[flang][preprocessor] Expand some keyword macros in quoted character …

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