FIX: better error when setting image's alpha value using path notation

+ allowing edge cases (0 and 255)
+ added range check for CHAR argument

Fixes: metaeducation/rebol-issues#2343

Author: Oldes

src/core/c-do.c

@@ -421,6 +421,8 @@ void Trace_Arg(REBINT num, REBVAL *arg, REBVAL *path)
 		Trap_Range(pvs->path);
 	case PE_BAD_SET_TYPE:
 		Trap2(RE_BAD_FIELD_SET, pvs->path, Of_Type(pvs->setval));
+	case PE_BAD_ARGUMENT:
+		Trap1(RE_INVALID_ARG, pvs->setval);
 	}
 
 	if (NOT_END(pvs->path+1)) Next_Path(pvs);

src/core/t-image.c

@@ -1284,9 +1284,10 @@ INLINE REBCNT ARGB_To_BGR(REBCNT i)
 	}
 
 	// Set the alpha only:
-	if (IS_INTEGER(val) && VAL_INT64(val) > 0 && VAL_INT64(val) < 255) n = VAL_INT32(val);
-	else if (IS_CHAR(val)) n = VAL_CHAR(val);
-	else return PE_BAD_RANGE;
+	if (IS_INTEGER(val) && VAL_INT64(val) >= 0 && VAL_INT64(val) <= 255) n = VAL_INT32(val);
+	else if (IS_CHAR(val) && VAL_CHAR(val) <= 255) n = VAL_CHAR(val); // char in R3 may be larger that 255, not like in R2
+	// Do we really want to be able change the alpha using a CHAR value? I don't think so, but keep it there so far; Oldes
+	else return PE_BAD_ARGUMENT;
 
 	dp = (REBCNT*)QUAD_SKIP(series, index);
 	*dp = (*dp & 0xffffff) | (n << 24);

src/include/sys-value.h

@@ -1005,7 +1005,8 @@ enum Path_Eval_Result {
 	PE_BAD_SELECT,
 	PE_BAD_SET,
 	PE_BAD_RANGE,
-	PE_BAD_SET_TYPE
+	PE_BAD_SET_TYPE,
+	PE_BAD_ARGUMENT
 };
 
 typedef REBINT (*REBPEF)(REBPVS *pvs); // Path evaluator function

src/tests/units/image-test.r3

@@ -60,6 +60,27 @@ Rebol [
 		--assert error? try [img/(3x2): 1.2.3]
 ===end-group===
 
+===start-group=== "set alpha using path"
+	;@@ https://github.com/rebol/rebol-issues/issues/2343
+	img: make image! 2x2
+	--test-- "set alpha using path and integer"
+		img/1: 0
+		--assert img/1 = 255.255.255.0
+		img/1: 127
+		--assert img/1 = 255.255.255.127
+		
+	--test-- "set alpha using path and char"
+		img/1: #"^@"
+		--assert img/1 = 255.255.255.0
+		img/1: to char! 127 
+		--assert img/1 = 255.255.255.127
+
+	--test-- "set alpha using path with out of range value"
+		--assert error? try [img/1: -1]
+		--assert error? try [img/1: 256]
+		--assert error? try [img/1: #"^(260)"]
+===end-group===
+
 ===start-group=== "image pixel assignment validity"
 	--test-- "image pixel 3-tuple assignment"
 		--assert 255.255.255 = img/1: 255.255.255