[flang-commits] [flang] [flang] Implement conditional expressions parser/semantics (F2023) (PR #186489)

[via flang-commits]

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@@ -61,6 +61,34 @@ TYPE_PARSER(parenthesized(
 TYPE_PARSER(construct<AcImpliedDoControl>(
     maybe(integerTypeSpec / "::"), loopBounds(scalarIntExpr)))
 
+// Conditional expression lookahead helper: checks if input starting with '('
+// contains '?' at nesting level 1. This avoids exponential backtracking when
+// parsing deeply nested parentheses that are not conditional expressions.
+struct ConditionalExprLookahead {
+  using resultType = Success;
+  constexpr ConditionalExprLookahead() {}
+  std::optional<Success> Parse(ParseState &state) const {
+    if (std::optional<const char *> at{state.PeekAtNextChar()}) {
+      if (**at != '(') {
+        return std::nullopt;
+      }
+      const char *const start{*at};
+      const char *const limit{start + state.BytesRemaining()};
+      int nestLevel{0};
+      for (const char *p{start}; p < limit; ++p) {
+        if (*p == '(') {
+          ++nestLevel;
+        } else if (*p == ')' && --nestLevel == 0) {
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jeanPerier wrote:

What if there are parentheses inside character literals?

```
integer function test(c)
  character(*) :: c
  test = (c.eq.")" ? 1 : 2)
end function
```

 

https://github.com/llvm/llvm-project/pull/186489