[clang] nonblocking/nonallocating attributes (was: nolock/noalloc) (PR #84983)

[via cfe-commits]

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@@ -1870,6 +1870,28 @@ bool Sema::IsFunctionConversion(QualType FromType, QualType ToType,
       FromFn = QT->getAs<FunctionType>();
       Changed = true;
     }
+
+    // For C, when called from checkPointerTypesForAssignment,
+    // we need not to alter FromFn, or else even an innocuous cast
+    // like dropping effects will fail. In C++ however we do want to
+    // alter FromFn. TODO: Is this correct?
+    if (getLangOpts().CPlusPlus) {
+      FromFPT =
+          dyn_cast<FunctionProtoType>(FromFn); // in case FromFn changed above
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Sirraide wrote:

No, that one’s different. 

The point here is that `cast` asserts if the value you’re casting is-not-a the type you’re casting to, whereas `dyn_cast` returns `nullptr`. As a result, the intended way to use `dyn_cast` is to follow it with a check to see if it returned `nullptr`; in this case, however, the next line immediately uses the result of the cast without doing any checks whatsoever, so this would just crash if `dyn_cast` returned null, whereas `cast` would instead assert, which is what we want. In other words, if you’re not doing null checks, you want to use `cast` instead of `dyn_cast`.

In this case, we’re in C++ mode (because of the surrounding `if`), so we should never have prototypeless functions; that means a `cast` here should be fine.

https://github.com/llvm/llvm-project/pull/84983