[clang][Sema] Warn on self move for inlined static cast

[MaxEW707]

There are code bases that inline std::move manually via static_cast.
Treat a static cast to an xvalue as an inlined std::move call and warn on a self move.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Max Winkler (MaxEW707)

Changes

There are code bases that inline std::move manually via static_cast.
Treat a static cast to an xvalue as an inlined std::move call and warn on a self move.

---

Full diff: https://github.com/llvm/llvm-project/pull/76646.diff

2 Files Affected:

• (modified) clang/lib/Sema/SemaChecking.cpp (+14-5)
• (modified) clang/test/SemaCXX/warn-self-move.cpp (+13)

diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 2a69325f029514..a21410434d8099 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -18843,17 +18843,26 @@ void Sema::DiagnoseSelfMove(const Expr *LHSExpr, const Expr *RHSExpr,
   LHSExpr = LHSExpr->IgnoreParenImpCasts();
   RHSExpr = RHSExpr->IgnoreParenImpCasts();
 
-  // Check for a call expression
+  // Check for a call expression or static_cast expression
   const CallExpr *CE = dyn_cast<CallExpr>(RHSExpr);
-  if (!CE || CE->getNumArgs() != 1)
+  const CXXStaticCastExpr *CXXSCE = dyn_cast<CXXStaticCastExpr>(RHSExpr);
+  if (!CE && !CXXSCE)
     return;
 
   // Check for a call to std::move
-  if (!CE->isCallToStdMove())
+  if (CE && (CE->getNumArgs() != 1 || !CE->isCallToStdMove()))
     return;
 
-  // Get argument from std::move
-  RHSExpr = CE->getArg(0);
+  // Check for a static_cast<T&&>(..) to an xvalue which we can treat as an
+  // inlined std::move
+  if (CXXSCE && !CXXSCE->isXValue())
+    return;
+
+  // Get argument from std::move or static_cast
+  if (CE)
+    RHSExpr = CE->getArg(0);
+  else
+    RHSExpr = CXXSCE->getSubExpr();
 
   const DeclRefExpr *LHSDeclRef = dyn_cast<DeclRefExpr>(LHSExpr);
   const DeclRefExpr *RHSDeclRef = dyn_cast<DeclRefExpr>(RHSExpr);
diff --git a/clang/test/SemaCXX/warn-self-move.cpp b/clang/test/SemaCXX/warn-self-move.cpp
index 0987e9b6bf6017..d0158626424142 100644
--- a/clang/test/SemaCXX/warn-self-move.cpp
+++ b/clang/test/SemaCXX/warn-self-move.cpp
@@ -16,6 +16,9 @@ void int_test() {
   x = std::move(x);  // expected-warning{{explicitly moving}}
   (x) = std::move(x);  // expected-warning{{explicitly moving}}
 
+  x = static_cast<int&&>(x);  // expected-warning{{explicitly moving}}
+  (x) = static_cast<int&&>(x);  // expected-warning{{explicitly moving}}
+
   using std::move;
   x = move(x); // expected-warning{{explicitly moving}} \
                    expected-warning {{unqualified call to 'std::move}}
@@ -26,6 +29,9 @@ void global_int_test() {
   global = std::move(global);  // expected-warning{{explicitly moving}}
   (global) = std::move(global);  // expected-warning{{explicitly moving}}
 
+  global = static_cast<int&&>(global);  // expected-warning{{explicitly moving}}
+  (global) = static_cast<int&&>(global);  // expected-warning{{explicitly moving}}
+
   using std::move;
   global = move(global); // expected-warning{{explicitly moving}} \
                              expected-warning {{unqualified call to 'std::move}}
@@ -35,11 +41,14 @@ class field_test {
   int x;
   field_test(field_test&& other) {
     x = std::move(x);  // expected-warning{{explicitly moving}}
+    x = static_cast<int&&>(x);  // expected-warning{{explicitly moving}}
     x = std::move(other.x);
     other.x = std::move(x);
     other.x = std::move(other.x);  // expected-warning{{explicitly moving}}
+    other.x = static_cast<int&&>(other.x);  // expected-warning{{explicitly moving}}
   }
   void withSuggest(int x) {
+    x = static_cast<int&&>(x); // expected-warning{{explicitly moving variable of type 'int' to itself; did you mean to move to member 'x'?}}
     x = std::move(x); // expected-warning{{explicitly moving variable of type 'int' to itself; did you mean to move to member 'x'?}}
   }
 };
@@ -50,11 +59,15 @@ struct C { C() {}; ~C() {} };
 void struct_test() {
   A a;
   a = std::move(a);  // expected-warning{{explicitly moving}}
+  a = static_cast<A&&>(a);  // expected-warning{{explicitly moving}}
 
   B b;
   b = std::move(b);  // expected-warning{{explicitly moving}}
+  b = static_cast<B&&>(b);  // expected-warning{{explicitly moving}}
   b.a = std::move(b.a);  // expected-warning{{explicitly moving}}
+  b.a = static_cast<A&&>(b.a);  // expected-warning{{explicitly moving}}
 
   C c;
   c = std::move(c);  // expected-warning{{explicitly moving}}
+  c = static_cast<C&&>(c);  // expected-warning{{explicitly moving}}
 }

[MaxEW707]

Ping

[shafik]

LGTM, any objections @tbaederr

[tbaederr]

Much better, thanks.

[MaxEW707]

Thanks all for the review :).

I will need one of you to commit on my behalf since I do not have write access.