[flang] Move ExprChecker into separate pass

DoConcurrentChecker depends on expressions being fully resolved so it
can't be in the same pass as ExprChecker. The same will probably
apply to AssignmentChecker when its finished.

Checks that don't depend on expressions can go in the first pass
with ExprChecker.

Original-commit: flang-compiler/f18@c0785ec06f
Reviewed-on: https://github.com/flang-compiler/f18/pull/315

flang/lib/semantics/semantics.cc

@@ -48,10 +48,12 @@ template<typename... C> struct SemanticsVisitor : public virtual C... {
     return true;
   }
   template<typename N> void Post(const N &node) { Leave(node); }
+  void Walk(const parser::Program &program) { parser::Walk(program, *this); }
 };
 
-using StatementSemantics =
-    SemanticsVisitor<ExprChecker, AssignmentChecker, DoConcurrentChecker>;
+using StatementSemanticsPass1 = SemanticsVisitor<ExprChecker>;
+using StatementSemanticsPass2 =
+    SemanticsVisitor<AssignmentChecker, DoConcurrentChecker>;
 
 SemanticsContext::SemanticsContext(
     const common::IntrinsicTypeDefaultKinds &defaultKinds)
@@ -102,8 +104,11 @@ bool Semantics::Perform() {
   if (AnyFatalError()) {
     return false;
   }
-  StatementSemantics visitor{context_};
-  parser::Walk(program_, visitor);
+  StatementSemanticsPass1{context_}.Walk(program_);
+  if (AnyFatalError()) {
+    return false;
+  }
+  StatementSemanticsPass2{context_}.Walk(program_);
   if (AnyFatalError()) {
     return false;
   }

flang/lib/semantics/semantics.h

@@ -122,8 +122,8 @@ private:
 
 // Base class for semantics checkers.
 struct BaseChecker {
-  template<typename C> void Enter(const C &x) {}
-  template<typename C> void Leave(const C &x) {}
+  template<typename N> void Enter(const N &) {}
+  template<typename N> void Leave(const N &) {}
 };
 
 }