[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

llvm-svn: 315787

2 files changed, 35 insertions, 1 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 31abdfa3f6c..e6f3aeae961 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/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/clang/test/SemaCXX/warn-unused-variables.cpp b/clang/test/SemaCXX/warn-unused-variables.cpp
index d7be785b35a..0974cf5a61d 100644
--- a/clang/test/SemaCXX/warn-unused-variables.cpp
+++ b/clang/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