checker, json: return error on invalid string input instead of empty string result

vlib/v/checker/fn.v

@@ -794,7 +794,20 @@ fn (mut c Checker) fn_call(mut node ast.CallExpr, mut continue_check &bool) ast.
 		}
 		c.expected_type = ast.string_type
 		node.args[1].typ = c.expr(mut node.args[1].expr)
-		if node.args[1].typ != ast.string_type {
+		if node.args[1].typ == ast.string_type {
+			if node.args[1].expr is ast.StringLiteral {
+				str_arg := node.args[1].expr
+				if str_arg.val.len > 2 && str_arg.val[0] != `{` && str_arg.val[0] != `[` {
+					c.error('json.decode: invalid json string', str_arg.pos)
+				}
+			} else if node.args[1].expr is ast.Ident && node.args[1].expr.obj is ast.Var {
+				str_var := (node.args[1].expr.obj as ast.Var).expr
+				str_var_val := str_var.str()
+				if str_var_val.len > 2 && str_var_val[1] != `{` && str_var_val[1] != `[` {
+					c.error('json.decode: invalid json string', str_var.pos())
+				}
+			}
+		} else {
 			c.error('json.decode: second argument needs to be a string', node.pos)
 		}
 		typ := expr as ast.TypeNode

vlib/v/checker/tests/json_decode_invalid_string.out

@@ -0,0 +1,12 @@
+vlib/v/checker/tests/json_decode_invalid_string.vv:7:8: error: json.decode: invalid json string
+    5 | }
+    6 | 
+    7 | txt := '"{"host": "localhost"}'
+      |        ~~~~~~~~~~~~~~~~~~~~~~~~
+    8 | json.decode(TestStruct, txt)!
+    9 | json.decode(TestStruct, txt.trim_string_right('"'))!
+vlib/v/checker/tests/json_decode_invalid_string.vv:10:25: error: json.decode: invalid json string
+    8 | json.decode(TestStruct, txt)!
+    9 | json.decode(TestStruct, txt.trim_string_right('"'))!
+   10 | json.decode(TestStruct, 'h{ost: "localhost"}')!
+      |                         ~~~~~~~~~~~~~~~~~~~~~

vlib/v/checker/tests/json_decode_invalid_string.vv

@@ -0,0 +1,10 @@
+import json
+
+struct TestStruct {
+	host string
+}
+
+txt := '"{"host": "localhost"}'
+json.decode(TestStruct, txt)!
+json.decode(TestStruct, txt.trim_string_right('"'))!
+json.decode(TestStruct, 'h{ost: "localhost"}')!