[flang] Extension: allow char string edit descriptors in input formats

FORMAT("J=",I3) is accepted by a few other Fortran compilers as a valid
format for input as well as for output.  The character string edit
descriptor "J=" is interpreted as if it had been 2X on input, causing
two characters to be skipped over.  The skipped characters don't have
to match the characters in the literal string.  An optional warning
is emitted under control of the -pedantic option.

Author: klausler

flang-rt/include/flang-rt/runtime/format-implementation.h

@@ -427,15 +427,24 @@ RT_API_ATTRS int FormatControl<CONTEXT>::CueUpNextDataEdit(
       } else {
         --chars;
       }
-      EmitAscii(context, format_ + start, chars);
+      if constexpr (std::is_base_of_v<InputStatementState, CONTEXT>) {
+        context.HandleRelativePosition(chars);
+      } else {
+        EmitAscii(context, format_ + start, chars);
+      }
     } else if (ch == 'H') {
       // 9HHOLLERITH
       if (!repeat || *repeat < 1 || offset_ + *repeat > formatLength_) {
         ReportBadFormat(context, "Invalid width on Hollerith in FORMAT",
             maybeReversionPoint);
         return 0;
       }
-      EmitAscii(context, format_ + offset_, static_cast<std::size_t>(*repeat));
+      if constexpr (std::is_base_of_v<InputStatementState, CONTEXT>) {
+        context.HandleRelativePosition(static_cast<std::size_t>(*repeat));
+      } else {
+        EmitAscii(
+            context, format_ + offset_, static_cast<std::size_t>(*repeat));
+      }
       offset_ += *repeat;
     } else if (ch >= 'A' && ch <= 'Z') {
       int start{offset_ - 1};

flang/docs/Extensions.md

@@ -424,6 +424,10 @@ end
 * A zero field width is allowed for logical formatted output (`L0`).
 * `OPEN(..., FORM='BINARY')` is accepted as a legacy synonym for
   the standard `OPEN(..., FORM='UNFORMATTED', ACCESS='STREAM')`.
+* A character string edit descriptor is allowed in an input format
+  with an optional compilation-time warning.  When executed, it
+  is treated as an 'nX' positioning control descriptor that skips
+  over the same number of characters, without comparison.
 
 ### Extensions supported when enabled by options
 

flang/include/flang/Common/format.h

@@ -430,11 +430,11 @@ template <typename CHAR> void FormatValidator<CHAR>::NextToken() {
       }
     }
     SetLength();
-    if (stmt_ == IoStmtKind::Read &&
-        previousToken_.kind() != TokenKind::DT) { // 13.3.2p6
-      ReportError("String edit descriptor in READ format expression");
-    } else if (token_.kind() != TokenKind::String) {
+    if (token_.kind() != TokenKind::String) {
       ReportError("Unterminated string");
+    } else if (stmt_ == IoStmtKind::Read &&
+        previousToken_.kind() != TokenKind::DT) { // 13.3.2p6
+      ReportWarning("String edit descriptor in READ format expression");
     }
     break;
   default:

flang/test/Semantics/io09.f90

@@ -1,8 +1,8 @@
-! RUN: %python %S/test_errors.py %s %flang_fc1
-  !ERROR: String edit descriptor in READ format expression
+! RUN: %python %S/test_errors.py %s %flang_fc1 -pedantic
+  !WARNING: String edit descriptor in READ format expression
   read(*,'("abc")')
 
-  !ERROR: String edit descriptor in READ format expression
+  !ERROR: Unterminated string
   !ERROR: Unterminated format expression
   read(*,'("abc)')