[flang] Warn on mismatched DATA substring sizes rather than crashing

[klausler]

Summary

When a DATA statement initializes a substring with a character constant
of the wrong length, do the right thing with blank padding or truncation,
and emit a warning. Current code is crashing due to an unhandled error
reported from the low-level data image initialization framework.

flang/include/flang/Evaluate/initial-image.h

Result Add(ConstantSubscript offset, std::size_t bytes,
     } else {
       auto elements{TotalElementCount(x.shape())};
       auto elementBytes{bytes > 0 ? bytes / elements : 0};
       if (elements * elementBytes != bytes) {
         return SizeMismatch;
       } else if (bytes == 0) {
         return Ok;
       } else {
+        Result result{Ok};
         for (auto at{x.lbounds()}; elements-- > 0; x.IncrementSubscripts(at)) {
           auto scalar{x.At(at)}; // this is a std string; size() in chars
-          // Subtle: an initializer for a substring may have been
-          // expanded to the length of the entire string.
           auto scalarBytes{scalar.size() * KIND};
-          if (scalarBytes < elementBytes ||
-              (scalarBytes > elementBytes && elements != 0)) {
-            return SizeMismatch;
+          if (scalarBytes != elementBytes) {
+            result = SizeMismatch;
+          }
+          // Blank padding when short
+          for (; scalarBytes < elementBytes; scalarBytes += KIND) {
+            scalar += ' ';
           }
           // TODO endianness
           std::memcpy(&data_.at(offset), scalar.data(), elementBytes);
           offset += elementBytes;
         }
-        return Ok;
+        return result;
       }
     }
   }
   Result Add(ConstantSubscript, std::size_t, const Constant<SomeDerived> &,
       FoldingContext &);
   template <typename T>
   Result Add(ConstantSubscript offset, std::size_t bytes, const Expr<T> &x,
       FoldingContext &c) {

flang/lib/Semantics/data-to-inits.cpp

if (expr->Rank() > 0) {
       case evaluate::InitialImage::NotAConstant:
         exprAnalyzer_.Say(
             "DATA statement value '%s' for '%s' is not a constant"_err_en_US,
             folded.AsFortran(), DescribeElement());
         break;
       case evaluate::InitialImage::OutOfRange:
         OutOfRangeError();
         break;
+      case evaluate::InitialImage::SizeMismatch:
+        exprAnalyzer_.Say(
+            "DATA statement value '%s' for '%s' has the wrong length"_warn_en_US,
+            folded.AsFortran(), DescribeElement());
+        break;
       default:
         CHECK(exprAnalyzer_.context().AnyFatalError());
         break;
       }
     } else {
       exprAnalyzer_.context().Say(
           "DATA statement value could not be converted to the type '%s' of the object '%s'"_err_en_US,
           designatorType->AsFortran(), DescribeElement());

flang/test/Semantics/data17.f90

+! RUN: %python %S/test_errors.py %s %flang_fc1
+character(4) a, b, c, d, e
+!WARNING: DATA statement value '"abcde"' for 'a' has the wrong length
+data a(1:4)/'abcde'/
+!WARNING: DATA statement value '"abc"' for 'b' has the wrong length
+data b(1:4)/'abc'/
+data c/'abcde'/ ! not a substring, conforms
+data d/'abc'/ ! not a substring, conforms
+!ERROR: DATA statement designator 'e(1_8:5_8)' is out of range
+data e(1:5)/'xyz'/
+end