Suppress -Wwarn-unused-variables when we don't know the constructor

This warning is supposed to be suppressed when the
constructor/destructor are non-trivial, since it might be a RAII type.
However, if the type has a trivial destructor and the constructor hasn't
been resolved (since it is called with dependent arguments), we were
still warning.

This patch suppresses the warning if the type could possibly have a
be a non-trivial constructor call.  Note that this does not take the
arity of the constructors into consideration, so it might suppress
the warning in cases where it isn't possible to call a non-trivial
constructor.

clang/lib/Sema/SemaDecl.cpp

@@ -1805,6 +1805,13 @@ static bool ShouldDiagnoseUnusedDecl(const NamedDecl *D) {
                 (VD->getInit()->isValueDependent() || !VD->evaluateValue()))
               return false;
           }
+
+          // Suppress the warning if we don't know how this is constructed, and
+          // it could possibly be non-trivial constructor.
+          if (Init->isTypeDependent())
+            for (const CXXConstructorDecl *Ctor : RD->ctors())
+              if (!Ctor->isTrivial())
+                return false;
         }
       }
     }

clang/test/SemaCXX/warn-unused-variables.cpp

@@ -238,4 +238,18 @@ void c() {
   a d(b::e ? "" : "");
 }
 }
+
+// Ensure we don't warn on dependent constructor calls.
+namespace dependent_ctor {
+struct S {
+  S() = default;
+  S(const S &) = default;
+  S(int);
+};
+
+template <typename T>
+void foo(T &t) {
+  S s{t};
+}
+}
 #endif