[clang] Warning for incorrect useof 'pure' attribute (PR #78200)

[Aaron Ballman via cfe-commits]

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@@ -692,6 +692,13 @@ def warn_maybe_falloff_nonvoid_function : Warning<
 def warn_falloff_nonvoid_function : Warning<
   "non-void function does not return a value">,
   InGroup<ReturnType>;
+def warn_pure_attr_on_cxx_constructor : Warning<
+  "constructor cannot be 'pure' (undefined behavior)">,
+  InGroup<IncorrectAttributeUsage>;
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AaronBallman wrote:

Also, I don't think it's always UB to mark a constructor as being pure, it's just not a sensible operation because a constructor does not produce a value. (For example, this constructor doesn't violate any principles of function purity except that it's not called for its value: `struct S { [[gnu::pure]] S() {} };`)

And I think the same logic applies to marking a destructor as pure; it doesn't make sense as an operation.

How about: `'%select{pure|const}0' attribute applied to a function which %select{returns 'void'|has no return value}1; attribute ignored`? Then it handles both situations, both attributes, and we don't need to invent a new warning group (it can go under `IgnoredAttributes`).

https://github.com/llvm/llvm-project/pull/78200