[flang] Portability warning and documentation for an obscure extension

[klausler]

Summary

A quotation mark can appear in a Fortran character literal by doubling
it; for example, PRINT *, "'""'" prints '"'. When those doubled
quotation marks are split by a free form line continuation, the
continuation line should have an ampersand before the second quotation
mark. But most compilers, including this one, allow the second
quotation mark to appear as the first character on the continuation
line, too.

So this works:

print *, "'"&

"'"

but it really should be written as:

print *, "'"&

&"'"

Emit a portability warning and document that we support this near-universal
extension.

[vzakhari]

LGTM

flang/docs/Extensions.md

 * An entity may appear in a `DATA` statement before its explicit
   type declaration under `IMPLICIT NONE(TYPE)`.
 * INCLUDE lines can start in any column, can be preceded in
   fixed form source by a '0' in column 6, can contain spaces
   between the letters of the word INCLUDE, and can have a
   numeric character literal kind prefix on the file name.
 * Intrinsic procedures TAND and ATAND. Constant folding is currently
   not supported for these procedures but this is planned.
+* When a pair of quotation marks in a character literal are split
+  by a line continuation in free form, the second quotation mark
+  may appear at the beginning of the continuation line without an
+  ampersand, althought one is required by the standard.
 
 ### Extensions supported when enabled by options
 
 * C-style backslash escape sequences in quoted CHARACTER literals
   (but not Hollerith) [-fbackslash]
 * Logical abbreviations `.T.`, `.F.`, `.N.`, `.A.`, `.O.`, and `.X.`
   [-flogical-abbreviations]
 * `.XOR.` as a synonym for `.NEQV.` [-fxor-operator]

 ## De Facto Standard Features
 
 * `EXTENDS_TYPE_OF()` returns `.TRUE.` if both of its arguments have the
   same type, a case that is technically implementation-defined.
 
 * `ENCODING=` is not in the list of changeable modes on an I/O unit,
   but every Fortran compiler allows the encoding to be changed on an
   open unit.
+

flang/lib/Parser/prescan.cpp

if (*at_ == quote && !isEscaped) {
       NextChar();
       if (InFixedFormSource()) {
         SkipSpaces();
       }
       if (*at_ != quote) {
         break;
       }
       inCharLiteral_ = true;
+      if (insertASpace_) {
+        Say(GetProvenanceRange(at_, end),
+            "Repeated quote mark in character literal continuation line should have been preceded by '&'"_port_en_US);
+        insertASpace_ = false;
+      }
     }
   }
   inCharLiteral_ = false;
 }
 
 void Prescanner::Hollerith(
     TokenSequence &tokens, int count, const char *start) {
   inCharLiteral_ = true;

flang/test/Parser/continuation-before-quote.f90

+! RUN: %flang_fc1 -fsyntax-only %s 2>&1 | FileCheck %s
+! Continuation between repeated quotation marks
+subroutine test
+!CHECK: portability: Repeated quote mark in character literal continuation line should have been preceded by '&'
+  print *, 'needs an '&
+'ampersand'''
+!CHECK-NOT: portability: Repeated quote mark in character literal continuation line should have been preceded by '&'
+  print *, 'has an '&
+&'ampersand'''
+end