[flang] Improve error message on bad LOGICAL compare operations

[PeteSteinfeld]

Summary

When comparing LOGICAL operands using ".eq." or ".ne." we were not
guiding users to the ".eqv." and ".neqv." operations.

flang/lib/Semantics/expression.cpp

if (analyzer.fatalErrors()) {
     return std::nullopt;
   } else {
     if (IsNullPointer(analyzer.GetExpr(0)) ||
         IsNullPointer(analyzer.GetExpr(1))) {
       context.Say("NULL() not allowed as an operand of a relational "
                   "operator"_err_en_US);
       return std::nullopt;
     }
-    analyzer.ConvertBOZ(0, analyzer.GetType(1));
-    analyzer.ConvertBOZ(1, analyzer.GetType(0));
+    std::optional<DynamicType> leftType{analyzer.GetType(0)};
+    std::optional<DynamicType> rightType{analyzer.GetType(1)};
+    analyzer.ConvertBOZ(0, rightType);
+    analyzer.ConvertBOZ(1, leftType);
     if (analyzer.IsIntrinsicRelational(opr)) {
       return AsMaybeExpr(Relate(context.GetContextualMessages(), opr,
           analyzer.MoveExpr(0), analyzer.MoveExpr(1)));
+    } else if (leftType && leftType->category() == TypeCategory::Logical &&
+        rightType && rightType->category() == TypeCategory::Logical) {
+      context.Say("LOGICAL operands must be compared using .EQV. or "
+                  ".NEQV."_err_en_US);
+      return std::nullopt;
     } else {
       return analyzer.TryDefinedOp(opr,
           "Operands of %s must have comparable types; have %s and %s"_err_en_US);
     }
   }
 }
 
 MaybeExpr ExpressionAnalyzer::Analyze(const parser::Expr::LT &x) {

flang/test/Semantics/resolve98.f90

+! RUN: %S/test_errors.sh %s %t %f18
+
+! Errors when comparing LOGICAL operands
+
+program testCompare
+  logical flag1, flag2
+  if (flag1 .eqv. .false.) continue
+  if (flag1 .neqv. flag2) continue
+  !ERROR: LOGICAL operands must be compared using .EQV. or .NEQV.
+  if (flag1 .eq. .false.) continue
+  !ERROR: LOGICAL operands must be compared using .EQV. or .NEQV.
+  if (flag1 .ne. flag2) continue
+end program testCompare