[flang][preprocessor] Expand some keyword macros in quoted character … (#96987)

…literals

To help port codes from compilers using pre-ANSI C preprocessors, which
didn't care much about context when replacing macros, support the
replacement of keyword macros in quoted character literals when (and
only when) the name of the keyword macro constitutes the entire
significant portion of a free form continuation line. See the new test
case for a motivating example.

Fixes #96781.

Author: klausler

flang/lib/Parser/prescan.cpp

@@ -820,6 +820,41 @@ void Prescanner::QuotedCharacterLiteral(
       }
       break;
     }
+    // Here's a weird edge case.  When there's a two or more following
+    // continuation lines at this point, and the entire significant part of
+    // the next continuation line is the name of a keyword macro, replace
+    // it in the character literal with its definition.  Example:
+    //   #define FOO foo
+    //   subroutine subr() bind(c, name="my_&
+    //     &FOO&
+    //     &_bar") ...
+    // produces a binding name of "my_foo_bar".
+    while (at_[1] == '&' && nextLine_ < limit_ && !InFixedFormSource()) {
+      const char *idStart{nextLine_};
+      if (const char *amper{SkipWhiteSpace(nextLine_)}; *amper == '&') {
+        idStart = amper + 1;
+      }
+      if (IsLegalIdentifierStart(*idStart)) {
+        std::size_t idLen{1};
+        for (; IsLegalInIdentifier(idStart[idLen]); ++idLen) {
+        }
+        if (idStart[idLen] == '&') {
+          CharBlock id{idStart, idLen};
+          if (preprocessor_.IsNameDefined(id)) {
+            TokenSequence ppTokens;
+            ppTokens.Put(id, GetProvenance(idStart));
+            if (auto replaced{
+                    preprocessor_.MacroReplacement(ppTokens, *this)}) {
+              tokens.Put(*replaced);
+              at_ = &idStart[idLen - 1];
+              NextLine();
+              continue; // try again on the next line
+            }
+          }
+        }
+      }
+      break;
+    }
     end = at_ + 1;
     NextChar();
     if (*at_ == quote && !isEscaped) {

flang/test/Preprocessing/kw-in-char.F90

@@ -0,0 +1,14 @@
+! RUN: %flang -E %s 2>&1 | FileCheck %s
+! CHECK: subroutine test_b_wrapper_c() bind(C, name="test_b_c_f")
+#define TEMP_LETTER b
+#define VAL c
+subroutine test_&
+TEMP_LETTER&
+_wrapper_&
+VAL&
+ () bind(C, name="test_&
+    &TEMP_LETTER&
+    &_&
+    &VAL&
+    &_f")
+end