[flang] Translate +x to (x), not x (#157513)

In expression semantics, don't completely delete the unary plus
operator, but instead translate it into parentheses. The distinction
matters in contexts where the bounds of x are significant or when x must
not be misinterpreted as a variable.

Fixes #157379.

Author: klausler

flang/lib/Semantics/expression.cpp

@@ -3700,7 +3700,10 @@ static MaybeExpr NumericUnaryHelper(ExpressionAnalyzer &context,
       analyzer.CheckForNullPointer();
       analyzer.CheckForAssumedRank();
       if (opr == NumericOperator::Add) {
-        return analyzer.MoveExpr(0);
+        // +x -> (x), not a bare x, because the bounds of the argument must
+        // not be exposed to allocatable assignments or structure constructor
+        // components.
+        return Parenthesize(analyzer.MoveExpr(0));
       } else {
         return Negation(context.GetContextualMessages(), analyzer.MoveExpr(0));
       }

flang/test/Evaluate/bug157379.f90

@@ -0,0 +1,13 @@
+! RUN: %flang_fc1 -fdebug-unparse %s 2>&1 | FileCheck %s
+program main
+  type t
+    integer, allocatable :: ia(:)
+  end type
+  type(t) x
+  integer, allocatable :: ja(:)
+  allocate(ja(2:2))
+  ja(2) = 2
+  !CHECK: x=t(ia=(ja))
+  x = t(+ja)            ! must be t(ia=(ja)), not t(ia=ja)
+  print *, lbound(x%ia) ! must be 1, not 2
+end