================
@@ -3058,6 +3058,133 @@ void Sema::NoteAllFoundTemplates(TemplateName Name) {
}
}
+static QualType commonTypeImpl(Sema &S, TemplateName BaseTemplate,
+ SourceLocation TemplateLoc,
+ ArrayRef<TemplateArgument> Ts) {
+ auto lookUpCommonType = [&](TemplateArgument T1,
+ TemplateArgument T2) -> QualType {
+ // Don't bother looking for other specializations if both types are
+ // builtins - users aren't allowed to specialize for them
----------------
ldionne wrote:
I believe it would make sense for such a diagnostic to work today, even without
the builtin. For example, it would be nice for the following code to be
diagnosed with a warning (or an error, whatever):
```c++
#include <type_traits>
template <>
struct std::common_type<int, int> { using type = long; }; // should warn
struct Foo { };
template <>
struct std::common_type<Foo> { using type = int; }; // should warn
// etc...
```
I don't think this should be tied to the implementation of the builtin, i.e. it
should be diagnosed where the user writes the invalid specialization, not where
we use `std::common_type_t`. As such, I think it would make sense to tackle
this as a follow-up and I filed an issue for it:
https://github.com/llvm/llvm-project/issues/101509
https://github.com/llvm/llvm-project/pull/99473
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