C++23 if consteval

- [if consteval](https://en.cppreference.com/w/cpp/language/if#Consteval_if) [P1938R3](https://wg21.link/P1938R3)
- linked SonarOpenCommunity#2536

cxx-squid/dox/diff-cpp20-cpp23_grammar.txt

@@ -204,13 +204,6 @@ label-seq:
    label
    label-seq label
 
-selection-statement:
-   if constexpropt ( init-statementopt condition ) statement
-   if constexpropt ( init-statementopt condition ) statement else statement
-   if !opt consteval compound-statement
-   if !opt consteval compound-statement else statement
-   switch ( init-statementopt condition ) statement
-
 **A.7 Declarations [gram.dcl]**
 
 declaration:

cxx-squid/src/main/java/org/sonar/cxx/parser/CxxGrammarImpl.java

@@ -969,7 +969,9 @@ private static void statements(LexerfulGrammarBuilder b) {
       b.firstOf(
         b.sequence(CxxKeyword.IF, b.optional(CxxKeyword.CONSTEXPR), "(", b.optional(initStatement), condition, ")",
                    statement, b.optional(CxxKeyword.ELSE, statement)), // C++
-        b.sequence(CxxKeyword.SWITCH, "(", b.optional(initStatement), condition, ")", statement)
+        b.sequence(CxxKeyword.IF, b.optional("!"), CxxKeyword.CONSTEVAL,
+                   compoundStatement, b.optional(CxxKeyword.ELSE, statement)), // C++23
+        b.sequence(CxxKeyword.SWITCH, "(", b.optional(initStatement), condition, ")", statement) // C++
       )
     );
 

cxx-squid/src/test/java/org/sonar/cxx/parser/StatementTest.java

@@ -90,7 +90,10 @@ void selectionStatement_reallife() {
       // fix #2286
       .matches("if(a() < 0 || c >= 1) {}")
       .matches("if(a() < 0 || c > 1) {}")
-      .matches("if(a() < 0 || c >> 1) {}");
+      .matches("if(a() < 0 || c >> 1) {}")
+      // C++23
+      .matches("if !consteval {}")
+      .matches("if !consteval {} else ;");
   }
 
   @Test
@@ -131,6 +134,7 @@ void selectionStatement() {
     mockRule(CxxGrammarImpl.statement);
     mockRule(CxxGrammarImpl.condition);
     mockRule(CxxGrammarImpl.initStatement);
+    mockRule(CxxGrammarImpl.compoundStatement);
 
     assertThatParser()
       .matches("if ( condition ) statement")
@@ -141,6 +145,10 @@ void selectionStatement() {
       .matches("if constexpr ( condition ) statement else statement")
       .matches("if ( initStatement condition ) statement else statement")
       .matches("if constexpr ( initStatement condition ) statement else statement")
+      .matches("if consteval compoundStatement")
+      .matches("if ! consteval compoundStatement")
+      .matches("if consteval compoundStatement else statement")
+      .matches("if ! consteval compoundStatement else statement")
       .matches("switch ( condition ) statement")
       .matches("switch ( initStatement condition ) statement");
   }

cxx-squid/src/test/resources/parser/own/C++23/if-consteval.cc

@@ -0,0 +1,40 @@
+constexpr bool is_constant_evaluated() noexcept
+{
+    if consteval { return true; } else { return false; }
+}
+
+constexpr bool is_runtime_evaluated() noexcept
+{
+    if !consteval { return true; } else { return false; }
+}
+
+consteval std::uint64_t ipow_ct(std::uint64_t base, std::uint8_t exp)
+{
+    if (!base) return base;
+    std::uint64_t res{1};
+    while (exp)
+    {
+        if (exp & 1) res *= base;
+        exp /= 2;
+        base *= base;
+    }
+    return res;
+}
+
+constexpr std::uint64_t ipow(std::uint64_t base, std::uint8_t exp)
+{
+    if consteval // use a compile-time friendly algorithm
+    {
+        return ipow_ct(base, exp);
+    }
+    else // use runtime evaluation
+    {
+        return std::pow(base, exp);
+    }
+}
+
+int main(int, const char* argv[])
+{
+    static_assert(ipow(0, 10) == 0 && ipow(2, 10) == 1024);
+    std::cout << ipow(std::strlen(argv[0]), 3) << '\n';
+}