[flang] Make F0.1 output editing of zero edge case consistent

[klausler]

Summary

The statement

PRINT '(2F0.1)', 0.0, 0.5

should emit consistent ".0 .5" output, not "0.0 .5".

[clementval]

LGTM

flang/runtime/edit-output.cpp

bool RealOutputEditing<binaryPrecision>::EditFOutput(const DataEdit &edit) {
   bool canIncrease{true};
   while (true) {
     decimal::ConversionToDecimalResult converted{
         Convert(extraDigits + fracDigits, rounding, flags)};
     if (IsInfOrNaN(converted)) {
       return EmitPrefix(edit, converted.length, editWidth) &&
           io_.Emit(converted.str, converted.length) && EmitSuffix(edit);
     }
-    int scale{IsZero() ? 1 : edit.modes.scale}; // kP
-    int expo{converted.decimalExponent + scale};
+    int expo{converted.decimalExponent + edit.modes.scale /*kP*/};
     int signLength{*converted.str == '-' || *converted.str == '+' ? 1 : 0};
     int convertedDigits{static_cast<int>(converted.length) - signLength};
+    if (IsZero()) { // don't treat converted "0" as significant digit
+      expo = 0;
+      convertedDigits = 0;
+    }
     int trailingOnes{0};
     if (expo > extraDigits && extraDigits >= 0 && canIncrease) {
       extraDigits = expo;
       if (!edit.digits.has_value()) { // F0
         fracDigits = sizeof buffer_ - extraDigits - 2; // sign & NUL
       }
       canIncrease = false; // only once
       continue;

flang/unittests/Runtime/NumericalFormatTest.cpp

TEST(IOApiTests, FormatDoubleValues) {
   using TestCaseTy = std::tuple<std::uint64_t,
       std::vector<std::tuple<const char *, const char *>>>;
   static const std::vector<TestCaseTy> testCases{
       {// -0
           0x8000000000000000,
           {
               {"(E9.1,';')", " -0.0E+00;"},
               {"(F4.0,';')", " -0.;"},
+              {"(F0.1,';')", "-.0;"},
               {"(G8.0,';')", "-0.0E+00;"},
               {"(G8.1,';')", " -0.    ;"},
               {"(G0,';')", "-0.;"},
               {"(E9.1,';')", " -0.0E+00;"},
           }},
       {// +Inf
           0x7ff0000000000000,
           {
static const std::vector<IndividualTestCaseTy> individualTestCases{
       {"(F5.3,';')", -9.9999, "*****;"},
       {"(F5.3,';')", -0.99999, "*****;"},
       {"(F5.3,';')", -0.099999, "-.100;"},
       {"(F5.3,';')", -0.0099999, "-.010;"},
       {"(F5.3,';')", -0.00099999, "-.001;"},
       {"(F5.3,';')", -0.0005, "-.001;"},
       {"(F5.3,';')", -0.00049999, "-.000;"},
       {"(F5.3,';')", -0.000099999, "-.000;"},
+      {"(F0.1,';')", 0.0, ".0;"},
   };
 
   for (auto const &[format, value, expect] : individualTestCases) {
     ASSERT_TRUE(CompareFormatReal(format, value, expect))
         << "Failed to format " << format << ", expected " << expect;
   }
 }