[clang] [Sema] Diagnose use of if/else-if condition variable inside else-if/else branch(s) (PR #156436)

[via cfe-commits]

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@@ -13660,10 +13594,11 @@ def warn_acc_var_referenced_lacks_op
               "reference has no effect">,
       InGroup<DiagGroup<"openacc-var-lacks-operation">>,
       DefaultError;
+def warn_out_of_scope_var_usage
+    : Warning<"variable %0 declared in 'if' block is always false or null here">,
+      InGroup<DiagGroup<"conditional-scope">>;
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arrowten wrote:

>Not really a fan of this wording. Perhaps we could use a %select here to decide whether this is a false or null (or perhaps even 0?). Or perhaps we should just say false always? What do others think?
I will try that.

>Also, a note pointing to the condition-var is probably a good idea too.
Its pointing to the "used" condition variable though.

https://github.com/llvm/llvm-project/pull/156436