PR libstdc++/77264
	* include/bits/basic_string.h (operator=(__sv_type)
	(append(__sv_type), assign(__sv_type), insert(size_type, __sv_type))
	(replace(size_type, size_type, __sv_type))
	(replace(const_iterator, const_iterator, __sv_type))
	(find(__sv_type, size_type), rfind(__sv_type, size_type))
	(compare(size_type, size_type, __sv_type)): Reformat.
	(_If_sv): Define helper for SFINAE constaints.
	(append(const _Tp&, size_type, size_type))
	(assign(const _Tp&, size_type, size_type))
	(insert(size_type, const _Tp&, size_type, size_type))
	(replace(size_type, size_type, const _Tp&, size_type, size_type)):
	Use _If_sv.
	* testsuite/21_strings/basic_string/modifiers/append/char/4.cc: Test
	SFINAE constraints.
	* testsuite/21_strings/basic_string/modifiers/append/wchar_t/4.cc:
	Likewise.
	* testsuite/21_strings/basic_string/modifiers/replace/char/7.cc:
	Likewise.
	* testsuite/21_strings/basic_string/modifiers/replace/wchar_t/7.cc:
	Likewise.
	* testsuite/21_strings/basic_string/modifiers/assign/char/4.cc:
	Likewise.
	* testsuite/21_strings/basic_string/modifiers/assign/wchar_t/4.cc:
	Likewise.
	* testsuite/21_strings/basic_string/modifiers/insert/char/3.cc:
	Likewise.
	* testsuite/21_strings/basic_string/modifiers/insert/wchar_t/3.cc:
	Likewise.
	* testsuite/21_strings/basic_string/modifiers/replace/char/7.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operations/compare/char/2.cc:
	Likewise.
	* testsuite/21_strings/basic_string/operations/compare/wchar_t/2.cc:
	Likewise.

From-SVN: r239532