flambda-backend: A + sign usually means Positive (#1536)

A + sign usually means Positive.

Author: goldfirere

parsing/parser.mly

@@ -4031,7 +4031,7 @@ signed_constant:
   | PLUS FLOAT        { let (f, m) = $2 in
                         Constant.value (Pconst_float(f, m)) }
   | PLUS HASH_INT     { unboxed_int $sloc $loc($2) Positive $2 }
-  | PLUS HASH_FLOAT   { unboxed_float $sloc Negative $2 }
+  | PLUS HASH_FLOAT   { unboxed_float $sloc Positive $2 }
 ;
 
 /* Identifiers and long identifiers */

testsuite/tests/typing-layouts/unboxed_floats.ml

@@ -44,3 +44,19 @@ Line 4, characters 6-9:
           ^^^
 Error: Unbound value add
 |}];;
+
+let x = #3.0
+let x = +#3.0
+let x = -#3.0
+
+let x = #3L
+let x = +#3L
+let x = -#3L
+
+[%%expect{|
+Line 1, characters 8-12:
+1 | let x = #3.0
+            ^^^^
+Error: Unboxed float literals aren't supported yet.
+|}]
+