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

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-rt/unittests/Runtime/NumericalFormatTest.cpp

@@ -882,6 +882,7 @@ TEST(IOApiTests, EditDoubleInputValues) {
       {"(F18.1)", "               125", 0x4029000000000000, 0},
       {"(F18.2)", "               125", 0x3ff4000000000000, 0},
       {"(F18.3)", "               125", 0x3fc0000000000000, 0},
+      {"('str',F3.0)", "xxx125", 0x405f400000000000, 0},
       {"(-1P,F18.0)", "               125", 0x4093880000000000, 0}, // 1250
       {"(1P,F18.0)", "               125", 0x4029000000000000, 0}, // 12.5
       {"(BZ,F18.0)", "              125 ", 0x4093880000000000, 0}, // 1250

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)')