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

[Erich Keane via cfe-commits]

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@@ -11792,6 +11792,32 @@ static bool CheckMultiVersionFunction(Sema &S, FunctionDecl *NewFD,
                                          OldDecl, Previous);
 }
 
+static void CheckFunctionDeclarationAttributesUsage(Sema &S,
+                                                    FunctionDecl *NewFD) {
+  const bool is_pure = NewFD->hasAttr<PureAttr>();
+  const bool is_const = NewFD->hasAttr<ConstAttr>();
+
+  if (is_pure && is_const) {
+    S.Diag(NewFD->getLocation(), diag::warn_const_attr_with_pure_attr);
+    NewFD->dropAttr<PureAttr>();
+  }
+  if (is_pure || is_const) {
+    if (isa<CXXConstructorDecl>(NewFD)) {
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erichkeane wrote:

I don't agree that a constructor 'returns a value'.  Both a constructor and a destructor are the same in that they are functions that modify their 1st parameter (the this parameter) or 'throw'.  They are effectively the same thing.

I don't see how a constructor could ever make sense with 'pure', same as it doesn't make sense on destructors.

We should be handling them the same.

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