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

[Caroline Newcombe via flang-commits]

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@@ -61,6 +61,39 @@ 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 {
+    ParseState scan{state};
+    if (!attempt("("_tok).Parse(scan)) {
+      return std::nullopt;
+    }
+    int nestLevel{1};
+    while (!scan.IsAtEnd()) {
+      if (attempt(charLiteralConstant).Parse(scan)) {
+        // Skip character literals; don't check contents.
+      } else if (attempt("("_tok).Parse(scan)) {
+        ++nestLevel;
+      } else if (attempt(")"_tok).Parse(scan)) {
+        if (--nestLevel == 0) {
+          return std::nullopt;
+        }
+      } else if (attempt("?"_tok).Parse(scan)) {
+        if (nestLevel == 1) {
+          return {Success{}};
+        }
+      } else {
+        scan.UncheckedAdvance();
+      }
+    }
+    return std::nullopt;
+  }
+};
+
 // R1001 primary ->
 //         literal-constant | designator | array-constructor |
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cenewcombe wrote:

Done

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