C++: Fix decltype resolving for template function

The last nail for std::unique_ptr (GCC variant, MSVC still doesn't work).

Use-case:
template<typename T>
static T f();

struct Foo { int bar; };

void fun()
{
    decltype(f<Foo>()) s;
    s.bar; // bar not highlighted
}

Task-number: QTCREATORBUG-14483
Task-number: QTCREATORBUG-8937
Change-Id: I5bab757400b070cf9dbb688a44fd8eafe95ddc61
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

src/libs/cplusplus/LookupContext.cpp

@@ -967,7 +967,7 @@ void CreateBindings::lookupInScope(const Name *name, Scope *scope,
             if (const TemplateNameId *instantiation = name->asTemplateNameId()) {
                 if (Template *specialization = s->asTemplate()) {
                     if (const Symbol *decl = specialization->declaration()) {
-                        if (decl->isFunction()) {
+                        if (decl->isFunction() || decl->isDeclaration()) {
                             Clone cloner(_control.data());
                             Subst subst(_control.data());
                             initializeSubst(cloner, subst, binding, scope,

src/plugins/cpptools/cppchecksymbols.cpp

@@ -1300,12 +1300,8 @@ bool CheckSymbols::maybeAddFunction(const QList<LookupItem> &candidates, NameAST
         isConstructor = isConstructorDeclaration(c);
 
         Function *funTy = c->type()->asFunctionType();
-        if (!funTy) {
-            //Try to find a template function
-            if (Template * t = r.type()->asTemplateType())
-                if ((c = t->declaration()))
-                    funTy = c->type()->asFunctionType();
-        }
+        if (!funTy) // Template function has an overridden type
+            funTy = r.type()->asFunctionType();
         if (!funTy)
             continue; // TODO: add diagnostic messages and color call-operators calls too?
 

src/plugins/cpptools/cppcompletion_test.cpp

@@ -2984,6 +2984,21 @@ void CppToolsPlugin::test_completion_data()
     ) << _("s.") << (QStringList()
             << QLatin1String("Foo")
             << QLatin1String("bar"));
+
+    QTest::newRow("typedefed_decltype_of_template_function") << _(
+            "template<typename T>\n"
+            "static T f();\n"
+            "\n"
+            "struct Foo { int bar; };\n"
+            "\n"
+            "void fun()\n"
+            "{\n"
+            "    decltype(f<Foo>()) s;\n"
+            "    @\n"
+            "}\n"
+    ) << _("s.") << (QStringList()
+            << QLatin1String("Foo")
+            << QLatin1String("bar"));
 }
 
 void CppToolsPlugin::test_completion_member_access_operator()