Fix failure of loplugin:useuniqueptr with older compilers

...where CompilerTest_compilerplugins_clang failed in
compilerplugins/clang/test/useuniqueptr.cxx due to Foo24's

  HTMLAttrs::const_iterator it = m_aSetAttrTab.begin();

and either the old compiler lacked Clang's recent
<https://reviews.llvm.org/D50666> "Fix Stmt::ignoreImplicit" (and the above
initialization expression happens to include a CXXBindTemporaryExpr, at least
with libstdc++), or an even older compiler was used in pre-C++17 mode, so the
above initialization expression happens to include an elidable CXXConstructExpr
copy constructor call.

Change-Id: I757a9ad76829e399b4fe2da1c82863909b8c9657
Reviewed-on: https://gerrit.libreoffice.org/61531
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

compilerplugins/clang/compat.hxx

@@ -193,6 +193,45 @@ inline llvm::StringRef getImmediateMacroNameForDiagnostics(
 #endif
 }
 
+inline clang::Expr const * IgnoreImplicit(clang::Expr const * expr) {
+#if CLANG_VERSION >= 80000
+    return expr->IgnoreImplicit();
+#else
+    using namespace clang;
+    // Copy from Clang's lib/AST/Stmt.cpp, including <https://reviews.llvm.org/D50666> "Fix
+    // Stmt::ignoreImplicit":
+    Stmt const *s = expr;
+
+    Stmt const *lasts = nullptr;
+
+    while (s != lasts) {
+        lasts = s;
+
+        if (auto *ewc = dyn_cast<ExprWithCleanups>(s))
+            s = ewc->getSubExpr();
+
+        if (auto *mte = dyn_cast<MaterializeTemporaryExpr>(s))
+            s = mte->GetTemporaryExpr();
+
+        if (auto *bte = dyn_cast<CXXBindTemporaryExpr>(s))
+            s = bte->getSubExpr();
+
+        if (auto *ice = dyn_cast<ImplicitCastExpr>(s))
+            s = ice->getSubExpr();
+    }
+
+    return static_cast<Expr const *>(s);
+#endif
+}
+
+inline bool CPlusPlus17(clang::LangOptions const & opts) {
+#if CLANG_VERSION >= 60000
+    return opts.CPlusPlus17;
+#else
+    return opts.CPlusPlus1z;
+#endif
+}
+
 // Work around <http://reviews.llvm.org/D22128>:
 //
 // SfxErrorHandler::GetClassString (svtools/source/misc/ehdl.cxx):

compilerplugins/clang/test/useuniqueptr.cxx

@@ -278,7 +278,6 @@ class Foo23
     }
 };
 
-#if CLANG_VERSION >= 60000
 class Foo24
 {
     typedef std::vector<int*> HTMLAttrs;
@@ -289,7 +288,6 @@ class Foo24
             delete *it; // expected-error {{rather manage with std::some_container<std::unique_ptr<T>> [loplugin:useuniqueptr]}}
     }
 };
-#endif
 
 //  ------------------------------------------------------------------------------------------------
 // tests for passing owning pointers to constructors

compilerplugins/clang/useuniqueptr.cxx

@@ -495,12 +495,16 @@ void UseUniquePtr::CheckLoopDelete(const FunctionDecl* functionDecl, const CXXDe
                     auto init = iterVarDecl->getInit();
                     if (init)
                     {
-                        init = init->IgnoreImplicit();
+                        init = compat::IgnoreImplicit(init);
+                        if (!compat::CPlusPlus17(compiler.getLangOpts()))
+                            if (auto x = dyn_cast<CXXConstructExpr>(init))
+                                if (x->isElidable())
+                                    init = compat::IgnoreImplicit(x->getArg(0));
                         if (auto x = dyn_cast<CXXConstructExpr>(init))
                             if (x->getNumArgs() == 1
                                 || (x->getNumArgs() >= 2 && isa<CXXDefaultArgExpr>(x->getArg(1))))
                             {
-                                init = x->getArg(0)->IgnoreImplicit();
+                                init = compat::IgnoreImplicit(x->getArg(0));
                             }
                         if (auto x = dyn_cast<CXXMemberCallExpr>(init))
                             init = x->getImplicitObjectArgument();