Make loplugin:stringadd slightly more aggressive

...by assuming that all const member functions are side-effect free.  (This
presumably means that some of the special cases in
StringAdd::isSideEffectFree are obsoleted by this more general case, but any
such removal is postponed to later clean-up.)

(Came across this when idly wondering why
8b7f948d9d79393bc6c1b11d239706666fd5d7de "sc, VmlFormControlExporter: avoid
OStringBuffer style" had not been found by the plugin before.)

Change-Id: I6bca10df53885b14a590543aabd61f23b3748572
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/118675
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

2 files changed, 54 insertions, 0 deletions

diff --git a/compilerplugins/clang/stringadd.cxx b/compilerplugins/clang/stringadd.cxx
index c25992928f1e..394139dbb354 100644
--- a/compilerplugins/clang/stringadd.cxx
+++ b/compilerplugins/clang/stringadd.cxx
@@ -378,6 +378,36 @@ bool StringAdd::isSideEffectFree(Expr const* expr)
                         }
                     }
                 }
+                // Aggressively assume that calls to const member functions are side effect free (if
+                // all of the call's sub-expressions are):
+                if (calleeMethodDecl->isConst())
+                {
+                    auto sef = true;
+                    // Other options besides CXXMemberCallExpr are e.g. CXXOperatorCallExpr which
+                    // does not have such a target expression:
+                    if (auto const mce = dyn_cast<CXXMemberCallExpr>(callExpr))
+                    {
+                        if (!isSideEffectFree(mce->getImplicitObjectArgument()))
+                        {
+                            sef = false;
+                        }
+                    }
+                    if (sef)
+                    {
+                        for (unsigned i = 0; i != callExpr->getNumArgs(); ++i)
+                        {
+                            if (!isSideEffectFree(callExpr->getArg(i)))
+                            {
+                                sef = false;
+                                break;
+                            }
+                        }
+                    }
+                    if (sef)
+                    {
+                        return true;
+                    }
+                }
             }
         if (auto calleeFunctionDecl = dyn_cast_or_null<FunctionDecl>(callExpr->getCalleeDecl()))
             if (calleeFunctionDecl && calleeFunctionDecl->getIdentifier())
diff --git a/compilerplugins/clang/test/stringadd.cxx b/compilerplugins/clang/test/stringadd.cxx
index fb805ce519b9..a20b64698433 100644
--- a/compilerplugins/clang/test/stringadd.cxx
+++ b/compilerplugins/clang/test/stringadd.cxx
@@ -235,4 +235,28 @@ void f2(char ch)
     s = s + OString(ch);
 }
 }
+
+namespace test10
+{
+struct C
+{
+    OString constStringFunction(int) const;
+    OString nonConstStringFunction();
+    int constIntFunction() const;
+    int nonConstIntFunction();
+};
+
+C getC();
+
+void f1(C c)
+{
+    OString s;
+    // expected-error@+1 {{simplify by merging with the preceding assignment [loplugin:stringadd]}}
+    s += c.constStringFunction(c.constIntFunction());
+    s += c.constStringFunction(c.nonConstIntFunction());
+    s += c.nonConstStringFunction();
+    s += getC().constStringFunction(c.constIntFunction());
+}
+}
+
 /* vim:set shiftwidth=4 softtabstop=4 expandtab cinoptions=b1,g0,N-s cinkeys+=0=break: */