[clang] [Clang][Sema] Reject template arguments not equivalent to their copies (part of P2308R1) (PR #193754)

[Yanzuo Liu via cfe-commits]

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@@ -7272,14 +7338,10 @@ ExprResult Sema::CheckTemplateArgument(NamedDecl *Param, QualType ParamType,
     return Arg;
   }
 
-  // Avoid making a copy when initializing a template parameter of class type
-  // from a template parameter object of the same type. This is going beyond
-  // the standard, but is required for soundness: in
-  //   template<A a> struct X { X *p; X<a> *q; };
-  // ... we need p and q to have the same type.
-  //
-  // Similarly, don't inject a call to a copy constructor when initializing
-  // from a template parameter of the same type.
+  // Fast path. A valid template parameter object is equivalent to its copy. No
+  // need to make a copy to check again when initializing a template parameter
+  // of class type from a template parameter object of the same type.
+  // TODO: Remove `ParamType->isRecordType()` in condition?
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zwuis wrote:

I thought this would improve performance but I'm not sure without benchmark. Removed the TODO.

https://github.com/llvm/llvm-project/pull/193754