[flang] Accept L0 (#121998)

Accept a zero field width for formatted logical output (L0),
interpreting it as if it had been L1.

Author: klausler

flang/docs/Extensions.md

@@ -410,6 +410,7 @@ end
 * A character length specifier in a component or entity declaration
   is accepted before an array specification (`ch*3(2)`) as well
   as afterwards.
+* A zero field width is allowed for logical formatted output (`L0`).
 
 ### Extensions supported when enabled by options
 

flang/include/flang/Common/format.h

@@ -463,10 +463,13 @@ template <typename CHAR> void FormatValidator<CHAR>::check_r(bool allowed) {
 template <typename CHAR> bool FormatValidator<CHAR>::check_w() {
   if (token_.kind() == TokenKind::UnsignedInteger) {
     wValue_ = integerValue_;
-    if (wValue_ == 0 &&
-        (*argString_ == 'A' || *argString_ == 'L' ||
-            stmt_ == IoStmtKind::Read)) { // C1306, 13.7.2.1p6
-      ReportError("'%s' edit descriptor 'w' value must be positive");
+    if (wValue_ == 0) {
+      if (*argString_ == 'A' || stmt_ == IoStmtKind::Read) {
+        // C1306, 13.7.2.1p6
+        ReportError("'%s' edit descriptor 'w' value must be positive");
+      } else if (*argString_ == 'L') {
+        ReportWarning("'%s' edit descriptor 'w' value should be positive");
+      }
     }
     NextToken();
     return true;

flang/test/Semantics/io07.f90

@@ -68,10 +68,10 @@
 6001 format(((I0, B0)))
 
      !ERROR: 'A' edit descriptor 'w' value must be positive
-     !ERROR: 'L' edit descriptor 'w' value must be positive
+     !WARNING: 'L' edit descriptor 'w' value should be positive
 6101 format((A0), ((L0)))
 
-     !ERROR: 'L' edit descriptor 'w' value must be positive
+     !WARNING: 'L' edit descriptor 'w' value should be positive
 6102 format((3(((L 0 0 0)))))
 
 7001 format(17G8.1, 17G8.1e3)

flang/test/Semantics/io08.f90

@@ -192,8 +192,7 @@
   !ERROR: 'A' edit descriptor 'w' value must be positive
   write(*,'(A0)')
 
-  !ERROR: 'L' edit descriptor 'w' value must be positive
-  write(*,'(L0)')
+  write(*,'(L0)') ! warning, not error
 
   !ERROR: Expected 'G' edit descriptor '.d' value
   write(*,'(G4)')

flang/unittests/Runtime/LogicalFormatTest.cpp

@@ -23,7 +23,7 @@ TEST(IOApiTests, LogicalFormatTest) {
   char buffer[bufferSize];
 
   // Create format for all types and values to be written
-  const char *format{"(L,L3,I3,L2,L2,I3,L2,A3,L2,L,F4.1,L2)"};
+  const char *format{"(L0,L3,I3,L2,L2,I3,L2,A3,L2,L,F4.1,L2)"};
   auto cookie{IONAME(BeginInternalFormattedOutput)(
       buffer, bufferSize, format, std::strlen(format))};