Name
	n3844, alx-0086r1 - comparable types

Principles
	-  Keep the language small and simple

Category
	Editorial

Author
	Christopher Bazley <chris.bazley.wg14@gmail.com>
	Alejandro Colomar <alx@kernel.org>

	Cc: Philipp Klaus Krause <philipp@informatik.uni-frankfurt.de>
	Cc: Aaron Ballman <aaron@aaronballman.com>

History
	<https://www.alejandro-colomar.es/src/alx/alx/std/wg14/alx-0086.git/>

	r0 (2026-02-06):
	-  Initial draft.  Fork from n3674.

	r1 (2026-02-10; n3844):
	-  Document the term access qualifiers.
	-  Add n3795 to See also.

Description
	Too often, we use the phrase "qualified or unqualified type".
	It implicitly does not include the _Atomic qualifier.  Let's use
	a specific term for that: "comparable type".

	This is a spin-off of n3674.

See also
	<https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3674.pdf>
	<https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3795.pdf>

Proposed wording
	Based on N3783.

    6.2.5  Types
	@@ p31+1
	+The
	+<b>const</b>,
	+<b>restrict</b>,
	+and
	+<b>volatile</b>
	+qualifiers
	+are collectively called
	+the <i>standard access qualifiers</i>.
	+There may also be implementation-defined access qualifiers.
	+The standard access qualifiers
	+and any implementation-defined access qualifiers
	+are collectively called
	+<i>access qualifiers<i>.
	
	@@ p32
	 Any type so far mentioned is an unqualified type.
	 Each unqualified type
	 has several qualified versions of its type,40)
	 corresponding to the combinations of
	-one, two, or all three of
	-the const, volatile, and restrict qualifiers.
	+some or all access qualifiers.
	 The qualified or unqualified versions of a type
	 are distinct types that belong to
	-the same type category
	+the same type category.
	-and have the same representation and alignment requirements.41)
	 An array and its element type
	 are always considered to be identically qualified.42)
	 Any other derived type
	 is not qualified by the qualifiers (if any)
	 of the type from which it is derived.

	## The above will be now covered by comparable type rules.
	## Types in the same type category are comparable types.

	@@ Footnote 41
	-41)
	-The same representation and alignment requirements
	-are meant to imply interchangeability as
	-arguments to functions,
	-return values from functions,
	-and members of unions.

    6.2.6.3  Pointer types and nullptr_t
	@@ p1
	 A pointer to void
	 shall have the same representation and alignment requirements
	 as a pointer to a character type.41)
	 Similarly,
	 pointers to
	-qualified or unqualified versions of compatible types
	+comparable types
	 shall have the same representation and alignment requirements.
	 All pointers to structure types
	 shall have the same representation and alignment requirements
	 as each other.
	 All pointers to union types
	 shall have the same representation and alignment requirements
	 as each other.
	 Pointers to other types
	 may not have the same representation or alignment requirements.

	## This guarantees that char** and const char*const* have the
	## same representation and alignment.

    6.2  Concepts
	## Add new subsections after 6.2.7 ("Compatible type and composite type")
	+6.2.7+1  Comparable type
	+1
	+	Two types are <i>comparable types</i>
	+	if they would be compatible types
	+	if all access qualifiers
	+	were removed from both types.
	+2
	+	Comparable complete object types
	+	shall have the same
	+	representation and alignment requirements.
	+	XXX)
	+
	+XXX)
	+	The same representation and alignment requirements
	+	are meant to imply interchangeability as
	+	arguments to functions,
	+	return values from functions,
	+	and members of unions.

    6.5.7  Additive operators
	@@ Constraints, p3
	 For subtraction, one of the following shall hold:
	  --	[...]
	  --	both operands are pointers to
	-	qualified or unqualified versions of compatible
	+	comparable
		complete object types; or
	  --	[...]

    6.5.9  Relational operators
	@@ Constraints, p2
	 One of the following shall hold:
	  --	[...]
	  --	both operands are pointers to
	-	qualified or unqualified versions of compatible
	+	comparable
		object types.

    6.5.10  Equality operators
	@@ Constraints, p2
	 One of the following shall hold:
	  --	[...]
	  --	both operands are pointers to
	-	qualified or unqualified versions of compatible
	+	comparable
		types.
	  --	one operand is a pointer to an object type
		and the other is a pointer to
		a qualified or unqualified version of void;
	  --	[...]
	  --	[...]
	  --	[...]

    6.5.16  Conditional operator
	@@ Constraints, p3
	 One of the following shall hold
	 for the second and third operands:110)
	  --	[...]
	  --	[...]
	  --	[...]
	  --	both operands are pointers to
	-	qualified or unqualified versions of compatible
	+	comparable
		types;
	  --	[...]
	  --	[...]
	  --	[...]

    6.5.17.2  Expressions :: Assignment operators :: Simple assignment
	@@ Constraints, p1
	 One of the following shall hold:113)
	  --	[...]
	  --	[...]
	  --	the left operand has
		atomic, qualified, or unqualified pointer type,
		and
		(considering the type the left operand would have
		after lvalue conversion)
		both operands are pointers to
	-	qualified or unqualified versions of compatible
	+	comparable
		types,
		and the type pointed to by the left operand
		has all the qualifiers
		of the type pointed to by the right operand;
	  --	[...]
	  --	[...]
	  --	[...]
	  --	[...]

	@@ Semantics, p3
	 If the value being stored in an object
	 is read from another object that
	 overlaps in any way the storage of the first object,
	 then the two objects
	 shall occupy exactly the same storage
	 and shall have
	-qualified or unqualified versions of a compatible
	+comparable
	 type;
	 otherwise, the behavior is undefined.

    7.16.2.2  The va_arg macro
	@@ Description, p2
	...
	 except for the following cases:
	  --	both ypes are pointers to
	-	qualified or unqualified versions of compatible
	+	comparable
		types;
	  --	[...]
	  --	[...]
	  --	[...]