[flang] Substrings with lower bound greater than upper bound

According to section 9.4.1, paragraph 3,
 If the starting point is greater than the ending point, the substring has
 length zero

But the compilers code for substring processing was failing a call to `CHECK()`
in this case.  I fixed this by just setting the number of items in the
resulting string to 0 for this situation.

Differential Revision: https://reviews.llvm.org/D87799

diff --git a/flang/lib/Evaluate/variable.cpp b/flang/lib/Evaluate/variable.cpp
index d87c716..c81f2b1 100644 (file)
--- a/flang/lib/Evaluate/variable.cpp
+++ b/flang/lib/Evaluate/variable.cpp
@@ -204,9 +204,11 @@ std::optional<Expr<SomeCharacter>> Substring::Fold(FoldingContext &context) {
       *ubi = *length;
     }
     if (lbi && literal) {
-      CHECK(*ubi >= *lbi);
       auto newStaticData{StaticDataObject::Create()};
-      auto items{*ubi - *lbi + 1};
+      auto items{0}; // If the lower bound is greater, the length is 0
+      if (*ubi >= *lbi) {
+        items = *ubi - *lbi + 1;
+      }
       auto width{(*literal)->itemBytes()};
       auto bytes{items * width};
       auto startByte{(*lbi - 1) * width};

diff --git a/flang/test/Semantics/resolve49.f90 b/flang/test/Semantics/resolve49.f90
index b0bca05..5ead078 100644 (file)
--- a/flang/test/Semantics/resolve49.f90
+++ b/flang/test/Semantics/resolve49.f90
@@ -17,6 +17,7 @@ program p2
   end type
   character :: a(10)
   character :: b(5)
+  character :: c(0)
   integer :: n
   n = 3
   b = a(n:7)
@@ -26,6 +27,7 @@ program p2
   a(n+3:) = b
   a(:n+2) = b
   n = iachar(1_'ABCDEFGHIJ'(1:1))
+  c = 'ABCDEFGHIJ'(1:0)
 end
 
 ! Test pointer assignment with bounds