[flang] Better error recovery for missing THEN in ELSE IF

[klausler]

Summary

The THEN keyword in the "ELSE IF (test) THEN" statement is useless
syntactically, and to omit it is a common error (at least for me!)
that has poor error recovery. This patch changes the parser to
cough up a simple "expected 'THEN'" and still recognize the rest of
the IF construct.

[clementval]

LGTM

flang/lib/Evaluate/check-expression.cpp

if (IsAllocatable(component)) {
     return IsNullPointer(expr) || IsInitialDataTarget(expr) ||
         IsInitialProcedureTarget(expr);
   } else {
     return (*this)(expr);
   }
 }
 
 bool IsConstantExprHelper::operator()(const ProcedureRef &call) const {
-  // LBOUND, UBOUND, and SIZE with DIM= arguments will have been reritten
+  // LBOUND, UBOUND, and SIZE with DIM= arguments will have been rewritten
   // into DescriptorInquiry operations.
   if (const auto *intrinsic{std::get_if<SpecificIntrinsic>(&call.proc().u)}) {
     if (intrinsic->name == "kind" ||
         intrinsic->name == IntrinsicProcTable::InvalidName) {
       // kind is always a constant, and we avoid cascading errors by considering
       // invalid calls to intrinsics to be constant
       return true;
     } else if (intrinsic->name == "lbound" && call.arguments().size() == 1) {

flang/lib/Parser/executable-parsers.cpp

 TYPE_CONTEXT_PARSER("IF construct"_en_US,
     construct<IfConstruct>(
         statement(construct<IfThenStmt>(maybe(name / ":"),
             "IF" >> parenthesized(scalarLogicalExpr) / "THEN")),
         block,
         many(construct<IfConstruct::ElseIfBlock>(
             unambiguousStatement(construct<ElseIfStmt>(
                 "ELSE IF" >> parenthesized(scalarLogicalExpr),
-                "THEN" >> maybe(name))),
+                recovery("THEN"_tok, ok) >> maybe(name))),
             block)),
         maybe(construct<IfConstruct::ElseBlock>(
             statement(construct<ElseStmt>("ELSE" >> maybe(name))), block)),
         statement(construct<EndIfStmt>(
             recovery("END IF" >> maybe(name), endStmtErrorRecovery)))))
 
 // R1139 if-stmt -> IF ( scalar-logical-expr ) action-stmt
 TYPE_CONTEXT_PARSER("IF statement"_en_US,

flang/test/Parser/elseif-then.f90

+! RUN: not %flang_fc1 -fsyntax-only %s 2>&1 | FileCheck %s
+! CHECK-NOT: expected '=>'
+! CHECK: error: expected 'THEN'
+if (.false.) then
+else if (.false.)
+else
+end if
+end