[flang] Adjust "doubled operator" expression extension (#93353)

Most Fortran compilers accept "doubled operators" as a language
extension. This is the use of a unary '+' or '-' operator that is not
the first unparenthesized operator in an expression, as in 'x*-y'.

This compiler has implemented this extension, but in a way that's
different from other compilers' behavior. I interpreted the unary
'+'/'-' as a unary operator in the sense of C/C++, giving it a higher
priority than any binary (dyadic) operator.

All other compilers with this extension, however, give a unary '+'/'-' a
lower precedence than exponentiation ('**'), a binary operator that
C/C++ lacks. And this interpretation makes more sense for Fortran,
anyway, where the standard conforming '-x**y' must mean '-(x**y)'
already.

This patch makes 'x*-y**z' parse as 'x*-(y**z)', not 'x*(-y)**z)', and
adds a test to ensure that it does.

1 files changed, 15 insertions, 9 deletions

diff --git a/flang/lib/Parser/expr-parsers.cpp b/flang/lib/Parser/expr-parsers.cpp
index b27366d..a47aae1 100644
--- a/flang/lib/Parser/expr-parsers.cpp
+++ b/flang/lib/Parser/expr-parsers.cpp
@@ -87,14 +87,8 @@ constexpr auto primary{instrumented("primary"_en_US,
 // R1002 level-1-expr -> [defined-unary-op] primary
 // TODO: Reasonable extension: permit multiple defined-unary-ops
 constexpr auto level1Expr{sourced(
-    first(primary, // must come before define op to resolve .TRUE._8 ambiguity
-        construct<Expr>(construct<Expr::DefinedUnary>(definedOpName, primary)),
-        extension<LanguageFeature::SignedPrimary>(
-            "nonstandard usage: signed primary"_port_en_US,
-            construct<Expr>(construct<Expr::UnaryPlus>("+" >> primary))),
-        extension<LanguageFeature::SignedPrimary>(
-            "nonstandard usage: signed primary"_port_en_US,
-            construct<Expr>(construct<Expr::Negate>("-" >> primary)))))};
+    primary || // must come before define op to resolve .TRUE._8 ambiguity
+    construct<Expr>(construct<Expr::DefinedUnary>(definedOpName, primary)))};
 
 // R1004 mult-operand -> level-1-expr [power-op mult-operand]
 // R1007 power-op -> **
@@ -105,7 +99,19 @@ struct MultOperand {
   static inline std::optional<Expr> Parse(ParseState &);
 };
 
-static constexpr auto multOperand{sourced(MultOperand{})};
+// Extension: allow + or - before a mult-operand
+// Such a unary operand has lower precedence than exponentiation,
+// so -x**2 is -(x**2), not (-x)**2; this matches all other
+// compilers with this extension.
+static constexpr auto standardMultOperand{sourced(MultOperand{})};
+static constexpr auto multOperand{standardMultOperand ||
+    extension<LanguageFeature::SignedMultOperand>(
+        "nonstandard usage: signed mult-operand"_port_en_US,
+        construct<Expr>(
+            construct<Expr::UnaryPlus>("+" >> standardMultOperand))) ||
+    extension<LanguageFeature::SignedMultOperand>(
+        "nonstandard usage: signed mult-operand"_port_en_US,
+        construct<Expr>(construct<Expr::Negate>("-" >> standardMultOperand)))};
 
 inline std::optional<Expr> MultOperand::Parse(ParseState &state) {
   std::optional<Expr> result{level1Expr.Parse(state)};