Implement -Wself-assign, which warns on code such as:

  int x = 42;
  x = x;  // Warns here.

The warning avoids macro expansions, templates, user-defined assignment
operators, and volatile types, so false positives are expected to be low.

The common (mis-)use of this code pattern is to silence unused variable
warnings, but a more idiomatic way of doing that is '(void)x;'.
A follow-up to this will add a note and fix-it hint suggesting this
replacement in cases where the StmtExpr consists precisely of the self
assignment.

llvm-svn: 122804

1 files changed, 47 insertions, 0 deletions

diff --git a/clang/test/SemaCXX/warn-self-assign.cpp b/clang/test/SemaCXX/warn-self-assign.cpp
new file mode 100644
index 00000000000..fcdb2ab6bc6
--- /dev/null
+++ b/clang/test/SemaCXX/warn-self-assign.cpp
@@ -0,0 +1,47 @@
+// RUN: %clang_cc1 -fsyntax-only -Wself-assign -verify %s
+
+void f() {
+  int a = 42, b = 42;
+  a = a; // expected-warning{{explicitly assigning}}
+  b = b; // expected-warning{{explicitly assigning}}
+  a = b;
+  b = a = b;
+  a = a = a; // expected-warning{{explicitly assigning}}
+  a = b = b = a;
+}
+
+// Dummy type.
+struct S {};
+
+void false_positives() {
+#define OP =
+#define LHS a
+#define RHS a
+  int a = 42;
+  // These shouldn't warn due to the use of the preprocessor.
+  a OP a;
+  LHS = a;
+  a = RHS;
+  LHS OP RHS;
+#undef OP
+#undef LHS
+#undef RHS
+
+  S s;
+  s = s; // Not a builtin assignment operator, no warning.
+
+  // Volatile stores aren't side-effect free.
+  volatile int vol_a;
+  vol_a = vol_a;
+  volatile int &vol_a_ref = vol_a;
+  vol_a_ref = vol_a_ref;
+}
+
+template <typename T> void g() {
+  T a;
+  a = a; // May or may not be a builtin assignment operator, no warning.
+}
+void instantiate() {
+  g<int>();
+  g<S>();
+}