Merge 2021-06 CWG Motion 9

P2201R1 Mixed string literal concatenation

Author: tkoeppe

source/compatibility.tex

@@ -24,6 +24,22 @@
 \effect
 Some identifiers are no longer well-formed.
 
+\diffref{lex.string}
+\change
+Concatenated \grammarterm{string-literal}s can no longer have
+conflicting \grammarterm{encoding-prefix}es.
+\rationale
+Removal of unimplemented conditionally-supported feature.
+\effect
+Concatenation of \grammarterm{string-literal}s
+with different \grammarterm{encoding-prefix}es
+is now ill-formed.
+\begin{example}
+\begin{codeblock}
+auto c = L"a" U"b";             // was conditionally-supported; now ill-formed
+\end{codeblock}
+\end{example}
+
 \rSec1[diff.cpp17]{\Cpp{} and ISO \CppXVII{}}
 
 \rSec2[diff.cpp17.general]{General}
@@ -1835,6 +1851,21 @@
 \howwide
 Programs which depend upon \tcode{sizeof('x')} are probably rare.
 
+\diffref{lex.string}
+\change
+Concatenated \grammarterm{string-literal}s can no longer have
+conflicting \grammarterm{encoding-prefix}es.
+\rationale
+Removal of non-portable feature.
+\effect
+Concatenation of \grammarterm{string-literal}s
+with different \grammarterm{encoding-prefix}es
+is now ill-formed.
+\difficulty
+Syntactic transformation.
+\howwide
+Seldom.
+
 \diffref{lex.string}
 \change
 String literals made const.\\

source/lex.tex

@@ -1732,10 +1732,7 @@
 In translation phase 6\iref{lex.phases}, adjacent \grammarterm{string-literal}{s} are concatenated. If
 both \grammarterm{string-literal}{s} have the same \grammarterm{encoding-prefix}, the resulting concatenated \grammarterm{string-literal} has
 that \grammarterm{encoding-prefix}. If one \grammarterm{string-literal} has no \grammarterm{encoding-prefix}, it is treated as a \grammarterm{string-literal} of
-the same \grammarterm{encoding-prefix} as the other operand. If a UTF-8 string literal token is adjacent to a
-wide string literal token, the program is ill-formed. Any other concatenations are
-conditionally-supported with \impldef{concatenation of some types of \grammarterm{string-literal}s}
-behavior.
+the same \grammarterm{encoding-prefix} as the other operand. Any other concatenations are ill-formed.
 \begin{note}
 This concatenation is an interpretation, not a conversion.
 Because the interpretation happens in translation phase 6