[flang][preprocessor] Finesse disabling of function-like macros (#71589)

During function-like macro expansion in a standard C/C++ preprocessor,
the macro being expanded is disabled from recursive macro expansion. The
implementation in this compiler's preprocessor, however, was too broad;
the macro expansion needs to be disabled for the "rescanning" phase
only, not for actual argument expansion.

(Also corrects an obsolete comment elsewhere that was noticed during
reduction of an original test case.)

flang/lib/Parser/preprocessor.cpp

@@ -397,9 +397,9 @@ std::optional<TokenSequence> Preprocessor::MacroReplacement(
           (n + 1 == argStart.size() ? k : argStart[n + 1] - 1) - at};
       args.emplace_back(TokenSequence(input, at, count));
     }
+    TokenSequence applied{def->Apply(args, prescanner)};
     def->set_isDisabled(true);
-    TokenSequence replaced{
+    TokenSequence replaced{ReplaceMacros(std::move(applied), prescanner)};
-        ReplaceMacros(def->Apply(args, prescanner), prescanner)};
     def->set_isDisabled(false);
     if (!replaced.empty()) {
       ProvenanceRange from{def->replacement().GetProvenanceRange()};

flang/lib/Semantics/check-call.cpp

@@ -1285,11 +1285,15 @@ static void CheckAssociated(evaluate::ActualArguments &arguments,
         return;
       }
       if (const auto &targetArg{arguments[1]}) {
-        // The standard requires that the POINTER= argument be a valid LHS for
+        // The standard requires that the TARGET= argument, when present,
-        // a pointer assignment when the TARGET= argument is present.  This,
+        // be a valid RHS for a pointer assignment that has the POINTER=
-        // perhaps unintentionally, excludes function results, including NULL(),
+        // argument as its LHS.  Some popular compilers misinterpret this
-        // from being used there, as well as INTENT(IN) dummy pointers.
+        // requirement more strongly than necessary, and actually validate
-        // Allow this usage as a benign extension with a portability warning.
+        // the POINTER= argument as if it were serving as the LHS of a pointer
+        // assignment.  This, perhaps unintentionally, excludes function
+        // results, including NULL(), from being used there, as well as
+        // INTENT(IN) dummy pointers.  Detect these conditions and emit
+        // portability warnings.
         if (!evaluate::ExtractDataRef(*pointerExpr) &&
             !evaluate::IsProcedurePointer(*pointerExpr)) {
           context.messages().Say(pointerArg->sourceLocation(),

flang/test/Preprocessing/disable-expansion.F90

@@ -0,0 +1,14 @@
+! RUN: %flang -E %s | FileCheck %s
+#define KWM a
+#define FLM(x) b FLM2(x) KWM c
+#define FLM2(x) d FLM(x) e
+! CHECK: a
+KWM
+! CHECK: b d FLM(y) e a c
+FLM(y)
+! CHECK: b d FLM(a) e a c
+FLM(KWM)
+! CHECK: b d FLM(b d FLM(y) e a c) e a c
+FLM(FLM(y))
+! CHECK: b d FLM(b d FLM(a) e a c) e a c
+FLM(FLM(KWM))