[flang][runtime] Improve confusing list-directed REAL(2) output

flang/runtime/edit-output.cpp

@@ -446,6 +446,7 @@ RT_API_ATTRS bool RealOutputEditing<KIND>::EditFOutput(const DataEdit &edit) {
       fracDigits = sizeof buffer_ - 2; // sign & NUL
     }
   }
+  bool emitTrailingZeroes{!(flags & decimal::Minimize)};
   // Multiple conversions may be needed to get the right number of
   // effective rounded fractional digits.
   bool canIncrease{true};
@@ -526,11 +527,18 @@ RT_API_ATTRS bool RealOutputEditing<KIND>::EditFOutput(const DataEdit &edit) {
     }
     int digitsBeforePoint{std::max(0, std::min(expo, convertedDigits))};
     int zeroesBeforePoint{std::max(0, expo - digitsBeforePoint)};
+    if (zeroesBeforePoint > 0 && (flags & decimal::Minimize)) {
+      // If a minimized result looks like an integer, emit all of
+      // its digits rather than clipping some to zeroes.
+      // This can happen with HUGE(0._2) == 65504._2.
+      flags &= ~decimal::Minimize;
+      continue;
+    }
     int zeroesAfterPoint{std::min(fracDigits, std::max(0, -expo))};
     int digitsAfterPoint{convertedDigits - digitsBeforePoint};
-    int trailingZeroes{flags & decimal::Minimize
-            ? 0
-            : std::max(0, fracDigits - (zeroesAfterPoint + digitsAfterPoint))};
+    int trailingZeroes{emitTrailingZeroes
+            ? std::max(0, fracDigits - (zeroesAfterPoint + digitsAfterPoint))
+            : 0};
     if (digitsBeforePoint + zeroesBeforePoint + zeroesAfterPoint +
             digitsAfterPoint + trailingZeroes ==
         0) {

flang/unittests/Runtime/NumericalFormatTest.cpp

@@ -958,3 +958,20 @@ TEST(IOApiTests, EditDoubleInputValues) {
                            << "', want " << want << ", got " << u.raw;
   }
 }
+
+// regression test for confusing digit minimization
+TEST(IOApiTests, ConfusingMinimization) {
+  char buffer[8]{};
+  auto cookie{IONAME(BeginInternalListOutput)(buffer, sizeof buffer)};
+  StaticDescriptor<0> staticDescriptor;
+  Descriptor &desc{staticDescriptor.descriptor()};
+  std::uint16_t x{0x7bff}; // HUGE(0._2)
+  desc.Establish(TypeCode{CFI_type_half_float}, sizeof x, &x, 0, nullptr);
+  desc.Check();
+  EXPECT_TRUE(IONAME(OutputDescriptor)(cookie, desc));
+  auto status{IONAME(EndIoStatement)(cookie)};
+  EXPECT_EQ(status, 0);
+  std::string got{std::string{buffer, sizeof buffer}};
+  EXPECT_TRUE(CompareFormattedStrings(" 65504. ", got))
+      << "expected ' 65504. ', got '" << got << '\''; // not 65500.!
+}