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

Fixes #62925.

The following code:
```cpp
#include <map>

int main() {
   std::map m1 = {std::pair{"foo", 2}, {"bar", 3}}; // guide #2
   std::map m2(m1.begin(), m1.end()); // guide #1
}
```
Is rejected by clang, but accepted by both gcc and msvc:
https://godbolt.org/z/6v4fvabb5 .

So basically CTAD with copy-list-initialization is rejected.

Note that this exact code is also used in a cppreference article:
https://en.cppreference.com/w/cpp/container/map/deduction_guides

I checked the C++11 and C++20 standard drafts to see whether suppressing
user conversion is the correct thing to do for user conversions. Based
on the standard I don't think that it is correct.

```
13.3.1.4 Copy-initialization of class by user-defined conversion [over.match.copy]
Under the conditions specified in 8.5, as part of a copy-initialization of an object of class type, a user-defined
conversion can be invoked to convert an initializer expression to the type of the object being initialized.
Overload resolution is used to select the user-defined conversion to be invoked
```
So we could use user defined conversions according to the standard.

```
If a narrowing conversion is required to initialize any of the elements, the
program is ill-formed.
```
We should not do narrowing.

```
In copy-list-initialization, if an explicit constructor is chosen, the initialization is ill-formed.
```
We should not use explicit constructors.

Author: spaits

clang/docs/ReleaseNotes.rst

@@ -207,6 +207,12 @@ C++20 Feature Support
   to update the ``__cpp_concepts`` macro to `202002L`. This enables
   ``<expected>`` from libstdc++ to work correctly with Clang.
 
+- User defined constructors are allowed for copy-list-initialization with CTAD.
+  The example code for deduction guides for std::map in
+  (`cppreference <https://en.cppreference.com/w/cpp/container/map/deduction_guides>`_)
+  will now work.
+  (#GH62925).
+
 C++23 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 

clang/include/clang/Sema/Initialization.h

@@ -603,7 +603,7 @@ class InitializationKind {
     /// Normal context
     IC_Normal,
 
-    /// Normal context, but allows explicit conversion functionss
+    /// Normal context, but allows explicit conversion functions
     IC_ExplicitConvs,
 
     /// Implicit context (value initialization)

clang/lib/Sema/SemaInit.cpp

@@ -10906,8 +10906,6 @@ QualType Sema::DeduceTemplateSpecializationFromInitializer(
     // FIXME: The "second phase of [over.match.list] case can also
     // theoretically happen here, but it's not clear whether we can
     // ever have a parameter of the right type.
-    bool SuppressUserConversions = Kind.isCopyInit();
-
     if (TD) {
       SmallVector<Expr *, 8> TmpInits;
       for (Expr *E : Inits)
@@ -10917,12 +10915,12 @@ QualType Sema::DeduceTemplateSpecializationFromInitializer(
           TmpInits.push_back(E);
       AddTemplateOverloadCandidate(
           TD, FoundDecl, /*ExplicitArgs=*/nullptr, TmpInits, Candidates,
-          SuppressUserConversions,
+          /*SuppressUserConversions=*/false,
           /*PartialOverloading=*/false, AllowExplicit, ADLCallKind::NotADL,
           /*PO=*/{}, AllowAggregateDeductionCandidate);
     } else {
       AddOverloadCandidate(GD, FoundDecl, Inits, Candidates,
-                           SuppressUserConversions,
+                           /*SuppressUserConversions=*/false,
                            /*PartialOverloading=*/false, AllowExplicit);
     }
   };

clang/test/SemaCXX/ctad-copy-init-list-narrowing.cpp

@@ -0,0 +1,46 @@
+// 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) {}
+};
+
+// 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>>) {}
+    map(std::initializer_list<pair<T, Y>>, int a) {}
+};
+
+} // namespace std
+
+// 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() {
+  std::map aOk = {{std::pair{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.
+  std::map aNarrowNested = {{std::pair{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}}
+
+  // Verify that narrowing conversion is disabled in the first level of nesting.
+  std::map aNarrow = {{std::pair{5, 'c'}, {5, 'c'}}, 5.0f}; // expected-error {{type 'float' cannot be narrowed to 'int' in initializer list}} // expected-note {{insert an explicit cast to silence this issue}}
+}