[Sema] Warn about unused variables if we can constant evaluate the initializer.

If the variable construction can be constant evaluated it doesn't have
side effects, so removing it is always safe. We only try to evaluate
variables that are unused, there should be no impact on compile time.

Differential Revision: https://reviews.llvm.org/D38678

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@315787 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Sema/SemaDecl.cpp b/lib/Sema/SemaDecl.cpp
index 31abdfa3f6c0d1cb183471ac83536c713287c0b0..e6f3aeae9617e52fc9e05c4f358cfe352d148268 100644 (file)
--- a/lib/Sema/SemaDecl.cpp
+++ b/lib/Sema/SemaDecl.cpp
@@ -1723,7 +1723,8 @@ static bool ShouldDiagnoseUnusedDecl(const NamedDecl *D) {
             dyn_cast<CXXConstructExpr>(Init);
           if (Construct && !Construct->isElidable()) {
             CXXConstructorDecl *CD = Construct->getConstructor();
-            if (!CD->isTrivial() && !RD->hasAttr<WarnUnusedAttr>())
+            if (!CD->isTrivial() && !RD->hasAttr<WarnUnusedAttr>() &&
+                !VD->evaluateValue())
               return false;
           }
         }

diff --git a/test/SemaCXX/warn-unused-variables.cpp b/test/SemaCXX/warn-unused-variables.cpp
index d7be785b35a6918c856573992c3c124a0b2d5bbf..0974cf5a61d699699714f67aebc77b7307c13d7f 100644 (file)
--- a/test/SemaCXX/warn-unused-variables.cpp
+++ b/test/SemaCXX/warn-unused-variables.cpp
@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -fsyntax-only -Wunused-variable -Wunused-label -Wno-c++1y-extensions -verify %s
+// RUN: %clang_cc1 -fsyntax-only -Wunused-variable -Wunused-label -Wno-c++1y-extensions -verify -std=c++11 %s
 template<typename T> void f() {
   T t;
   t = 17;
@@ -194,3 +195,35 @@ void test() {
 }
 
 }
+
+#if __cplusplus >= 201103L
+namespace with_constexpr {
+template <typename T>
+struct Literal {
+  T i;
+  Literal() = default;
+  constexpr Literal(T i) : i(i) {}
+};
+
+struct NoLiteral {
+  int i;
+  NoLiteral() = default;
+  constexpr NoLiteral(int i) : i(i) {}
+  ~NoLiteral() {}
+};
+
+static Literal<int> gl1;          // expected-warning {{unused variable 'gl1'}}
+static Literal<int> gl2(1);       // expected-warning {{unused variable 'gl2'}}
+static const Literal<int> gl3(0); // expected-warning {{unused variable 'gl3'}}
+
+template <typename T>
+void test(int i) {
+  Literal<int> l1;     // expected-warning {{unused variable 'l1'}}
+  Literal<int> l2(42); // expected-warning {{unused variable 'l2'}}
+  Literal<int> l3(i);  // no-warning
+  Literal<T> l4(0);    // no-warning
+  NoLiteral nl1;       // no-warning
+  NoLiteral nl2(42);   // no-warning
+}
+}
+#endif