[std] Reword "necessary", "permitted", "allowed", "may" in notes.

This is so that notes do not (inappropriately) state requirements or
permissions.

Author: tkoeppe

source/basic.tex

@@ -6202,15 +6202,15 @@
 The ``inter-thread happens before'' relation describes arbitrary
 concatenations of ``sequenced before'', ``synchronizes with'' and
 ``dependency-ordered before'' relationships, with two exceptions. The first
-exception is that a concatenation is not permitted to end with
+exception is that a concatenation never ends with
 ``dependency-ordered before'' followed by ``sequenced before''. The reason for
 this limitation is that a consume operation participating in a
 ``dependency-ordered before'' relationship provides ordering only with respect
 to operations to which this consume operation actually carries a dependency. The
 reason that this limitation applies only to the end of such a concatenation is
 that any subsequent release operation will provide the required ordering for a
-prior consume operation. The second exception is that a concatenation is not
-permitted to consist entirely of ``sequenced before''. The reasons for this
+prior consume operation. The second exception is that a concatenation never
+consist entirely of ``sequenced before''. The reasons for this
 limitation are (1) to permit ``inter-thread happens before'' to be transitively
 closed and (2) the ``happens before'' relation, defined below, provides for
 relationships consisting entirely of ``sequenced before''.
@@ -6290,7 +6290,7 @@
 \begin{note}
 This states that operations on
 ordinary objects are not visibly reordered. This is not actually detectable
-without data races, but it is necessary to ensure that data races, as defined
+without data races, but is needed to ensure that data races, as defined
 below, and with suitable restrictions on the use of atomics, correspond to data
 races in a simple interleaved (sequentially consistent) execution.
 \end{note}
@@ -6401,7 +6401,7 @@
 However, this applies only to data-race-free programs, and data-race-free
 programs cannot observe most program transformations that do not change
 single-threaded program semantics. In fact, most single-threaded program
-transformations continue to be allowed, since any program that behaves
+transformations remain possible, since any program that behaves
 differently as a result has undefined behavior.
 \end{note}
 
@@ -6538,7 +6538,7 @@
 the implementation ensures that the thread will eventually make progress for as
 long as it has not terminated.
 \begin{note}
-This is regardless of whether or not other threads of execution (if any)
+This applies regardless of whether or not other threads of execution (if any)
 have been or are making progress. To eventually fulfill this requirement means that
 this will happen in an unspecified but finite amount of time.
 \end{note}

source/classes.tex

@@ -3074,8 +3074,7 @@
 structs that share a common initial sequence\iref{class.mem}, and
 if a non-static data member of an object of this standard-layout union type
 is active and is one of the standard-layout structs,
-it is permitted to inspect the common initial sequence
-of any of the standard-layout struct members;
+the common initial sequence of any of the standard-layout struct members can be inspected;
 see~\ref{class.mem}.
 \end{note}
 

source/declarations.tex

@@ -1558,7 +1558,7 @@
 \begin{codeblock}
 friend class T;
 \end{codeblock}
-is ill-formed. However, the similar declaration \tcode{friend T;} is allowed\iref{class.friend}.
+is ill-formed. However, the similar declaration \tcode{friend T;} is well-formed.\iref{class.friend}.
 \end{note}
 
 \pnum
@@ -4684,7 +4684,7 @@
 \tcode{X}.
 The form
 \tcode{()}
-is permitted in certain other initialization contexts\iref{expr.new,
+can appear in certain other initialization contexts\iref{expr.new,
 expr.type.conv,class.base.init}.
 \end{note}
 
@@ -4752,7 +4752,7 @@
 is sequenced before those associated with the initialization of $e_j$.
 \begin{note}
 By contrast with direct-list-initialization,
-narrowing conversions\iref{dcl.init.list} are permitted,
+narrowing conversions\iref{dcl.init.list} can appear,
 designators are not permitted,
 a temporary object bound to a reference
 does not have its lifetime extended\iref{class.temporary}, and
@@ -9332,7 +9332,7 @@
 If a function \tcode{f} is called where \tcode{f} was previously declared with the \tcode{noreturn}
 attribute and \tcode{f} eventually returns, the behavior is undefined.
 \begin{note}
-The function may
+The function can
 terminate by throwing an exception.
 \end{note}
 

source/expressions.tex

@@ -6080,7 +6080,7 @@
 inheritance relationship between the two classes.
 \begin{note}
 For example, if the classes were defined later in the translation unit,
-a multi-pass compiler would be permitted to interpret a cast between
+a multi-pass compiler could validly interpret a cast between
 pointers to the classes as if the class types were complete at the point
 of the cast.
 \end{note}

source/iostreams.tex

@@ -13839,8 +13839,8 @@
 \begin{note}
 Some operating systems have no unambiguous way to distinguish between native format and generic format arguments.
 This is by design as it simplifies use for operating systems that do not require
-disambiguation. An implementation for an operating system where disambiguation
-is required is permitted to distinguish between the formats.
+disambiguation. It is possible that an implementation for an operating system
+where disambiguation is needed distinguishes between the formats.
 \end{note}
 
 \pnum

source/lib-intro.tex

@@ -3605,8 +3605,8 @@
 provided by an implementation.
 \begin{note}
 In particular,
-an implementation is not allowed to provide
-an additional declaration of that function or function template
+a conforming implementation does not provide
+any additional declarations of that function or function template
 at namespace scope.
 \end{note}
 \begin{note}

source/numerics.tex

@@ -9472,7 +9472,8 @@
 if \tcode{X} cannot be converted to \tcode{int}
 by integral promotion\iref{conv.prom}, the program is ill-formed.
 \begin{note}
-Arguments that can be promoted to \tcode{int} are permitted for compatibility with C.
+Allowing arguments that can be promoted to \tcode{int}
+provides compatibility with C.
 \end{note}
 \end{itemdescr}
 

source/overloading.tex

@@ -2141,8 +2141,8 @@
 or when converting to the left operand of an assignment operation
 only standard conversion sequences are allowed.
 \begin{note}
-When converting to the explicit object parameter, if any,
-user-defined conversion sequences are allowed.
+When a conversion to the explicit object parameter occurs,
+it can include user-defined conversion sequences.
 \end{note}
 
 \pnum

source/support.tex

@@ -3915,10 +3915,9 @@
 returns an \tcode{exception_ptr} object that refers to the thrown exception or,
 if this is not possible, to an instance of \tcode{bad_exception}.
 \begin{note}
-The
-copy constructor of the thrown exception can also fail, so the implementation is allowed
-to substitute a \tcode{bad_exception} object to avoid infinite
-recursion.
+The copy constructor of the thrown exception can also fail,
+so the implementation can substitute a \tcode{bad_exception} object
+to avoid infinite recursion.
 \end{note}
 \end{itemdescr}
 

source/templates.tex

@@ -769,7 +769,7 @@
 \keyword{typename}
 prefix, the
 \keyword{template}
-prefix is allowed
+prefix is well-formed
 even when lookup for the name would already find a template.
 \end{note}
 \begin{example}