[clang:diagnostics] Turning off warn_self_assignment_overloaded for user-defined compound assignments

Fixes 42469 https://github.com/llvm/llvm-project/issues/42469

Only check self assignment on BO_Assign when BuildOverloadedBinOp.

clang/lib/Sema/SemaExpr.cpp

@@ -15657,13 +15657,22 @@ static ExprResult BuildOverloadedBinOp(Sema &S, Scope *Sc, SourceLocation OpLoc,
                                        Expr *LHS, Expr *RHS) {
   switch (Opc) {
   case BO_Assign:
+    // In the non-overloaded case, we warn about self-assignment (x = x) for
+    // both simple assignment and certain compound assignments where algebra
+    // tells us the operation yields a constant result.  When the operator is
+    // overloaded, we can't do the latter because we don't want to assume that
+    // those algebraic identities still apply; for example, a path-building
+    // library might use operator/= to append paths.  But it's still reasonable
+    // to assume that simple assignment is just moving/copying values around
+    // and so self-assignment is likely a bug.
+    DiagnoseSelfAssignment(S, LHS, RHS, OpLoc, false);
+    [[fallthrough]];
   case BO_DivAssign:
   case BO_RemAssign:
   case BO_SubAssign:
   case BO_AndAssign:
   case BO_OrAssign:
   case BO_XorAssign:
-    DiagnoseSelfAssignment(S, LHS, RHS, OpLoc, false);
     CheckIdentityFieldAssignment(LHS, RHS, OpLoc, S);
     break;
   default:

clang/test/SemaCXX/warn-self-assign-overloaded.cpp

@@ -53,15 +53,15 @@ void f() {
 
 #ifndef DUMMY
   a *= a;
-  a /= a; // expected-warning {{explicitly assigning}}
-  a %= a; // expected-warning {{explicitly assigning}}
+  a /= a;
+  a %= a;
   a += a;
-  a -= a; // expected-warning {{explicitly assigning}}
+  a -= a;
   a <<= a;
   a >>= a;
-  a &= a; // expected-warning {{explicitly assigning}}
-  a |= a; // expected-warning {{explicitly assigning}}
-  a ^= a; // expected-warning {{explicitly assigning}}
+  a &= a;
+  a |= a;
+  a ^= a;
 #endif
 }