checker: report clear error for comma in declaration assignment instead of panicking (#28145)

Author: waterWang

vlib/v/checker/assign.v

@@ -158,6 +158,14 @@ fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 		c.inside_recheck = old_recheck
 	}
 	for i, mut right in node.right {
+		// early check: if right-side has more values than left-side for a declaration
+		// (e.g. `x := a, b` — comma not valid in expression), produce a clear error
+		// and exit before any out-of-bounds access on node.left
+		if is_decl && i >= node.left.len {
+			c.error('unexpected `,` in expression, use `;` or a new line to separate statements',
+				right.pos())
+			return
+		}
 		if right in [ast.ArrayInit, ast.CallExpr, ast.ComptimeCall, ast.DumpExpr, ast.IfExpr,
 			ast.LockExpr, ast.MapInit, ast.MatchExpr, ast.ParExpr, ast.SelectorExpr, ast.StructInit] {
 			if right in [ast.ArrayInit, ast.IfExpr, ast.MapInit, ast.MatchExpr, ast.StructInit]

vlib/v/checker/tests/comma_in_decl_assign_err.out

@@ -0,0 +1,19 @@
+vlib/v/checker/tests/comma_in_decl_assign_err.vv:3:24: error: unexpected `,` in expression, use `;` or a new line to separate statements
+    1 | fn main() {
+    2 |     a := [1, 2, 3]
+    3 |     mut b := a.clone(), a.clone()
+      |                           ~~~~~~~
+    4 |     println(b)
+    5 | }
+vlib/v/checker/tests/comma_in_decl_assign_err.vv:4:10: error: invalid variable `b`
+    2 |     a := [1, 2, 3]
+    3 |     mut b := a.clone(), a.clone()
+    4 |     println(b)
+      |             ^
+    5 | }
+vlib/v/checker/tests/comma_in_decl_assign_err.vv:4:2: error: `println` can not print void expressions
+    2 |     a := [1, 2, 3]
+    3 |     mut b := a.clone(), a.clone()
+    4 |     println(b)
+      |     ~~~~~~~~~~
+    5 | }

vlib/v/checker/tests/comma_in_decl_assign_err.vv

@@ -0,0 +1,5 @@
+fn main() {
+	a := [1, 2, 3]
+	mut b := a.clone(), a.clone()
+	println(b)
+}