FIX: transcode/error of an invalid serialized constructor triggers the error it should insert

resolves: Oldes/Rebol-issues#1857

Author: Oldes

src/core/l-scan.c

@@ -1661,12 +1661,20 @@ extern REBSER *Scan_Full_Block(SCAN_STATE *scan_state, REBYTE mode_char);
 		case TOKEN_CONSTRUCT:
 			block = Scan_Full_Block(scan_state, ']');
 			value = BLK_TAIL(emitbuf);
-			emitbuf->tail++; // Protect the block from GC
-			if (!Construct_Value(value, block)) {
-				if (IS_END(value)) Set_Block(value, block);
-				Trap1(RE_MALCONSTRUCT, value);
+			// make sure that there was not an error... transcode "#["
+			if (!IS_ERROR(value)){
+				emitbuf->tail++; // Protect the block from GC
+				if (!Construct_Value(value, block)) {
+					if (IS_END(value)) Set_Block(value, block);
+					if (GET_FLAG(scan_state->opts, SCAN_RELAX)) {
+						block = Make_Error(RE_MALCONSTRUCT, value, 0, 0);
+						SET_ERROR(value, RE_PAST_END, block);
+					} else {
+						Trap1(RE_MALCONSTRUCT, value);
+					}
+				}
+				emitbuf->tail--; // Unprotect
 			}
-			emitbuf->tail--; // Unprotect
 			break;
 
 		case TOKEN_MAP:
@@ -1720,6 +1728,7 @@ extern REBSER *Scan_Full_Block(SCAN_STATE *scan_state, REBYTE mode_char);
 				goto exit_block;
 			}
 		}
+		
 
 		// Check for end of path:
 		if (mode_char == '/') {

src/tests/units/lexer-test.r3

@@ -49,6 +49,29 @@ Rebol [
 			value = 2 line = 2
 		]
 
+	--test-- "transcode/error"
+		--assert all [
+			block? blk: transcode/error "1 2d"
+			blk/1 = 1
+			error? blk/2
+			blk/2/id = 'invalid
+		]
+		;@@ https://github.com/Oldes/Rebol-issues/issues/1857
+		--assert all [
+			error? e: transcode/one/error "#[block! 1]"
+			e/id = 'malconstruct
+		]
+		--assert all [
+			error? e: transcode/one/error "#[block! [1d]"
+			e/id = 'malconstruct
+		]
+		--assert all [
+			error? e: transcode/one/error "#["
+			e/id = 'missing
+			e/arg1 = 'end-of-script
+		]
+
+
 ===end-group===
 
 ===start-group=== "Invalid construction"