[flang] Allow tabs as white space in formats

[vdonaldson]

Summary

The fortran standard views blanks in IO formats as white space in
non-string contexts. Other compilers extend this to also view horizontal
tabs as white space. Some compilers additionally add other white space
characters to this group.

Add recognition of horizontal and vertical tabs to runtime format
validation code to match what the runtime code currently does.

[klausler]

Looks great; thanks for taking care of this.

[vdonaldson]

https://reviews.llvm.org/D119987

flang/include/flang/Common/format.h

private:
   char argString_[3]{}; // 1-2 character msg arg; usually edit descriptor name
   bool formatHasErrors_{false};
   bool unterminatedFormatError_{false};
   bool suppressMessageCascade_{false};
   bool reporterExit_{false};
   int maxNesting_{0}; // max level of nested parentheses
 };
 
+template <typename CHAR> static inline bool IsWhite(CHAR c) {
+  // White space.  ' ' is standard.  Other characters are extensions.
+  // Extension candidates:
+  //   '\t' (horizontal tab)
+  //   '\n' (new line)
+  //   '\v' (vertical tab)
+  //   '\f' (form feed)
+  //   '\r' (carriage ret)
+  return c == ' ' || c == '\t' || c == '\v';
+}
+
 template <typename CHAR> CHAR FormatValidator<CHAR>::NextChar() {
   for (++cursor_; cursor_ < end_; ++cursor_) {
-    if (*cursor_ != ' ') {
+    if (!IsWhite(*cursor_)) {
       return toupper(*cursor_);
     }
   }
   cursor_ = end_; // don't allow cursor_ > end_
   return ' ';
 }
 
 template <typename CHAR> CHAR FormatValidator<CHAR>::LookAheadChar() {
   for (laCursor_ = cursor_ + 1; laCursor_ < end_; ++laCursor_) {
-    if (*laCursor_ != ' ') {
+    if (!IsWhite(*cursor_)) {
       return toupper(*laCursor_);
     }
   }
   laCursor_ = end_; // don't allow laCursor_ > end_
   return ' ';
 }
 
 // After a call to LookAheadChar, set token kind and advance cursor to laCursor.

flang/test/Semantics/io08.f90

! RUN: %python %S/test_errors.py %s %flang_fc1
   write(*,'("""abc""")')
   write(*,'("a""""bc", 2x)')
   write(*,'(3Habc)')
   write(*,'(3Habc, 2X, 3X)')
   write(*,'(987654321098765432X)')
   write(*,'($)')
   write(*,'(\)')
   write(*,'(RZ,RU,RP,RN,RD,RC,SS,SP,S,3G15.3e2)')
+  write(*, '(' // achar( 9) // ')') ! horizontal tab
+  write(*, '(' // achar(11) // ')') ! vertical tab
+  write(*, '(' // achar(32) // ')') ! space
 
   ! C1302 warnings; no errors
   write(*,'(3P7I2)')
   write(*,'(5X i3)')
   write(*,'(XEN)')
 
   !ERROR: Empty format expression
   write(*,"")