bpo-43892: Validate the first term of complex literal value patterns (GH-25735)

diff --git a/Parser/pegen.c b/Parser/pegen.c
index 4d6e69e..e32b271 100644
--- a/Parser/pegen.c
+++ b/Parser/pegen.c
@@ -2352,7 +2352,17 @@
 _PyPegen_ensure_imaginary(Parser *p, expr_ty exp)
 {
     if (exp->kind != Constant_kind || !PyComplex_CheckExact(exp->v.Constant.value)) {
-        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(exp, "Imaginary number required in complex literal");
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(exp, "imaginary number required in complex literal");
+        return NULL;
+    }
+    return exp;
+}
+
+expr_ty
+_PyPegen_ensure_real(Parser *p, expr_ty exp)
+{
+    if (exp->kind != Constant_kind || PyComplex_CheckExact(exp->v.Constant.value)) {
+        RAISE_SYNTAX_ERROR_KNOWN_LOCATION(exp, "real number required in complex literal");
         return NULL;
     }
     return exp;