QList::iterator: use templates for advancing operators

Because of the addition of the operator T*(), the expression "it + N"
where N was not exactly qsizetype but any other integer type was a
compilation failure because of ambiguous overload resolution.

With GCC it's apparently a warning:

 warning: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second:
 note: candidate 1: ‘QList<T>::iterator QList<T>::iterator::operator+(qsizetype) const [with T = char; qsizetype = long long int]’
 note: candidate 2: ‘operator+(char*, ptrdiff_t {aka long int})’ (built-in)

With Clang, it's an error:

 error: use of overloaded operator '+' is ambiguous (with operand types 'QList<int>::const_iterator' and 'ptrdiff_t' (aka 'long'))
 note: candidate function
        inline const_iterator operator+(qsizetype j) const { return const_iterator(i+j); }
 note: built-in candidate operator+(const int *, long)

Fixes: QTBUG-96128
Change-Id: Ie72b0dd0fbe84d2caae0fffd16a06f23dd56b060
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
(cherry picked from commit 878b2047b52c93e904eb46ef1044819a8b5614ab)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

2 files changed, 156 insertions, 10 deletions

diff --git a/src/corelib/tools/qlist.h b/src/corelib/tools/qlist.h
index 0329dd37da..ddc85f2f22 100644
--- a/src/corelib/tools/qlist.h
+++ b/src/corelib/tools/qlist.h
@@ -162,13 +162,19 @@ public:
         inline iterator operator++(int) { T *n = i; ++i; return n; }
         inline iterator &operator--() { i--; return *this; }
         inline iterator operator--(int) { T *n = i; i--; return n; }
-        inline iterator &operator+=(qsizetype j) { i+=j; return *this; }
-        inline iterator &operator-=(qsizetype j) { i-=j; return *this; }
-        inline iterator operator+(qsizetype j) const { return iterator(i+j); }
-        inline iterator operator-(qsizetype j) const { return iterator(i-j); }
-        friend inline iterator operator+(qsizetype j, iterator k) { return k + j; }
         inline qsizetype operator-(iterator j) const { return i - j.i; }
         inline operator T*() const { return i; }
+
+        template <typename Int> std::enable_if_t<std::is_integral_v<Int>, iterator>
+        &operator+=(Int j) { i+=j; return *this; }
+        template <typename Int> std::enable_if_t<std::is_integral_v<Int>, iterator>
+        &operator-=(Int j) { i-=j; return *this; }
+        template <typename Int> std::enable_if_t<std::is_integral_v<Int>, iterator>
+        operator+(Int j) const { return iterator(i+j); }
+        template <typename Int> std::enable_if_t<std::is_integral_v<Int>, iterator>
+        operator-(Int j) const { return iterator(i-j); }
+        template <typename Int> friend std::enable_if_t<std::is_integral_v<Int>, iterator>
+        operator+(Int j, iterator k) { return k + j; }
     };
 
     class const_iterator {
@@ -206,13 +212,19 @@ public:
         inline const_iterator operator++(int) { const T *n = i; ++i; return n; }
         inline const_iterator &operator--() { i--; return *this; }
         inline const_iterator operator--(int) { const T *n = i; i--; return n; }
-        inline const_iterator &operator+=(qsizetype j) { i+=j; return *this; }
-        inline const_iterator &operator-=(qsizetype j) { i-=j; return *this; }
-        inline const_iterator operator+(qsizetype j) const { return const_iterator(i+j); }
-        inline const_iterator operator-(qsizetype j) const { return const_iterator(i-j); }
-        friend inline const_iterator operator+(qsizetype j, const_iterator k) { return k + j; }
         inline qsizetype operator-(const_iterator j) const { return i - j.i; }
         inline operator const T*() const { return i; }
+
+        template <typename Int> std::enable_if_t<std::is_integral_v<Int>, const_iterator>
+        &operator+=(Int j) { i+=j; return *this; }
+        template <typename Int> std::enable_if_t<std::is_integral_v<Int>, const_iterator>
+        &operator-=(Int j) { i-=j; return *this; }
+        template <typename Int> std::enable_if_t<std::is_integral_v<Int>, const_iterator>
+        operator+(Int j) const { return const_iterator(i+j); }
+        template <typename Int> std::enable_if_t<std::is_integral_v<Int>, const_iterator>
+        operator-(Int j) const { return const_iterator(i-j); }
+        template <typename Int> friend std::enable_if_t<std::is_integral_v<Int>, const_iterator>
+        operator+(Int j, const_iterator k) { return k + j; }
     };
     using Iterator = iterator;
     using ConstIterator = const_iterator;
diff --git a/tests/auto/corelib/tools/qlist/tst_qlist.cpp b/tests/auto/corelib/tools/qlist/tst_qlist.cpp
index 48931a81ea..aa8d70b9fe 100644
--- a/tests/auto/corelib/tools/qlist/tst_qlist.cpp
+++ b/tests/auto/corelib/tools/qlist/tst_qlist.cpp
@@ -2408,30 +2408,97 @@ void tst_QList::iterators() const
     idx = 0;
     auto it = v.begin();
     QCOMPARE(*it, idx);
+    // idx == 0
 
     std::advance(it, 7);
     idx += 7;
     QCOMPARE(*it, idx);
+    // idx == 7
 
     it++;
     idx++;
     QCOMPARE(*it, idx);
+    // idx == 8
 
     ++it;
     ++idx;
     QCOMPARE(*it, idx);
+    // idx == 9
 
     std::advance(it, -3);
     idx -= 3;
     QCOMPARE(*it, idx);
+    // idx == 6
 
     it--;
     idx--;
     QCOMPARE(*it, idx);
+    // idx == 5
 
     --it;
     --idx;
     QCOMPARE(*it, idx);
+    // idx == 4
+
+    it = it + 1;
+    idx = idx + 1;
+    QCOMPARE(*it, idx);
+    // idx == 5
+
+    it = it + ptrdiff_t(1);
+    idx = idx + 1;
+    QCOMPARE(*it, idx);
+    // idx == 6
+
+    it = it + qsizetype(1);
+    idx = idx + 1;
+    QCOMPARE(*it, idx);
+    // idx == 7
+
+    it = it - qsizetype(1);
+    idx = idx - 1;
+    QCOMPARE(*it, idx);
+    // idx == 6
+
+    it = it - ptrdiff_t(1);
+    idx = idx - 1;
+    QCOMPARE(*it, idx);
+    // idx == 5
+
+    it = it - 1;
+    idx = idx - 1;
+    QCOMPARE(*it, idx);
+    // idx == 4
+
+    it -= 1;
+    idx -= 1;
+    QCOMPARE(*it, idx);
+    // idx == 3
+
+    it -= qsizetype(1);
+    idx -= 1;
+    QCOMPARE(*it, idx);
+    // idx == 2
+
+    it -= ptrdiff_t(1);
+    idx -= 1;
+    QCOMPARE(*it, idx);
+    // idx == 1
+
+    it += ptrdiff_t(1);
+    idx += 1;
+    QCOMPARE(*it, idx);
+    // idx == 2
+
+    it += qsizetype(1);
+    idx += 1;
+    QCOMPARE(*it, idx);
+    // idx == 3
+
+    it += 1;
+    idx += 1;
+    QCOMPARE(*it, idx);
+    // idx == 4
 
     *it = idx + 1;
     QCOMPARE(*it, idx + 1);
@@ -2488,30 +2555,97 @@ void tst_QList::constIterators() const
     qsizetype idx = 0;
     auto it = v.cbegin();
     QCOMPARE(*it, idx);
+    // idx == 0
 
     std::advance(it, 7);
     idx += 7;
     QCOMPARE(*it, idx);
+    // idx == 7
 
     it++;
     idx++;
     QCOMPARE(*it, idx);
+    // idx == 8
 
     ++it;
     ++idx;
     QCOMPARE(*it, idx);
+    // idx == 9
 
     std::advance(it, -3);
     idx -= 3;
     QCOMPARE(*it, idx);
+    // idx == 6
 
     it--;
     idx--;
     QCOMPARE(*it, idx);
+    // idx == 5
 
     --it;
     --idx;
     QCOMPARE(*it, idx);
+    // idx == 4
+
+    it = it + 1;
+    idx = idx + 1;
+    QCOMPARE(*it, idx);
+    // idx == 5
+
+    it = it + ptrdiff_t(1);
+    idx = idx + 1;
+    QCOMPARE(*it, idx);
+    // idx == 6
+
+    it = it + qsizetype(1);
+    idx = idx + 1;
+    QCOMPARE(*it, idx);
+    // idx == 7
+
+    it = it - qsizetype(1);
+    idx = idx - 1;
+    QCOMPARE(*it, idx);
+    // idx == 6
+
+    it = it - ptrdiff_t(1);
+    idx = idx - 1;
+    QCOMPARE(*it, idx);
+    // idx == 5
+
+    it = it - 1;
+    idx = idx - 1;
+    QCOMPARE(*it, idx);
+    // idx == 4
+
+    it -= 1;
+    idx -= 1;
+    QCOMPARE(*it, idx);
+    // idx == 3
+
+    it -= qsizetype(1);
+    idx -= 1;
+    QCOMPARE(*it, idx);
+    // idx == 2
+
+    it -= ptrdiff_t(1);
+    idx -= 1;
+    QCOMPARE(*it, idx);
+    // idx == 1
+
+    it += ptrdiff_t(1);
+    idx += 1;
+    QCOMPARE(*it, idx);
+    // idx == 2
+
+    it += qsizetype(1);
+    idx += 1;
+    QCOMPARE(*it, idx);
+    // idx == 3
+
+    it += 1;
+    idx += 1;
+    QCOMPARE(*it, idx);
+    // idx == 4
 
     // stl-style reverse iterators
     idx = v.size() - 1;