Improve checking for subtraction operator

Author: robertoraggi

src/parser/cxx/type_checker.cc

@@ -1624,8 +1624,6 @@ auto TypeChecker::Visitor::usual_arithmetic_conversion(ExpressionAST*& expr,
   (void)lvalue_to_rvalue_conversion(other);
   adjust_cv(other);
 
-  if (control()->is_same(expr->type, other->type)) return expr->type;
-
   ExpressionAST* savedExpr = expr;
   ExpressionAST* savedOther = other;
 
@@ -1991,8 +1989,15 @@ void TypeChecker::Visitor::check_subtraction(BinaryExpressionAST* ast) {
   }
 
   if (control()->is_pointer(ast->rightExpression->type)) {
-    if (control()->is_same(ast->leftExpression->type,
-                           ast->rightExpression->type)) {
+    auto leftElementType =
+        control()->get_element_type(ast->leftExpression->type);
+    (void)strip_cv(leftElementType);
+
+    auto rightElementType =
+        control()->get_element_type(ast->rightExpression->type);
+    (void)strip_cv(rightElementType);
+
+    if (control()->is_same(leftElementType, rightElementType)) {
       ast->type = control()->getLongIntType();  // TODO: ptrdiff_t
     } else {
       error(ast->opLoc,

tests/unit_tests/sema/subtraction_01.cc

@@ -0,0 +1,33 @@
+// RUN: %cxx -verify -fcheck %s
+
+auto main() -> int {
+  char* p;
+  const char* cp;
+  void* vp;
+  int i = 0;
+  short s = 0;
+
+  static_assert(__is_same(int, decltype('a' - '0')));
+  static_assert(__is_same(int, decltype('a' - 0)));
+  static_assert(__is_same(int, decltype('a' - s)));
+  static_assert(__is_same(long, decltype('a' - 1l)));
+  static_assert(__is_same(unsigned, decltype('a' - 1u)));
+
+  static_assert(__is_same(decltype(p), decltype(p - 0)));
+  static_assert(__is_same(decltype(p), decltype(p - i)));
+
+  static_assert(__is_same(long, decltype(p - p)));
+  static_assert(__is_same(long, decltype(p - cp)));
+
+  // clang-format off
+
+  // expected-error@1 {{'char*' and 'void*' are not pointers to compatible types}}
+  p - vp;
+
+  // expected-error@1 {{invalid operands to binary expression 'int' and 'char*'}}
+  0 - p;
+
+  // clang-format on
+
+  return 0;
+}