checker: identify return on comptime generic match nested in if else (fix #25461) (#25462)

Author: dy-tea

vlib/v/checker/match.v

@@ -445,12 +445,21 @@ fn (mut c Checker) match_expr(mut node ast.MatchExpr) ast.Type {
 			}
 		}
 		first_iteration = false
-		if has_return := c.has_return(branch.stmts) {
-			if has_return {
+		if node.is_comptime {
+			// branches may not have been processed by c.stmts()
+			if has_top_return(branch.stmts) {
 				nbranches_with_return++
 			} else {
 				nbranches_without_return++
 			}
+		} else {
+			if has_return := c.has_return(branch.stmts) {
+				if has_return {
+					nbranches_with_return++
+				} else {
+					nbranches_without_return++
+				}
+			}
 		}
 	}
 	if nbranches_with_return > 0 {

vlib/v/checker/return.v

@@ -367,6 +367,14 @@ fn has_top_return(stmts []ast.Stmt) bool {
 					if has_top_return(stmt.expr.stmts) {
 						return true
 					}
+				} else if stmt.expr is ast.IfExpr {
+					if has_top_return_in_if_expr(stmt.expr) {
+						return true
+					}
+				} else if stmt.expr is ast.MatchExpr {
+					if has_top_return_in_match_expr(stmt.expr) {
+						return true
+					}
 				}
 			}
 			else {}
@@ -375,6 +383,57 @@ fn has_top_return(stmts []ast.Stmt) bool {
 	return false
 }
 
+fn has_top_return_in_if_expr(if_expr ast.IfExpr) bool {
+	if if_expr.branches.len < 2 || !if_expr.has_else {
+		return false
+	}
+	for branch in if_expr.branches {
+		if !has_top_return(branch.stmts) {
+			return false
+		}
+	}
+	return true
+}
+
+fn has_top_return_in_match_expr(match_expr ast.MatchExpr) bool {
+	if match_expr.branches.len == 0 {
+		return false
+	}
+	if match_expr.is_comptime {
+		mut has_else := false
+		for branch in match_expr.branches {
+			if branch.is_else {
+				has_else = true
+				break
+			}
+			for expr in branch.exprs {
+				if expr is ast.Ident && expr.name == '\$else' {
+					has_else = true
+					break
+				}
+			}
+			if has_else {
+				break
+			}
+		}
+		if has_else {
+			for branch in match_expr.branches {
+				if !has_top_return(branch.stmts) {
+					return false
+				}
+			}
+			return true
+		}
+		return false
+	}
+	for branch in match_expr.branches {
+		if !has_top_return(branch.stmts) {
+			return false
+		}
+	}
+	return true
+}
+
 fn (mut c Checker) check_noreturn_fn_decl(mut node ast.FnDecl) {
 	if !node.is_noreturn {
 		return

vlib/v/tests/comptime/comptime_match_generic_inside_if_test.v

@@ -0,0 +1,26 @@
+fn strange[T](a T, b T) T {
+	if a != 0 {
+		$match T {
+			u8 {
+				return a
+			}
+			i8 {
+				if a > 0 {
+					return a
+				} else {
+					return b
+				}
+			}
+			$else {
+				$compile_error('unknown')
+			}
+		}
+	} else {
+		return b
+	}
+}
+
+fn test_main() {
+	assert strange[u8](0, 1) == 1
+	assert strange[i8](0, 1) == 1
+}