[Clang][Sema] Check unexpected else statement in cond-update-stmt

[tianshilei1992]

Summary

In 'cond-update-stmt', else statement is not expected. This patch adds
the check in Sema.

[ABataev]

LG

clang/include/clang/Basic/DiagnosticSemaKinds.td

 def err_omp_atomic_compare : Error<
   "the statement for 'atomic compare' must be a compound statement of form '{x = expr ordop x ? expr : x;}', '{x = x ordop expr? expr : x;}',"
   " '{x = x == e ? d : x;}', '{x = e == x ? d : x;}', or 'if(expr ordop x) {x = expr;}', 'if(x ordop expr) {x = expr;}', 'if(x == e) {x = d;}',"
   " 'if(e == x) {x = d;}' where 'x' is an lvalue expression with scalar type, 'expr', 'e', and 'd' are expressions with scalar type,"
   " and 'ordop' is one of '<' or '>'.">;
 def note_omp_atomic_compare: Note<
   "%select{expected compound statement|expected exactly one expression statement|expected assignment statement|expected conditional operator|expect result value to be at false expression|"
   "expect binary operator in conditional expression|expect '<', '>' or '==' as order operator|expect comparison in a form of 'x == e', 'e == x', 'x ordop expr', or 'expr ordop x'|"
-  "expect lvalue for result value|expect scalar value|expect integer value}0">;
+  "expect lvalue for result value|expect scalar value|expect integer value|unexpected 'else' statement}0">;
 def err_omp_atomic_several_clauses : Error<
   "directive '#pragma omp atomic' cannot contain more than one 'read', 'write', 'update', 'capture', or 'compare' clause">;
 def err_omp_several_mem_order_clauses : Error<
   "directive '#pragma omp %0' cannot contain more than one %select{'seq_cst', 'relaxed', |}1'acq_rel', 'acquire' or 'release' clause">;
 def err_omp_atomic_incompatible_mem_order_clause : Error<
   "directive '#pragma omp atomic%select{ %0|}1' cannot be used with '%2' clause">;
 def note_omp_previous_mem_order_clause : Note<
   "'%0' clause used here">;

clang/lib/Sema/SemaOpenMP.cpp

enum ErrorTy {
     /// Invalid comparison (not x == e, e == x, x ordop expr, or expr ordop x).
     InvalidComparison,
     /// X is not a lvalue.
     XNotLValue,
     /// Not a scalar.
     NotScalar,
     /// Not an integer.
     NotInteger,
+    /// 'else' statement is not expected.
+    UnexpectedElse,
     /// No error.
     NoError,
   };
 
   struct ErrorInfoTy {
     ErrorTy Error;
     SourceLocation ErrorLoc;
     SourceRange ErrorRange;
bool OpenMPAtomicCompareChecker::checkCondUpdateStmt(IfStmt *S,
   }
   default:
     ErrorInfo.Error = ErrorTy::InvalidBinaryOp;
     ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = Cond->getExprLoc();
     ErrorInfo.ErrorRange = ErrorInfo.NoteRange = Cond->getSourceRange();
     return false;
   }
 
+  if (S->getElse()) {
+    ErrorInfo.Error = ErrorTy::UnexpectedElse;
+    ErrorInfo.ErrorLoc = ErrorInfo.NoteLoc = S->getElse()->getBeginLoc();
+    ErrorInfo.ErrorRange = ErrorInfo.NoteRange = S->getElse()->getSourceRange();
+    return false;
+  }
+
   return true;
 }
 
 bool OpenMPAtomicCompareChecker::checkCondExprStmt(Stmt *S,
                                                    ErrorInfoTy &ErrorInfo) {
   auto *BO = dyn_cast<BinaryOperator>(S);
   if (!BO) {
     ErrorInfo.Error = ErrorTy::NotAnAssignment;

clang/test/OpenMP/atomic_messages.c

 // omp51-error@+3 {{the statement for 'atomic compare' must be a compound statement of form '{x = expr ordop x ? expr : x;}', '{x = x ordop expr? expr : x;}', '{x = x == e ? d : x;}', '{x = e == x ? d : x;}', or 'if(expr ordop x) {x = expr;}', 'if(x ordop expr) {x = expr;}', 'if(x == e) {x = d;}', 'if(e == x) {x = d;}' where 'x' is an lvalue expression with scalar type, 'expr', 'e', and 'd' are expressions with scalar type, and 'ordop' is one of '<' or '>'.}}
 // omp51-note@+2 {{expected exactly one expression statement}}
 #pragma omp atomic compare
   {
     if (x > d)
       x = e;
     d = e;
   }
+// omp51-error@+7 {{the statement for 'atomic compare' must be a compound statement of form '{x = expr ordop x ? expr : x;}', '{x = x ordop expr? expr : x;}', '{x = x == e ? d : x;}', '{x = e == x ? d : x;}', or 'if(expr ordop x) {x = expr;}', 'if(x ordop expr) {x = expr;}', 'if(x == e) {x = d;}', 'if(e == x) {x = d;}' where 'x' is an lvalue expression with scalar type, 'expr', 'e', and 'd' are expressions with scalar type, and 'ordop' is one of '<' or '>'.}}
+// omp51-note@+6 {{unexpected 'else' statement}}
+#pragma omp atomic compare
+  {
+    if (x > e)
+      x = e;
+    else
+      d = e;
+  }
   float fx = 0.0f;
   float fd = 0.0f;
   float fe = 0.0f;
 // omp51-error@+5 {{the statement for 'atomic compare' must be a compound statement of form '{x = expr ordop x ? expr : x;}', '{x = x ordop expr? expr : x;}', '{x = x == e ? d : x;}', '{x = e == x ? d : x;}', or 'if(expr ordop x) {x = expr;}', 'if(x ordop expr) {x = expr;}', 'if(x == e) {x = d;}', 'if(e == x) {x = d;}' where 'x' is an lvalue expression with scalar type, 'expr', 'e', and 'd' are expressions with scalar type, and 'ordop' is one of '<' or '>'.}}
 // omp51-note@+4 {{expect integer value}}
 #pragma omp atomic compare
   {
     if (fx > fe)