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

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Gábor Spaits (spaits)

<details>
<summary>Changes</summary>

This PR would solve issue #<!-- -->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 did some debugging and found out, that the reason why this does not work in clang is that, in the function `Sema::DeduceTemplateSpecializationFromInitializer` we explicitly suppress user conversion, if the initialization type is a copy initialization.

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.

Also a note:
In `InitializationKind::InitKind` there is no init kind called `IK_CopyList` but there is `IK_DirectList`, should we add `IK_CopyList`? Or it is unnecessary for now?

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


3 Files Affected:

- (modified) clang/include/clang/Sema/Initialization.h (+1-1) 
- (modified) clang/lib/Sema/SemaInit.cpp (+2-4) 
- (added) clang/test/SemaCXX/ctad-copy-init-list-narrowing.cpp (+90) 


``````````diff
diff --git a/clang/include/clang/Sema/Initialization.h b/clang/include/clang/Sema/Initialization.h
index f443e327eaf32..4b876db436b48 100644
--- a/clang/include/clang/Sema/Initialization.h
+++ b/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)
diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index ed8b226a6b39f..211b6887befa3 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -10892,8 +10892,6 @@ 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)
@@ -10903,12 +10901,12 @@ 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);
     }
   };
diff --git a/clang/test/SemaCXX/ctad-copy-init-list-narrowing.cpp b/clang/test/SemaCXX/ctad-copy-init-list-narrowing.cpp
new file mode 100644
index 0000000000000..21b1137158d5a
--- /dev/null
+++ b/clang/test/SemaCXX/ctad-copy-init-list-narrowing.cpp
@@ -0,0 +1,90 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wno-unused-value -std=c++20 %s
+namespace std
+{
+  typedef long unsigned int size_t;
+}
+
+namespace std
+{
+  template<class _E>
+    class initializer_list
+    {
+    public:
+      typedef _E value_type;
+      typedef const _E& reference;
+      typedef const _E& const_reference;
+      typedef size_t size_type;
+      typedef const _E* iterator;
+      typedef const _E* const_iterator;
+
+    private:
+      iterator _M_array;
+      size_type _M_len;
+
+
+      constexpr initializer_list(const_iterator __a, size_type __l)
+      : _M_array(__a), _M_len(__l) { }
+
+    public:
+      constexpr initializer_list() noexcept
+      : _M_array(0), _M_len(0) { }
+
+
+      constexpr size_type
+      size() const noexcept { return _M_len; }
+
+
+      constexpr const_iterator
+      begin() const noexcept { return _M_array; }
+
+
+      constexpr const_iterator
+      end() const noexcept { return begin() + size(); }
+    };
+
+  template<class _Tp>
+    constexpr const _Tp*
+    begin(initializer_list<_Tp> __ils) noexcept
+    { return __ils.begin(); }
+
+  template<class _Tp>
+    constexpr const _Tp*
+    end(initializer_list<_Tp> __ils) noexcept
+    { return __ils.end(); }
+}
+
+template<class T, class Y>
+class pair{
+    private:
+    T fst;
+    Y snd;
+    public:
+    pair(T f, Y s) : fst(f), snd(s) {}
+};
+
+template<class T, class Y>
+class map {
+    public:
+    map(std::initializer_list<pair<T, Y>>, int a = 4, int b = 5) {}
+};
+
+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) {}
+};
+
+int main() {
+    map mOk ={pair{5, 'a'}, {6, 'b'}, {7, 'c'}};
+    map mNarrow ={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}}
+
+    A aOk = {{Contained{5, 'c'}, {5, 'c'}}, 5};
+    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}}
+    A aNarrow = {{Contained{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}}
+}

``````````

</details>


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