991a667 100644
--- a/compilerplugins/clang/test/vclwidgets.cxx
+++ b/compilerplugins/clang/test/vclwidgets.cxx
@@ -22,7 +22,7 @@ struct Widget : public VclReferenceBase
         Widget* p = mpParent;
         (void)p;
         // test against false+
-        p = true ? mpParent.get() : nullptr;
+        p = (true) ? mpParent.get() : nullptr;
     }
 
     ~Widget() override
diff --git a/compilerplugins/clang/unnecessaryparen.cxx b/compilerplugins/clang/unnecessaryparen.cxx
index 8a94051d5bf4..02b71694e6ac 100644
--- a/compilerplugins/clang/unnecessaryparen.cxx
+++ b/compilerplugins/clang/unnecessaryparen.cxx
@@ -224,6 +224,12 @@ void UnnecessaryParen::VisitSomeStmt(const Stmt *parent, const Expr* cond, Strin
     if (parenExpr) {
         if (parenExpr->getLocStart().isMacroID())
             return;
+        // Used to silence -Wunreachable-code:
+        if (isa<CXXBoolLiteralExpr>(parenExpr->getSubExpr())
+            && stmtName == "if")
+        {
+            return;
+        }
         // assignments need extra parentheses or they generate a compiler warning
         auto binaryOp = dyn_cast<BinaryOperator>(parenExpr->getSubExpr());
         if (binaryOp && binaryOp->getOpcode() == BO_Assign)