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

[Chuanqi Xu via cfe-commits]

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@@ -11889,6 +11889,13 @@ bool Sema::CheckFunctionDeclaration(Scope *S, FunctionDecl *NewFD,
     NewFD->setInvalidDecl();
   }
 
+  if (NewFD->hasAttr<PureAttr>() || NewFD->hasAttr<ConstAttr>()) {
+    if (isa<CXXConstructorDecl>(NewFD))
+      Diag(NewFD->getLocation(), diag::warn_pure_attr_on_cxx_constructor);
+    else if (NewFD->getReturnType()->isVoidType())
+      Diag(NewFD->getLocation(), diag::warn_pure_function_returns_void);
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ChuanqiXu9 wrote:

Then the title and the diagnostic might be confusing if the user are using `[[gnu::const]]`.  You can look at the use of `%select` in `clang/include/clang/Basic/DiagnosticSemaKinds.td` to improve this. Also we need a test for `[[gnu::const]]` too.

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