Reject explicit conversion functions for list-initialized reference temporaries (CWG 2267) [GH #344]

src/il.h

@@ -2823,6 +2823,10 @@ typedef int a_conv_context_set;
 #define CCO_FORCE_DEPENDENCE ((a_conv_context_set)0x20000000)
 			/* TRUE if overload resolution should treat the call
 			   as dependent in any case. */
+#define CCO_EXPLICIT_CONV_FUNCTIONS_RETURN_REF ((a_conv_context_set)0x40000000)
+			/* Used with CCO_ALLOW_EXPLICIT_CONV_FUNCTIONS to
+			   restrict the explicit conversion functions allowed
+			   to those that return a reference. */
 
 
 /*

src/overload.c

@@ -16401,7 +16401,10 @@ ultimate binding is to an rvalue reference .
         }  /* if */
       } else if (conv_context & CCO_ALLOW_EXPLICIT_CONV_FUNCTIONS) {
         /* We've been told specifically to allow explicit conversion
-           functions. */
+           functions (possibly only those that return a reference). */
+        if (conv_context & CCO_EXPLICIT_CONV_FUNCTIONS_RETURN_REF) {
+          require_reference_return = TRUE;
+        }  /* if */
       } else {
         goto reject_function;
       }  /* if */
@@ -28193,6 +28196,32 @@ will be an lvalue instead of the usual prvalue.
            in reproducing the source when the cp_gen_be is used. */
         rmake_lvalue_temp = TRUE;
       }  /* if */
+      if (is_direct_init &&
+          (clang_version_is(any_version) || gnu_version_is(<90000) ||
+           ms_version_is(<1929) ||
+           (ms_version_is(any_version) && ms_permissive))) {
+        /* Core issue 2267 clarified that the temporary is copy-list-
+           initialized even when the reference is direct-list-initialized,
+           and so explicit conversion functions cannot be used to initialize
+           it.  For example:
+              struct A { explicit operator int(); } a;
+              int &&r{a};  // Error.
+           Clang, g++ before version 9, and MSVC in permissive mode (and
+           before version 19.29 also in non-permissive mode) accept such
+           cases, though. */
+        rconv_context |= CCO_ALLOW_EXPLICIT_CONV_FUNCTIONS;
+      } else if (is_direct_init && gnu_version_is(>= 140000)) {
+        /* g++ 14 and later bind the reference directly to the result of a
+           conversion function that returns a reference (a proposed resolution
+           of Core issue 1996), and that binding may use an explicit conversion
+           function:
+              struct B { explicit operator int&&(); };
+              int &&r{B{}};  // Okay with g++ 14.
+           Approximate that by allowing such conversion functions for the
+           initialization of the temporary. */
+        rconv_context |= CCO_ALLOW_EXPLICIT_CONV_FUNCTIONS |
+                         CCO_EXPLICIT_CONV_FUNCTIONS_RETURN_REF;
+      }  /* if */
       /* The cost of a reference initialization in overload resolution is the
          cost of the underlying initialization of the temporary, so pass
          arg_match down to the next level.  See below for class cases. */
@@ -28201,7 +28230,7 @@ will be an lvalue instead of the usual prvalue.
                             /*is_direct_init=*/FALSE,
                             /*check_narrowing=*/TRUE,
                             /*warning_on_narrowing=*/FALSE,
-                            rconv_context | CCO_ALLOW_EXPLICIT_CONV_FUNCTIONS,
+                            rconv_context,
                             /*fill_in_dtor=*/TRUE,
                             /*force_temp=*/TRUE,
                             rmake_lvalue_temp,

src/pending_changes/gh-344-list-init-ref-explicit-conv.txt

@@ -0,0 +1,36 @@
+[GH #344]
+Explicit conversion functions and list-initialization of references
+
+When a reference is list-initialized from an element whose type is not
+reference-related to the referenced type, a temporary is created and the
+reference is bound to that temporary.  The resolution of Core issue 2267
+clarified that this temporary is copy-list-initialized even when the reference
+itself is direct-list-initialized, so explicit conversion functions cannot be
+used to initialize it.  The front end nevertheless accepted them, even for
+copy-list-initialization (including when passing an argument).  That is now
+fixed.  For example:
+
+  struct A { explicit operator int() const; } a;
+  struct B { explicit operator int&&(); };
+  void f(int&&);
+  int &&r1{a};      // Now an error.
+  int &&r2{B{}};    // Now an error.
+  int &&r3 = {a};   // Now an error in all modes.
+  void g() {
+    f({a});         // Now an error in all modes.
+  }
+
+The change for EDGcpfe/23402 (in version 6.2) implemented the resolution of
+Core issue 2267 for initializations with parentheses (e.g., "int &&r(a);"),
+but not for list-initializations.  Those were still governed by the change for
+EDGcpfe/21369 (in version 5.1), which allowed explicit conversion functions to
+initialize the temporary in every list-initialization of a reference (to
+accept cases like r2 above).  That allowance is now limited to the emulation
+modes described below.
+
+Because Clang, GCC before version 9, and Microsoft C++ in permissive mode
+(and before version 19.29 also in non-permissive mode) accept the
+direct-list-initialization cases, those are still accepted in the
+corresponding emulation modes.  GCC 14 and later accept them only when the
+conversion function returns a reference (like r2 above), and so does the
+front end when emulating those versions.

tests/tests/edg/ccs/gnats21369.sft.cpp

@@ -1,5 +1,5 @@
 //type:cp
-//options_all:--c++11
+//options_all:--c++11 --gnu=80000
 
 struct A {
   explicit operator int() { return int{}; }

tests/tests/imported/clang/cpp/SemaCXX/.cxx2a-explicit-bool.rto/default.1.1.txt

@@ -356,10 +356,22 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
   int ai01 = {A<true>()}; // expected-error {{no viable conversion}}
               ^
 
+"Test_name.c", line 258: error: no suitable conversion function from "conversion::A<true>" to "const int" exists
+  const int& ai11 = {A<true>()}; // expected-error {{no viable conversion}}
+                     ^
+
+"Test_name.c", line 259: error: no suitable conversion function from "conversion::A<true>" to "int" exists
+  int&& ai31 = {A<true>()}; // expected-error {{no viable conversion}}
+                ^
+
 "Test_name.c", line 260: error: no suitable conversion function from "conversion::A<true>" to "int" exists
   int ai41 = {A_true}; // expected-error {{no viable conversion}}
               ^
 
+"Test_name.c", line 261: error: no suitable conversion function from "conversion::A<true>" to "const int" exists
+  const int& ai51 = {A_true}; // expected-error {{no viable conversion}}
+                     ^
+
 "Test_name.c", line 296: warning: 'explicit(bool)' is a C++20 feature
     explicit(enable_ifv<is_same<B, T2>::value, a>::value)
     ^
@@ -815,4 +827,4 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
     explicit(1L) S(char, char, char);
     ^
 
-118 errors detected in the compilation of "Test_name.c".
+121 errors detected in the compilation of "Test_name.c".

tests/tests/imported/clang/cpp/SemaCXX/.cxx2a-explicit-bool.rto/default.2.1.txt

@@ -296,10 +296,22 @@ fe_only -DTEST_NUMBER=2 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
   int ai01 = {A<true>()}; // expected-error {{no viable conversion}}
               ^
 
+"Test_name.c", line 258: error: no suitable conversion function from "conversion::A<true>" to "const int" exists
+  const int& ai11 = {A<true>()}; // expected-error {{no viable conversion}}
+                     ^
+
+"Test_name.c", line 259: error: no suitable conversion function from "conversion::A<true>" to "int" exists
+  int&& ai31 = {A<true>()}; // expected-error {{no viable conversion}}
+                ^
+
 "Test_name.c", line 260: error: no suitable conversion function from "conversion::A<true>" to "int" exists
   int ai41 = {A_true}; // expected-error {{no viable conversion}}
               ^
 
+"Test_name.c", line 261: error: no suitable conversion function from "conversion::A<true>" to "const int" exists
+  const int& ai51 = {A_true}; // expected-error {{no viable conversion}}
+                     ^
+
 "Test_name.c", line 305: error: no suitable conversion function from "conversion2::A<true>" to "int" exists
   int ai0 = A<true>(); // expected-error {{no viable conversion}}
             ^
@@ -683,4 +695,4 @@ fe_only -DTEST_NUMBER=2 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
     explicit(ptr) S(long);             // expected-error {{conversion from 'const int *' to 'bool' is not allowed in a converted constant expression}}
              ^
 
-121 errors detected in the compilation of "Test_name.c".
+124 errors detected in the compilation of "Test_name.c".

tests/tests/imported/clang/cpp/SemaCXX/.explicit.rto/default.1.1.txt

@@ -123,6 +123,10 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
       int copyList4 = {n}; // expected-error {{no viable conversion}}
                        ^
 
+"Test_name.c", line 201: error: no suitable conversion function from "NotBool" to "const bool" exists
+      const bool &copyList6 = {n}; // expected-error {{no viable conversion}}
+                               ^
+
 "Test_name.c", line 203: error: no suitable conversion function from "NotBool" to "const int" exists
       const int &copyList8 = {n}; // expected-error {{no viable conversion}}
                               ^
@@ -171,4 +175,4 @@ fe_only -DTEST_NUMBER=1 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
         switch(EC){}
                ^
 
-30 errors detected in the compilation of "Test_name.c".
+31 errors detected in the compilation of "Test_name.c".

tests/tests/imported/clang/cpp/SemaCXX/.explicit.rto/default.2.1.txt

@@ -131,6 +131,10 @@ fe_only -DTEST_NUMBER=2 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
       int copyList4 = {n}; // expected-error {{no viable conversion}}
                        ^
 
+"Test_name.c", line 201: error: no suitable conversion function from "NotBool" to "const bool" exists
+      const bool &copyList6 = {n}; // expected-error {{no viable conversion}}
+                               ^
+
 "Test_name.c", line 203: error: no suitable conversion function from "NotBool" to "const int" exists
       const int &copyList8 = {n}; // expected-error {{no viable conversion}}
                               ^
@@ -171,4 +175,4 @@ fe_only -DTEST_NUMBER=2 --clang -tused -e 200 --no_wrap_diagnostics --clang_vers
         switch(EC){}
                ^
 
-31 errors detected in the compilation of "Test_name.c".
+32 errors detected in the compilation of "Test_name.c".

tests/tests/regressions/.gh-344.rto/default.1.1.txt

@@ -0,0 +1,34 @@
+fe_only -DTEST_NUMBER=1 --c++11 Test_name.c
+"Test_name.c", line 21: error: no suitable conversion function from "E" to "int" exists
+  int &&r1{e};
+           ^
+
+"Test_name.c", line 22: error: no suitable conversion function from "E" to "const int" exists
+  const int &r2{e};
+                ^
+
+"Test_name.c", line 23: error: no suitable conversion function from "ER" to "int" exists
+  int &&r3{ER{}};
+           ^
+
+"Test_name.c", line 26: error: no suitable conversion function from "E" to "int" exists
+  int &&r4 = {e};
+              ^
+
+"Test_name.c", line 27: error: no suitable conversion function from "ER" to "int" exists
+  int &&r5 = {ER{}};
+              ^
+
+"Test_name.c", line 29: error: no suitable conversion function from "E" to "int" exists
+    h({e});
+       ^
+
+"Test_name.c", line 30: error: no suitable conversion function from "ER" to "int" exists
+    h({ER{}});
+       ^
+
+"Test_name.c", line 31: error: no suitable conversion function from "E" to "const int" exists
+    hc({e});
+        ^
+
+8 errors detected in the compilation of "Test_name.c".

tests/tests/regressions/.gh-344.rto/default.2.1.txt

@@ -0,0 +1,34 @@
+fe_only -DTEST_NUMBER=2 --c++20 Test_name.c
+"Test_name.c", line 21: error: no suitable conversion function from "E" to "int" exists
+  int &&r1{e};
+           ^
+
+"Test_name.c", line 22: error: no suitable conversion function from "E" to "const int" exists
+  const int &r2{e};
+                ^
+
+"Test_name.c", line 23: error: no suitable conversion function from "ER" to "int" exists
+  int &&r3{ER{}};
+           ^
+
+"Test_name.c", line 26: error: no suitable conversion function from "E" to "int" exists
+  int &&r4 = {e};
+              ^
+
+"Test_name.c", line 27: error: no suitable conversion function from "ER" to "int" exists
+  int &&r5 = {ER{}};
+              ^
+
+"Test_name.c", line 29: error: no suitable conversion function from "E" to "int" exists
+    h({e});
+       ^
+
+"Test_name.c", line 30: error: no suitable conversion function from "ER" to "int" exists
+    h({ER{}});
+       ^
+
+"Test_name.c", line 31: error: no suitable conversion function from "E" to "const int" exists
+    hc({e});
+        ^
+
+8 errors detected in the compilation of "Test_name.c".

tests/tests/regressions/.gh-344.rto/default.3.1.txt

@@ -0,0 +1,22 @@
+fe_only -DTEST_NUMBER=3 --g++ --c++17 --gnu_version=80500 Test_name.c
+"Test_name.c", line 26: error: no suitable conversion function from "E" to "int" exists
+  int &&r4 = {e};
+              ^
+
+"Test_name.c", line 27: error: no suitable conversion function from "ER" to "int" exists
+  int &&r5 = {ER{}};
+              ^
+
+"Test_name.c", line 29: error: no suitable conversion function from "E" to "int" exists
+    h({e});
+       ^
+
+"Test_name.c", line 30: error: no suitable conversion function from "ER" to "int" exists
+    h({ER{}});
+       ^
+
+"Test_name.c", line 31: error: no suitable conversion function from "E" to "const int" exists
+    hc({e});
+        ^
+
+5 errors detected in the compilation of "Test_name.c".

tests/tests/regressions/.gh-344.rto/default.4.1.txt

@@ -0,0 +1,34 @@
+fe_only -DTEST_NUMBER=4 --g++ --c++17 --gnu_version=130300 Test_name.c
+"Test_name.c", line 21: error: no suitable conversion function from "E" to "int" exists
+  int &&r1{e};
+           ^
+
+"Test_name.c", line 22: error: no suitable conversion function from "E" to "const int" exists
+  const int &r2{e};
+                ^
+
+"Test_name.c", line 23: error: no suitable conversion function from "ER" to "int" exists
+  int &&r3{ER{}};
+           ^
+
+"Test_name.c", line 26: error: no suitable conversion function from "E" to "int" exists
+  int &&r4 = {e};
+              ^
+
+"Test_name.c", line 27: error: no suitable conversion function from "ER" to "int" exists
+  int &&r5 = {ER{}};
+              ^
+
+"Test_name.c", line 29: error: no suitable conversion function from "E" to "int" exists
+    h({e});
+       ^
+
+"Test_name.c", line 30: error: no suitable conversion function from "ER" to "int" exists
+    h({ER{}});
+       ^
+
+"Test_name.c", line 31: error: no suitable conversion function from "E" to "const int" exists
+    hc({e});
+        ^
+
+8 errors detected in the compilation of "Test_name.c".

tests/tests/regressions/.gh-344.rto/default.5.1.txt

@@ -0,0 +1,30 @@
+fe_only -DTEST_NUMBER=5 --g++ --c++17 --gnu_version=150200 Test_name.c
+"Test_name.c", line 21: error: no suitable conversion function from "E" to "int" exists
+  int &&r1{e};
+           ^
+
+"Test_name.c", line 22: error: no suitable conversion function from "E" to "const int" exists
+  const int &r2{e};
+                ^
+
+"Test_name.c", line 26: error: no suitable conversion function from "E" to "int" exists
+  int &&r4 = {e};
+              ^
+
+"Test_name.c", line 27: error: no suitable conversion function from "ER" to "int" exists
+  int &&r5 = {ER{}};
+              ^
+
+"Test_name.c", line 29: error: no suitable conversion function from "E" to "int" exists
+    h({e});
+       ^
+
+"Test_name.c", line 30: error: no suitable conversion function from "ER" to "int" exists
+    h({ER{}});
+       ^
+
+"Test_name.c", line 31: error: no suitable conversion function from "E" to "const int" exists
+    hc({e});
+        ^
+
+7 errors detected in the compilation of "Test_name.c".

tests/tests/regressions/.gh-344.rto/default.6.1.txt

@@ -0,0 +1,22 @@
+fe_only -DTEST_NUMBER=6 --clang --c++17 --clang_version=180100 Test_name.c
+"Test_name.c", line 26: error: no suitable conversion function from "E" to "int" exists
+  int &&r4 = {e};
+              ^
+
+"Test_name.c", line 27: error: no suitable conversion function from "ER" to "int" exists
+  int &&r5 = {ER{}};
+              ^
+
+"Test_name.c", line 29: error: no suitable conversion function from "E" to "int" exists
+    h({e});
+       ^
+
+"Test_name.c", line 30: error: no suitable conversion function from "ER" to "int" exists
+    h({ER{}});
+       ^
+
+"Test_name.c", line 31: error: no suitable conversion function from "E" to "const int" exists
+    hc({e});
+        ^
+
+5 errors detected in the compilation of "Test_name.c".