Adjust the constant expression rules for lists, maps, and conditional expressions (#4313)

Adjust the constant expression rules for lists, sets, maps, constant object expressions, and conditional expressions. For the latter, it is specified that a conditional expression which is not potentially constant is also not constant. For the others, the rules in section 'Constants' are simplified (it's redundant with the same rule as stated in other sections).

There is no breaking change and no implementation effort, because the newly specified behavior is the already implemented behavior (for the analyzer as well as the CFE).

Author: eernstg

specification/dartLangSpec.tex

@@ -41,6 +41,12 @@
 % version of the language which will actually be specified by the next stable
 % release of this document.
 %
+% Apr 2025
+% - Change the rules about constants to be more consistent: Every constant
+%   expression is also a potentially constant expression. Also, eliminate
+%   redundancies in the rules about constant collection literals and constant
+%   object expressions (they are now defined just once, outside 'Constants').
+%
 % Sep 2024
 % - Clarify the extension applicability rule to explicitly state that it
 %   is concerned with an instance member with the same basename, not a
@@ -8653,48 +8659,20 @@ \subsection{Constants}
   is a potentially constant expression.
 
 \item
-  A constant object expression (\ref{const}),
-  \code{\CONST{} $C$<$T_1,\ \ldots,\ T_k$>(\metavar{arguments})} or
-  \code{\CONST{} $C$<$T_1,\ \ldots,\ T_k$>.\id(\metavar{arguments})},
-  or either expression without the leading \CONST{} that occurs in
-  a constant context,
-  is a potentially constant expression.
-  It is further a constant expression if the invocation evaluates to an object.
-  % \ref{const} requires each actual argument to be a constant expression,
-  % but here we also catch errors during evaluation, e.g., `C(1, 0)` where
-  % `C(double x, double y): z = x / y;`.
-  It is a compile-time error if a constant object expression is
-  not a constant expression (\ref{const}).
+  A constant object expression (\ref{const})
+  is a potentially constant and constant expression.
 
 \item
-  A constant list literal (\ref{lists}),
-  \code{\CONST{} <$T$>[$e_1$, \ldots, $e_n$]}, or
-  \code{<$T$>[$e_1$, \ldots, $e_n$]}
-  that occurs in a constant context,
-  is a potentially constant expression if $T$ is a constant type expression,
-  and $e_1$, \ldots{} , $e_n$ are constant expressions.
-  It is further a constant expression
-  if the list literal evaluates to an object.
+  A constant list literal (\ref{lists})
+  is a potentially constant and constant expression.
 
 \item
-  A constant set literal (\ref{sets}),
-  \code{\CONST{} <$T$>\{$e_1$, \ldots, $e_n$\}}, or
-  \code{<$T$>\{$e_1$, \ldots, $e_n$\}}
-  that occurs in a constant context,
-  is a potentially constant expression
-  if $T$ is a constant type expression,
-  and $e_1$, \ldots{} , $e_n$ are constant expressions.
-  It is further a constant expression
-  if the set literal evaluates to an object.
+  A constant set literal (\ref{sets})
+  is a potentially constant and constant expression.
 
 \item
-  A constant map literal (\ref{maps}),
-  \code{\CONST{} <$K$, $V$>\{$k_1$: $v_1$, \ldots, $k_n$: $v_n$\}}, or
-  \code{<$K$, $V$>\{$k_1$: $v_1$, \ldots, $k_n$: $v_n$\}}
-  that occurs in a constant context,
-  is a potentially constant expression.
-  It is further a constant expression
-  if the map literal evaluates to an object.
+  A constant map literal (\ref{maps})
+  is a potentially constant and constant expression.
 
 \item
   A parenthesized expression \code{($e$)} is
@@ -8813,7 +8791,7 @@ \subsection{Constants}
 \item An expression of the form \code{$e_1$\,?\,$e_2$\,:\,$e_3$}
   is potentially constant if $e_1$, $e_2$, and $e_3$
   are all potentially constant expressions.
-  It is constant if $e_1$ is a constant expression and either
+  It is further constant if $e_1$ is a constant expression and either
   \begin{enumerate}
   \item $e_1$ evaluates to \TRUE{} and $e_2$ is a constant expression, or
   \item $e_1$ evaluates to \FALSE{} and $e_3$ is a constant expression.