[clang] [Clang] Reapply CWG2369 "Ordering between constraints and substitution" (PR #122423)

[Erich Keane via cfe-commits]

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@@ -522,6 +522,12 @@ enum class TemplateSubstitutionKind : char {
     llvm::PointerUnion<Decl *, DeclArgumentPack *> *
     findInstantiationOf(const Decl *D);
 
+    /// Similar to \p findInstantiationOf(), but it wouldn't assert if the
+    /// instantiation was not found within the current instantiation scope. This
+    /// is helpful for on-demand declaration instantiation.
+    llvm::PointerUnion<Decl *, DeclArgumentPack *> *
+    findInstantiationUnsafe(const Decl *D);
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erichkeane wrote:

Instead of `unsafe`, I'd suggest something like `getInstantiationOfIfExists` and `getInstantiationOf` (or similar) rather than `unsafe`.  `unsafe` means, to me, that there is a precondition that if you don't setup correctly, it fails.  Not that it migth just return null.

Finally, as interface, is there a reason these are returning a pointer to a pointer union?  Seems oen should return by value, the other in an `optional`.

https://github.com/llvm/llvm-project/pull/122423