[clang] [clang] CTAD alias: Respect explicit deduction guides defined after the first use of the alias template. (PR #125478)

[Shafik Yaghmour via cfe-commits]

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@@ -237,8 +237,17 @@ static_assert(__is_same(decltype(s.t), int));
 // explicit deduction guide.
 Foo(int) -> Foo<X>;
 AFoo s2{i};
-// FIXME: the type should be X because of the above explicit deduction guide.
-static_assert(__is_same(decltype(s2.t), int));
+static_assert(__is_same(decltype(s2.t), X));
+
+
+template<class T>
+using BFoo = AFoo<T>;
+static_assert(__is_same(decltype(BFoo(i).t), X));
+
+
+Foo(double) -> Foo<int>;
+static_assert(__is_same(decltype(AFoo(1.0).t), int));
+static_assert(__is_same(decltype(BFoo(1.0).t), int));
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shafik wrote:

Can we get more complicated test cases, w/ multiple template params and multiple constructor arguments etc e.g.: https://compiler-explorer.com/z/x8PhEqWeo

We should really be trying harder to stress test our implementation.

Does it fix this case: https://godbolt.org/z/b84rzhjjK if not we need to open a bug.

https://github.com/llvm/llvm-project/pull/125478