diagnose comparisons between two arrays (P2865), fix for [GH #370]

src/error_msg.txt

@@ -3680,5 +3680,8 @@ ec_interpolated_id_is_not_identifier;;"%sq does not spell an identifier"
 ec_interpolated_operand_is_not_token_sequence;;"the operand of a \"\\{...}\" interpolator is not a token sequence"
 ec_tag_redefined_differently;;"this declaration of %n does not declare the same type as the declaration %p"
 ec_bad_gcc_unroll_pragma_value;;"the argument of \"#pragma GCC unroll\" must be a non-negative integral constant less than 65535"
+ec_array_comparison_deprecated;;"comparison between two arrays is deprecated"
+ec_array_comparison_not_allowed;;"comparison between two arrays is not allowed"
+ec_array_comparison_compares_addresses;;"comparison between two arrays compares their addresses"
  ###############################################################################
 

src/expr.c

@@ -32487,6 +32487,62 @@ different enum types (or, in C, have different affiliated enum types).
 }  /* diagnose_comparison_if_different_enum_types */
 
 
+static a_boolean is_array_comparison_operand(an_operand *opnd)
+/*
+Return TRUE if opnd, an operand of a built-in equality or relational operator,
+is an array: either it has array type, or it is a class operand that overload
+resolution converted to an array and then to a pointer (e.g., through a
+conversion function returning a reference to an array), possibly followed by
+an implicit qualification conversion.
+*/
+{
+  a_boolean is_array = is_array_type(opnd->type);
+
+  if (!is_array && is_expression_operand(opnd)) {
+    an_expr_node_ptr expr = opnd->variant.expression;
+    /* Skip implicit pointer conversions (e.g., "int *" to "const int *");
+       a cast written in the source ends the search. */
+    while (is_cast_operation_node(expr) && expr->compiler_generated &&
+           is_pointer_type(expr->type)) {
+      expr = expr->variant.operation.operands;
+    }  /* while */
+    is_array = is_operation_node(expr) &&
+               node_operator_is(expr, eok_array_to_pointer);
+  }  /* if */
+  return is_array;
+}  /* is_array_comparison_operand */
+
+
+static void check_for_array_comparison(an_operand         *opnd1,
+                                       an_operand         *opnd2,
+                                       a_source_position  *operator_pos)
+/*
+opnd1 and opnd2 are the operands of a built-in equality or relational
+operator at operator_pos, before the array-to-pointer conversion.  If both
+are arrays, issue a diagnostic: such comparisons are deprecated in C++20 and
+not allowed in C++26 (P2865).  Before C++20, issue a remark (a warning when
+emulating Clang 20 or later, which warns by default).
+*/
+{
+  if (!C_mode() && is_array_comparison_operand(opnd1) &&
+      is_array_comparison_operand(opnd2)) {
+    if (!cpp20_mode) {
+      expr_pos_diagnostic(clangcpp_version_is(>=200000) ? es_warning
+                                                        : es_remark,
+                          ec_array_comparison_compares_addresses,
+                          operator_pos);
+    } else if (cpp26_mode && !gpp_version_is(<150000) &&
+               !clangcpp_version_is(<200000)) {
+      /* GCC 15 and Clang 20 implement P2865.  Earlier versions only issue a
+         deprecation warning. */
+      expr_pos_error(ec_array_comparison_not_allowed, operator_pos);
+    } else {
+      expr_pos_warning(ec_array_comparison_deprecated, operator_pos);
+    }  /* if */
+  }  /* if */
+}  /* check_for_array_comparison */
+
+
 static void process_rel_operator(an_operand               *opnd1,
                                  an_operand               *opnd2,
                                  a_token_kind             operator_token,
@@ -32532,6 +32588,7 @@ describe the location of the operator.
   }  /* if */
   if (!processed) {
     /* Non-operator-function cases. */
+    check_for_array_comparison(opnd1, opnd2, operator_pos);
     /* The first operand must be an arithmetic or enum type or a pointer. */
     do_operand_transformations(opnd1, TOPT_NO_OPTIONS);
     opnd1_is_pointer = FALSE;
@@ -33036,6 +33093,7 @@ operator_tsn describe the location of the operator.
   }  /* if */
   if (!processed) {
     /* Non-operator-function cases. */
+    check_for_array_comparison(opnd1, opnd2, operator_pos);
     /* The first operand must be an arithmetic or enum type or a pointer. */
     do_operand_transformations(opnd1, TOPT_NO_OPTIONS);
     if (is_arithmetic_or_enum_type(opnd1->type)) {

src/pending_changes/gh-370-array-comparisons.txt

@@ -0,0 +1,17 @@
+[GH #370]
+C++26: comparison between two arrays is not allowed
+
+A comparison between two arrays using an equality or relational operator is
+deprecated in C++20 and not allowed in C++26 (P2865).  The front end did not
+diagnose it.  It is now a warning in C++20 and C++23, and an error in C++26
+(only a warning when emulating GCC versions before 15 or Clang versions before
+20, which do the same).  Before C++20 it is a remark (a warning when
+emulating Clang 20 or later, which warns by default).  This includes class
+operands that are converted to arrays by a conversion function.  For
+example, with --c++26:
+
+  int a[2], b[2];
+  bool x = a == b;      // Now an error.
+  bool y = a == &b[0];  // Okay: one operand is a pointer.
+  struct C { using A = int[2]; operator A &() const; } c;
+  bool z = c == c;      // Now an error.

tests/expectations/edg_x86_64/imported.log

@@ -1441,7 +1441,6 @@ clang/cpp/SemaCXX/vtordisp-mode.sft.cpp:fp:-DTEST_NUMBER=3 --c++11 -DVTORDISP_MO
 clang/cpp/SemaCXX/warn-array-comparion.sft.cpp:fn:-DTEST_NUMBER=1 --c++17:FAIL:
 clang/cpp/SemaCXX/warn-array-comparion.sft.cpp:fn:-DTEST_NUMBER=2 --c++20:FAIL:
 clang/cpp/SemaCXX/warn-array-comparion.sft.cpp:fn:-DTEST_NUMBER=3 --c++20:FAIL:
-clang/cpp/SemaCXX/warn-array-comparion.sft.cpp:fn:-DTEST_NUMBER=4 --c++26:FAIL:
 clang/cpp/SemaCXX/warn-bool-conversion.sft.cpp:fn:-DTEST_NUMBER=1 --c++03:FAIL:
 clang/cpp/SemaCXX/warn-cast-function-type-strict.sft.cpp:fp:-DTEST_NUMBER=1:FAIL:
 clang/cpp/SemaCXX/warn-cast-function-type.sft.cpp:fp:-DTEST_NUMBER=1:FAIL:
@@ -1453,7 +1452,6 @@ clang/cpp/SemaCXX/warn-memsize-comparison.sft.cpp:fn:-DTEST_NUMBER=1:FAIL:
 clang/cpp/SemaCXX/warn-nrvo.sft.cpp:fp:-DTEST_NUMBER=1:FAIL:
 clang/cpp/SemaCXX/warn-self-comparisons.sft.cpp:fn:-DTEST_NUMBER=1 --c++17:FAIL:
 clang/cpp/SemaCXX/warn-self-comparisons.sft.cpp:fn:-DTEST_NUMBER=2 --c++20:FAIL:
-clang/cpp/SemaCXX/warn-self-comparisons.sft.cpp:fn:-DTEST_NUMBER=3 --c++26:FAIL:
 clang/cpp/SemaCXX/warn-static-const-float.sft.cpp:fn:-DTEST_NUMBER=1 --c++03 -DEXT --c++03 -DNONE --c++03 -DNONE --c++03 -DNONE:FAIL:
 clang/cpp/SemaCXX/warn-static-const-float.sft.cpp:fn:-DTEST_NUMBER=2 --c++11 -DERR:FAIL:
 clang/cpp/SemaCXX/warn-static-const-float.sft.cpp:fn:-DTEST_NUMBER=3 --c++11 -DERR:FAIL:

tests/tests/changes/.GH370.rto/default.1.1.txt

@@ -0,0 +1,34 @@
+fe_only -DTEST_NUMBER=1 --c++26 Test_name.c
+"Test_name.c", line 10: error: comparison between two arrays is not allowed
+  bool x1 = a == b;  // Two arrays: diagnosed.
+              ^
+
+"Test_name.c", line 11: error: comparison between two arrays is not allowed
+  bool x2 = a != b;
+              ^
+
+"Test_name.c", line 12: error: comparison between two arrays is not allowed
+  bool x3 = a < b;
+              ^
+
+"Test_name.c", line 13: error: comparison between two arrays is not allowed
+  bool x4 = a >= b;
+              ^
+
+"Test_name.c", line 22: error: comparison between two arrays is not allowed
+    bool z1 = c == c;    // Two arrays: diagnosed.
+                ^
+
+"Test_name.c", line 23: error: comparison between two arrays is not allowed
+    bool z2 = c < c;
+                ^
+
+"Test_name.c", line 24: error: comparison between two arrays is not allowed
+    bool z3 = c == a;
+                ^
+
+"Test_name.c", line 33: error: comparison between two arrays is not allowed
+  bool h1(C c) { return c == ca; }              // Two arrays: diagnosed.
+                          ^
+
+8 errors detected in the compilation of "Test_name.c".

tests/tests/changes/.GH370.rto/default.2.1.txt

@@ -0,0 +1,34 @@
+fe_only -DTEST_NUMBER=2 --c++26 --gnu_version 150000 Test_name.c
+"Test_name.c", line 10: error: comparison between two arrays is not allowed
+  bool x1 = a == b;  // Two arrays: diagnosed.
+              ^
+
+"Test_name.c", line 11: error: comparison between two arrays is not allowed
+  bool x2 = a != b;
+              ^
+
+"Test_name.c", line 12: error: comparison between two arrays is not allowed
+  bool x3 = a < b;
+              ^
+
+"Test_name.c", line 13: error: comparison between two arrays is not allowed
+  bool x4 = a >= b;
+              ^
+
+"Test_name.c", line 22: error: comparison between two arrays is not allowed
+    bool z1 = c == c;    // Two arrays: diagnosed.
+                ^
+
+"Test_name.c", line 23: error: comparison between two arrays is not allowed
+    bool z2 = c < c;
+                ^
+
+"Test_name.c", line 24: error: comparison between two arrays is not allowed
+    bool z3 = c == a;
+                ^
+
+"Test_name.c", line 33: error: comparison between two arrays is not allowed
+  bool h1(C c) { return c == ca; }              // Two arrays: diagnosed.
+                          ^
+
+8 errors detected in the compilation of "Test_name.c".

tests/tests/changes/.GH370.rto/default.3.1.txt

@@ -0,0 +1,34 @@
+fe_only -DTEST_NUMBER=3 --c++26 --clang_version 200000 Test_name.c
+"Test_name.c", line 10: error: comparison between two arrays is not allowed
+  bool x1 = a == b;  // Two arrays: diagnosed.
+              ^
+
+"Test_name.c", line 11: error: comparison between two arrays is not allowed
+  bool x2 = a != b;
+              ^
+
+"Test_name.c", line 12: error: comparison between two arrays is not allowed
+  bool x3 = a < b;
+              ^
+
+"Test_name.c", line 13: error: comparison between two arrays is not allowed
+  bool x4 = a >= b;
+              ^
+
+"Test_name.c", line 22: error: comparison between two arrays is not allowed
+    bool z1 = c == c;    // Two arrays: diagnosed.
+                ^
+
+"Test_name.c", line 23: error: comparison between two arrays is not allowed
+    bool z2 = c < c;
+                ^
+
+"Test_name.c", line 24: error: comparison between two arrays is not allowed
+    bool z3 = c == a;
+                ^
+
+"Test_name.c", line 33: error: comparison between two arrays is not allowed
+  bool h1(C c) { return c == ca; }              // Two arrays: diagnosed.
+                          ^
+
+8 errors detected in the compilation of "Test_name.c".

tests/tests/changes/.GH370_a.rto/default.1.1.txt

@@ -0,0 +1,33 @@
+fe_only -DTEST_NUMBER=1 --c++20 Test_name.c
+"Test_name.c", line 13: warning: comparison between two arrays is deprecated
+  bool x1 = a == b;  // Two arrays: diagnosed.
+              ^
+
+"Test_name.c", line 14: warning: comparison between two arrays is deprecated
+  bool x2 = a != b;
+              ^
+
+"Test_name.c", line 15: warning: comparison between two arrays is deprecated
+  bool x3 = a < b;
+              ^
+
+"Test_name.c", line 16: warning: comparison between two arrays is deprecated
+  bool x4 = a >= b;
+              ^
+
+"Test_name.c", line 25: warning: comparison between two arrays is deprecated
+    bool z1 = c == c;    // Two arrays: diagnosed.
+                ^
+
+"Test_name.c", line 26: warning: comparison between two arrays is deprecated
+    bool z2 = c < c;
+                ^
+
+"Test_name.c", line 27: warning: comparison between two arrays is deprecated
+    bool z3 = c == a;
+                ^
+
+"Test_name.c", line 36: warning: comparison between two arrays is deprecated
+  bool h1(C c) { return c == ca; }              // Two arrays: diagnosed.
+                          ^
+

tests/tests/changes/.GH370_a.rto/default.2.1.txt

@@ -0,0 +1,33 @@
+fe_only -DTEST_NUMBER=2 --c++26 --gnu_version 140000 Test_name.c
+"Test_name.c", line 13: warning: comparison between two arrays is deprecated
+  bool x1 = a == b;  // Two arrays: diagnosed.
+              ^
+
+"Test_name.c", line 14: warning: comparison between two arrays is deprecated
+  bool x2 = a != b;
+              ^
+
+"Test_name.c", line 15: warning: comparison between two arrays is deprecated
+  bool x3 = a < b;
+              ^
+
+"Test_name.c", line 16: warning: comparison between two arrays is deprecated
+  bool x4 = a >= b;
+              ^
+
+"Test_name.c", line 25: warning: comparison between two arrays is deprecated
+    bool z1 = c == c;    // Two arrays: diagnosed.
+                ^
+
+"Test_name.c", line 26: warning: comparison between two arrays is deprecated
+    bool z2 = c < c;
+                ^
+
+"Test_name.c", line 27: warning: comparison between two arrays is deprecated
+    bool z3 = c == a;
+                ^
+
+"Test_name.c", line 36: warning: comparison between two arrays is deprecated
+  bool h1(C c) { return c == ca; }              // Two arrays: diagnosed.
+                          ^
+

tests/tests/changes/.GH370_a.rto/default.3.1.txt

@@ -0,0 +1,33 @@
+fe_only -DTEST_NUMBER=3 --c++26 --clang_version 190000 Test_name.c
+"Test_name.c", line 13: warning: comparison between two arrays is deprecated
+  bool x1 = a == b;  // Two arrays: diagnosed.
+              ^
+
+"Test_name.c", line 14: warning: comparison between two arrays is deprecated
+  bool x2 = a != b;
+              ^
+
+"Test_name.c", line 15: warning: comparison between two arrays is deprecated
+  bool x3 = a < b;
+              ^
+
+"Test_name.c", line 16: warning: comparison between two arrays is deprecated
+  bool x4 = a >= b;
+              ^
+
+"Test_name.c", line 25: warning: comparison between two arrays is deprecated
+    bool z1 = c == c;    // Two arrays: diagnosed.
+                ^
+
+"Test_name.c", line 26: warning: comparison between two arrays is deprecated
+    bool z2 = c < c;
+                ^
+
+"Test_name.c", line 27: warning: comparison between two arrays is deprecated
+    bool z3 = c == a;
+                ^
+
+"Test_name.c", line 36: warning: comparison between two arrays is deprecated
+  bool h1(C c) { return c == ca; }              // Two arrays: diagnosed.
+                          ^
+

tests/tests/changes/.GH370_a.rto/default.4.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=4 --c++17 Test_name.c

tests/tests/changes/.GH370_a.rto/default.5.1.txt

@@ -0,0 +1,57 @@
+fe_only -DTEST_NUMBER=5 --c++17 --remarks Test_name.c
+"Test_name.c", line 13: remark: comparison between two arrays compares their addresses
+  bool x1 = a == b;  // Two arrays: diagnosed.
+              ^
+
+"Test_name.c", line 14: remark: comparison between two arrays compares their addresses
+  bool x2 = a != b;
+              ^
+
+"Test_name.c", line 15: remark: comparison between two arrays compares their addresses
+  bool x3 = a < b;
+              ^
+
+"Test_name.c", line 16: remark: comparison between two arrays compares their addresses
+  bool x4 = a >= b;
+              ^
+
+"Test_name.c", line 25: remark: comparison between two arrays compares their addresses
+    bool z1 = c == c;    // Two arrays: diagnosed.
+                ^
+
+"Test_name.c", line 26: remark: comparison between two arrays compares their addresses
+    bool z2 = c < c;
+                ^
+
+"Test_name.c", line 27: remark: comparison between two arrays compares their addresses
+    bool z3 = c == a;
+                ^
+
+"Test_name.c", line 25: remark: variable "z1" was declared but never referenced
+    bool z1 = c == c;    // Two arrays: diagnosed.
+         ^
+
+"Test_name.c", line 26: remark: variable "z2" was declared but never referenced
+    bool z2 = c < c;
+         ^
+
+"Test_name.c", line 27: remark: variable "z3" was declared but never referenced
+    bool z3 = c == a;
+         ^
+
+"Test_name.c", line 28: remark: variable "z4" was declared but never referenced
+    bool z4 = c == p;    // Array and pointer: not diagnosed.
+         ^
+
+"Test_name.c", line 29: remark: variable "z5" was declared but never referenced
+    bool z5 = q == q;    // Conversions to pointers: not diagnosed.
+         ^
+
+"Test_name.c", line 30: remark: variable "z6" was declared but never referenced
+    bool z6 = +c == +c;  // Explicit decay: not diagnosed.
+         ^
+
+"Test_name.c", line 36: remark: comparison between two arrays compares their addresses
+  bool h1(C c) { return c == ca; }              // Two arrays: diagnosed.
+                          ^
+