Correct class names for KeysView, ValuesView and ItemsView in bind_map (#4353)

* Create templated abstract classes KeysView, ValuesView and ItemsView, and implement them on-the-fly when wrapping any specific map type

* We don't want to wrap different ValuesView objects for double values and const double, for example, as both wrappers will be named ValuesView[float]

* Fallback to C++ names if key or values types are not wrapped

* Added a test for .keys(), .values() and .items() returning the same types for similarly-typed maps

* Fixed wrong use of auto in a declarator list: the two descriptions might have different types

* Fixes for clang-tidy issues: explicit single-argument constructor, using the 'override' keyword when overriding functions

* Bugfix for old versions of clang++, which seem to have trouble with the struct being defined inside a module, which was also needlessly ugly anyway

* Bugfix for clang++, which doesn't have some of the names in runtime uness they are specified to be static

* A fix for clang-tidy performance-inefficient-string-concatenation issues - I personally think this looks uglier, but it's probably worth it for clang-tidy to be happy

* Possible fix for clang++ linking issues - make the descriptions static constexpr to make sure they are known before linking

* Correct names for previously-wrapped types as keys/values of maps

* Bugfix - typo in type info names which caused things to segfault

* Apply suggestions from code review

Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>

* Use detail::remove_cvref_t instead of doing remove_cv and remove_reference separately

* Avoid names with double underscore, as they are reserved

* Improved testing for KeysView, ValuesView and ItemsView: check type names + stricter asserts

* Moved description logic to helper function in type_caster_base.h

* style: pre-commit fixes

* Fix a clang-tidy issue: do not use 'else' after 'return'

* Apply suggestion by @Skylion007, with additional trivial simplification.

Co-authored-by: Amir <aimir@local>
Co-authored-by: aimir <aimir@localhost>
Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>

tests/test_stl_binders.py

@@ -309,3 +309,29 @@ def test_map_delitem():
     del um["ua"]
     assert sorted(list(um)) == ["ub"]
     assert sorted(list(um.items())) == [("ub", 2.6)]
+
+
+def test_map_view_types():
+    map_string_double = m.MapStringDouble()
+    unordered_map_string_double = m.UnorderedMapStringDouble()
+    map_string_double_const = m.MapStringDoubleConst()
+    unordered_map_string_double_const = m.UnorderedMapStringDoubleConst()
+
+    assert map_string_double.keys().__class__.__name__ == "KeysView[str]"
+    assert map_string_double.values().__class__.__name__ == "ValuesView[float]"
+    assert map_string_double.items().__class__.__name__ == "ItemsView[str, float]"
+
+    keys_type = type(map_string_double.keys())
+    assert type(unordered_map_string_double.keys()) is keys_type
+    assert type(map_string_double_const.keys()) is keys_type
+    assert type(unordered_map_string_double_const.keys()) is keys_type
+
+    values_type = type(map_string_double.values())
+    assert type(unordered_map_string_double.values()) is values_type
+    assert type(map_string_double_const.values()) is values_type
+    assert type(unordered_map_string_double_const.values()) is values_type
+
+    items_type = type(map_string_double.items())
+    assert type(unordered_map_string_double.items()) is items_type
+    assert type(map_string_double_const.items()) is items_type
+    assert type(unordered_map_string_double_const.items()) is items_type