all: `if` guard optional: define `err` in `else` branch (#5853)

vlib/v/ast/ast.v

@@ -433,7 +433,7 @@ pub:
 	left     Expr // `a` in `a := if ...`
 	pos      token.Position
 pub mut:
-	branches []IfBranch
+	branches []IfBranch // includes all `else if` branches
 	is_expr  bool
 	typ      table.Type
 	has_else bool

vlib/v/gen/cgen.v

@@ -2368,12 +2368,15 @@ fn (mut g Gen) if_expr(node ast.IfExpr) {
 		return
 	}
 	mut is_guard := false
-	mut guard_vars := []string{len: node.branches.len}
+	mut guard_idx := 0
+	mut guard_vars := []string{}
 	for i, branch in node.branches {
 		cond := branch.cond
 		if cond is ast.IfGuardExpr {
 			if !is_guard {
 				is_guard = true
+				guard_idx = i
+				guard_vars = []string{ len: node.branches.len }
 				g.writeln('{ /* if guard */ ')
 			}
 			var_name := g.new_tmp_var()
@@ -2385,8 +2388,15 @@ fn (mut g Gen) if_expr(node ast.IfExpr) {
 		if i > 0 {
 			g.write('} else ')
 		}
+		// if last branch is `else {`
 		if i == node.branches.len - 1 && node.has_else {
 			g.writeln('{')
+			// define `err` only for simple `if val := opt {...} else {`
+			if is_guard && guard_idx == i - 1 {
+				cvar_name := guard_vars[guard_idx]
+				g.writeln('\tstring err = ${cvar_name}.v_error;')
+				g.writeln('\tint errcode = ${cvar_name}.ecode;')
+			}
 		} else {
 			match branch.cond as cond {
 				ast.IfGuardExpr {

vlib/v/parser/if.v

@@ -17,6 +17,7 @@ fn (mut p Parser) if_expr() ast.IfExpr {
 	mut branches := []ast.IfBranch{}
 	mut has_else := false
 	mut comments := []ast.Comment{}
+	mut prev_guard := false
 	for p.tok.kind in [.key_if, .key_else] {
 		p.inside_if = true
 		start_pos := p.tok.position()
@@ -28,26 +29,46 @@ fn (mut p Parser) if_expr() ast.IfExpr {
 			if p.tok.kind == .key_if {
 				p.next()
 			} else {
+				// else {
 				has_else = true
 				p.inside_if = false
 				end_pos := p.prev_tok.position()
 				body_pos := p.tok.position()
+				// only declare `err` if previous branch was an `if` guard
+				if prev_guard {
+					p.open_scope()
+					p.scope.register('errcode', ast.Var{
+						name: 'errcode'
+						typ: table.int_type
+						pos: body_pos
+						is_used: true
+					})
+					p.scope.register('err', ast.Var{
+						name: 'err'
+						typ: table.string_type
+						pos: body_pos
+						is_used: true
+					})
+				}
 				branches << ast.IfBranch{
 					stmts: p.parse_block()
 					pos: start_pos.extend(end_pos)
 					body_pos: body_pos.extend(p.tok.position())
 					comments: comments
 				}
+				if prev_guard {
+					p.close_scope()
+				}
 				comments = []
 				break
 			}
 		}
 		mut cond := ast.Expr{}
-		mut is_or := false
+		mut is_guard := false
 		// `if x := opt() {`
 		if p.peek_tok.kind == .decl_assign {
-			is_or = true
 			p.open_scope()
+			is_guard = true
 			var_pos := p.tok.position()
 			var_name := p.check_name()
 			p.check(.decl_assign)
@@ -61,13 +82,14 @@ fn (mut p Parser) if_expr() ast.IfExpr {
 				var_name: var_name
 				expr: expr
 			}
+			prev_guard = true
 		} else {
+			prev_guard = false
 			cond = p.expr(0)
 		}
 		mut left_as_name := ''
-		is_infix := cond is ast.InfixExpr
-		if is_infix {
-			infix := cond as ast.InfixExpr
+		if cond is ast.InfixExpr as infix {
+			// if sum is T
 			is_is_cast := infix.op == .key_is
 			is_ident := infix.left is ast.Ident
 			left_as_name = if is_is_cast && p.tok.kind == .key_as {
@@ -84,7 +106,7 @@ fn (mut p Parser) if_expr() ast.IfExpr {
 		body_pos := p.tok.position()
 		p.inside_if = false
 		stmts := p.parse_block()
-		if is_or {
+		if is_guard {
 			p.close_scope()
 		}
 		branches << ast.IfBranch{

vlib/v/tests/option_test.v

@@ -3,13 +3,20 @@ fn opt_err_with_code() ?string {
 }
 
 fn test_err_with_code() {
+	if w := opt_err_with_code() {
+		assert false
+		_ := w
+	} else {
+		assert err == 'hi'
+		assert errcode == 137
+	}
 	v := opt_err_with_code() or {
 		assert err == 'hi'
 		assert errcode == 137
 		return
 	}
 	assert false
-	println(v) // suppress not used error
+	_ := v
 }
 
 fn opt_err() ?string {
@@ -69,7 +76,7 @@ fn test_if_else_opt() {
 	if _ := err_call(false) {
 		assert false
 	} else {
-		assert true
+		assert err.len != 0
 	}
 }
 
@@ -150,12 +157,12 @@ fn test_or_return() {
 	if _ := or_return_error() {
 		assert false
 	} else {
-		assert true
+		assert err.len != 0
 	}
 	if _ := or_return_none() {
 		assert false
 	} else {
-		assert true
+		assert err.len == 0
 	}
 }