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

[klausler]

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.

[llvmbot]

@llvm/pr-subscribers-flang-semantics

Author: Peter Klausler (klausler)

Changes

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.

---

Full diff: https://github.com/llvm/llvm-project/pull/157513.diff

2 Files Affected:

• (modified) flang/lib/Semantics/expression.cpp (+4-1)
• (added) flang/test/Evaluate/bug157379.f90 (+13)

diff --git a/flang/lib/Semantics/expression.cpp b/flang/lib/Semantics/expression.cpp
index ccccf60adae5d..3f048ab6f7a4d 100644
--- a/flang/lib/Semantics/expression.cpp
+++ b/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));
       }
diff --git a/flang/test/Evaluate/bug157379.f90 b/flang/test/Evaluate/bug157379.f90
new file mode 100644
index 0000000000000..53aac4c29d3e4
--- /dev/null
+++ b/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