[flang][runtime] Don't always accept a bare exponent letter (#151597)

For more accurate compatibility with other compilers' extensions, accept
a bare exponent letter as valid real input to a formatted READ statement
only in a fixed-width input field. So it works with (G1.0) editing, but
not (G)/(D)/(E)/(F) or list-directed input.

Fixes https://github.com/llvm/llvm-project/issues/151465.

3 files changed, 21 insertions, 6 deletions

diff --git a/flang-rt/lib/runtime/edit-input.cpp b/flang-rt/lib/runtime/edit-input.cpp
index 3a8abf3..80cc085 100644
--- a/flang-rt/lib/runtime/edit-input.cpp
+++ b/flang-rt/lib/runtime/edit-input.cpp
@@ -396,8 +396,9 @@ static RT_API_ATTRS ScannedRealInput ScanRealInput(
       }
     }
   } else if (first == radixPointChar || (first >= '0' && first <= '9') ||
-      (bzMode && (first == ' ' || first == '\t')) || first == 'E' ||
-      first == 'D' || first == 'Q') {
+      (bzMode && (first == ' ' || first == '\t')) ||
+      (remaining.has_value() &&
+          (first == 'D' || first == 'E' || first == 'Q'))) {
     if (first == '0') {
       next = io.NextInField(remaining, edit);
       if (next && (*next == 'x' || *next == 'X')) { // 0X...
@@ -462,6 +463,14 @@ static RT_API_ATTRS ScannedRealInput ScanRealInput(
         // optional exponent letter and the exponent value.
         io.SkipSpaces(remaining);
         next = io.NextInField(remaining, edit);
+        if (!next) {
+          if (remaining.has_value()) {
+            // bare exponent letter accepted in fixed-width field
+            hasGoodExponent = true;
+          } else {
+            return {}; // error
+          }
+        }
       }
     }
     if (next &&
diff --git a/flang-rt/unittests/Runtime/NumericalFormatTest.cpp b/flang-rt/unittests/Runtime/NumericalFormatTest.cpp
index 4e5fdcc..033002c 100644
--- a/flang-rt/unittests/Runtime/NumericalFormatTest.cpp
+++ b/flang-rt/unittests/Runtime/NumericalFormatTest.cpp
@@ -965,7 +965,10 @@ TEST(IOApiTests, EditDoubleInputValues) {
       {"(RU,E9.1)", " 1.0E-325", 0x1, 0},
       {"(E9.1)", "-1.0E-325", 0x8000000000000000, 0},
       {"(RD,E9.1)", "-1.0E-325", 0x8000000000000001, 0},
-      {"(F7.0))", "+NaN(q)", 0x7ff8000000000000, 0},
+      {"(F7.0)", "+NaN(q)", 0x7ff8000000000000, 0},
+      {"(G)", "D", 0, IostatBadRealInput},
+      {"(G0)", "D", 0, IostatErrorInFormat},
+      {"(G1.0)", "D", 0, 0},
   };
   for (auto const &[format, data, want, iostat] : testCases) {
     auto cookie{IONAME(BeginInternalFormattedInput)(
@@ -988,13 +991,13 @@ TEST(IOApiTests, EditDoubleInputValues) {
     // union value
     IONAME(GetIoMsg)(cookie, iomsg, bufferSize - 1);
     auto status{IONAME(EndIoStatement)(cookie)};
-    ASSERT_EQ(status, iostat)
+    EXPECT_EQ(status, iostat)
         << '\'' << format << "' failed reading '" << data << "', status "
         << static_cast<int>(status) << " != expected " << iostat << " iomsg '"
         << iomsg << "'";
 
     // Ensure raw uint64 value matches expected conversion from double
-    ASSERT_EQ(u.raw, want) << '\'' << format << "' failed reading '" << data
+    EXPECT_EQ(u.raw, want) << '\'' << format << "' failed reading '" << data
                            << "', want " << want << ", got " << u.raw;
   }
 }
diff --git a/flang/docs/Extensions.md b/flang/docs/Extensions.md
index d697842..11c6717 100644
--- a/flang/docs/Extensions.md
+++ b/flang/docs/Extensions.md
@@ -479,6 +479,9 @@ end
 * Old-style `PARAMETER pi=3.14` statement without parentheses
   [-falternative-parameter-statement]
 * `UNSIGNED` type (-funsigned)
+* Default exponent of zero, e.g. `3.14159E`, on a READ from a
+  fixed-width input field.  Includes the case with only an
+  exponent letter for compatibility with other compilers.
 
 ### Extensions and legacy features deliberately not supported
 
@@ -492,7 +495,7 @@ end
 * `VIRTUAL` as synonym for `DIMENSION`
 * `ENCODE` and `DECODE` as synonyms for internal I/O
 * `IMPLICIT AUTOMATIC`, `IMPLICIT STATIC`
-* Default exponent of zero, e.g. `3.14159E`
+* Default exponent of zero, e.g. `3.14159E`, on a literal constant
 * Characters in defined operators that are neither letters nor digits
 * `B` suffix on unquoted octal constants
 * `Z` prefix on unquoted hexadecimal constants (dangerous)