QSpan: make adl_begin() etc SFINAE-friendly

The definition of iterator_t, and, therefore, of is_compatible_range
depends on this, otherwise say, 0, is being treated as a valid range
and hits a hard error in adl_begin() when trying to call begin(int&).

TIL: decltype(auto) does _not_ SFINAE.

Fix by calculating the return type manually, re-enabing SFINAE.

Manual conflict resolutions:
 - qspan.h -> qspan_p.h

Change-Id: Icacd70554f4050ecaeb396c9ae60bc4f21a220c9
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit 94e122c)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 1b559b2)
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/corelib/tools/qspan_p.h

@@ -88,14 +88,25 @@ using is_qualification_conversion =
 template <typename From, typename To>
 constexpr inline bool is_qualification_conversion_v = is_qualification_conversion<From, To>::value;
 
+namespace AdlTester {
+#define MAKE_ADL_TEST(what) \
+    using std:: what; /* bring into scope */ \
+    template <typename T> using what ## _result = decltype( what (std::declval<T&&>())); \
+    /* end */
+MAKE_ADL_TEST(begin)
+MAKE_ADL_TEST(data)
+MAKE_ADL_TEST(size)
+#undef MAKE_ADL_TEST
+}
+
 // Replacements for std::ranges::XXX(), but only bringing in ADL XXX()s,
 // not doing the extra work C++20 requires
 template <typename Range>
-decltype(auto) adl_begin(Range &&r) { using std::begin; return begin(r); }
+AdlTester::begin_result<Range> adl_begin(Range &&r) { using std::begin; return begin(r); }
 template <typename Range>
-decltype(auto) adl_data(Range &&r)  { using std::data; return data(r); }
+AdlTester::data_result<Range>  adl_data(Range &&r)  { using std::data; return data(r); }
 template <typename Range>
-decltype(auto) adl_size(Range &&r)  { using std::size; return size(r); }
+AdlTester::size_result<Range>  adl_size(Range &&r)  { using std::size; return size(r); }
 
 // Replacement for std::ranges::iterator_t (which depends on C++20 std::ranges::begin)
 // This one uses adl_begin() instead.

tests/auto/corelib/tools/qspan/tst_qspan.cpp

@@ -96,6 +96,9 @@ static_assert(!std::is_convertible_v<QSpan<const int, 42>, std::span<int, 42>>);
 static_assert(!std::is_convertible_v<QSpan<const int, 0>, std::span<int, 0>>);
 #endif // __cpp_lib_span
 
+// Spans don't convert from nonsense:
+static_assert(!std::is_constructible_v<QSpan<const int>, int&&>);
+
 class tst_QSpan : public QObject
 {
     Q_OBJECT