[flang] Prep for review

Original-commit: flang-compiler/f18@e86eae005e
Reviewed-on: https://github.com/flang-compiler/f18/pull/200

flang/lib/parser/parse-tree-visitor.h

@@ -75,6 +75,9 @@ template<typename T, typename M> void Walk(std::optional<T> &x, M &mutator) {
     Walk(*x, mutator);
   }
 }
+// For most lists, just traverse the elements; but when a list constitutes
+// a Block (i.e., std::list<ExecutionPartConstruct>), also invoke the
+// visitor/mutator on the list itself.
 template<typename T, typename V> void Walk(const std::list<T> &x, V &visitor) {
   for (const auto &elem : x) {
     Walk(elem, visitor);
@@ -85,7 +88,6 @@ template<typename T, typename M> void Walk(std::list<T> &x, M &mutator) {
     Walk(elem, mutator);
   }
 }
-// When a list constitutes a Block, invoke the visitor or mutator.
 template<typename V> void Walk(const Block &x, V &visitor) {
   if (visitor.Pre(x)) {
     for (const auto &elem : x) {

flang/lib/semantics/canonicalize-do.cc

@@ -14,8 +14,6 @@
 
 #include "canonicalize-do.h"
 #include "../parser/parse-tree-visitor.h"
-#include <variant>
-#include <vector>
 
 namespace Fortran::parser {
 
@@ -118,4 +116,5 @@ void CanonicalizeDo(Program &program) {
   CanonicalizationOfDoLoops canonicalizationOfDoLoops{labelInfos};
   Walk(program, canonicalizationOfDoLoops);
 }
+
 }  // namespace Fortran::parser

flang/tools/f18/f18.cc

@@ -207,6 +207,7 @@ std::string CompileFortran(std::string path, Fortran::parser::Options options,
   if (driver.measureTree) {
     MeasureParseTree(parseTree);
   }
+  // TODO: Change this predicate to just "if (!driver.debugNoSemantics)"
   if (driver.debugSemantics || driver.debugResolveNames || driver.dumpSymbols ||
       driver.dumpUnparseWithSymbols || driver.debugExpressions) {
     semantics.Perform(parseTree);
@@ -410,6 +411,7 @@ int main(int argc, char *const argv[]) {
     } else if (arg == "-fdebug-instrumented-parse") {
       options.instrumentedParse = true;
     } else if (arg == "-fdebug-semantics") {
+      // TODO: Enable by default once basic tests pass
       driver.debugSemantics = true;
     } else if (arg == "-funparse") {
       driver.dumpUnparse = true;