[clang] [Sema][CTAD] Allow user defined conversion for copy-list-initialization (PR #94752)

[Haojian Wu via cfe-commits]

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@@ -0,0 +1,65 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wno-unused-value -std=c++20 %s
+
+namespace std {
+  typedef decltype(sizeof(int)) size_t;
+
+  template <typename E>
+  struct initializer_list
+  {
+    const E *p;
+    size_t n;
+    initializer_list(const E *p, size_t n) : p(p), n(n) {}
+  };
+
+  struct string {
+    string(const char *);
+  };
+
+  // Classes to use to reproduce the exact scenario present in 62925.
+template<class T, class Y>
+class pair{
+    public:
+    pair(T f, Y s) {}
+};
+
+template<class T, class Y>
+class map {
+    public:
+    map(std::initializer_list<pair<T, Y>>, int a = 4, int b = 5) {}
+};
+
+} // namespace std
+
+
+// Classes to test different levels of nestings and conversions.
+template<class T, class Y>
+class Contained {
+  public:
+  Contained(T, Y) {}
+};
+
+template<class T, class Y>
+class A {
+  public:
+  A(std::initializer_list<Contained<T, Y> >, int) {}
+};
+
+
+// This is the almost the exact code that was in issue #62925.
+void testOneLevelNesting() {
+  std::map mOk = {std::pair{5, 'a'}, {6, 'b'}, {7, 'c'}};
+
+  // Verify that narrowing conversion is disabled in the first level of nesting.
+  std::map mNarrow = {std::pair{5, 'a'}, {6.0f, 'b'}, {7, 'c'}}; // expected-error {{type 'float' cannot be narrowed to 'int' in initializer list}} // expected-note {{insert an explicit cast to silence this issue}}
+}
+
+void testMultipleLevelNesting() {
+  A aOk = {{Contained{5, 'c'}, {5, 'c'}}, 5};
+
+  // Verify that narrowing conversion is disabled when it is not in a nested
+  // in another std::initializer_list, but it happens in the most outer one.
+  A aNarrowNested = {{Contained{5, 'c'}, {5.0f, 'c'}}, 5}; // expected-error {{type 'float' cannot be narrowed to 'int' in initializer list}} // expected-note {{insert an explicit cast to silence this issue}}
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hokein wrote:

Class `Contained` and `pair` look the same (also for `A` and `map`).

Can we reuse the `pair` and `map` to test the "mutiple level nesting" case?  e.g. `std::map mNaarowNested = {{Contained{5, 'c'}, {5.0f, 'c'}}, 5}};` (you probably need to add a constructor `map(std::initializer_list<pair<T, Y>>, int)`).

https://github.com/llvm/llvm-project/pull/94752