parser: warn about ambiguous infix/prefix op token (#6491)

vlib/v/ast/ast.v

@@ -422,6 +422,8 @@ pub fn (i &Ident) var_info() IdentVar {
 	}
 }
 
+// left op right
+// See: token.Kind.is_infix
 pub struct InfixExpr {
 pub:
 	op          token.Kind
@@ -434,6 +436,7 @@ pub mut:
 	auto_locked string
 }
 
+// ++, --
 pub struct PostfixExpr {
 pub:
 	op          token.Kind
@@ -443,6 +446,7 @@ pub mut:
 	auto_locked string
 }
 
+// See: token.Kind.is_prefix
 pub struct PrefixExpr {
 pub:
 	op         token.Kind

vlib/v/parser/pratt.v

@@ -269,12 +269,14 @@ pub fn (mut p Parser) expr_with_left(left ast.Expr, precedence int, is_stmt_iden
 				pos: pos
 			}
 		} else if p.tok.kind.is_infix() {
+			if p.tok.kind.is_prefix() && p.tok.line_nr != p.prev_tok.line_nr {
 				// return early for deref assign `*x = 2` goes to prefix expr
-			if p.tok.kind == .mul &&
+				if p.tok.kind == .mul && p.peek_tok2.kind == .assign {
-				p.tok.line_nr != p.prev_tok.line_nr &&
-				p.peek_tok2.kind == .assign {
 					return node
 				}
+				// added 10/2020: LATER this will be parsed as PrefixExpr instead
+				p.warn_with_pos('move infix `$p.tok.kind` operator before new line (if infix intended) or use brackets for a prefix expression', p.tok.position())
+			}
 			// continue on infix expr
 			node = p.infix_expr(node)
 			// return early `if bar is SumType as b {`

vlib/v/parser/tests/prefix_first.out

@@ -0,0 +1,28 @@
+vlib/v/parser/tests/prefix_first.vv:15:3: warning: move infix `-` operator before new line (if infix intended) or use brackets for a prefix expression
+   13 |     _ = if true {
+   14 |         v = 1
+   15 |         -1
+      |         ^
+   16 |     } else {1}
+   17 | }
+vlib/v/parser/tests/prefix_first.vv:26:3: warning: move infix `&` operator before new line (if infix intended) or use brackets for a prefix expression
+   24 |     _ = opt() or {
+   25 |         _ = 1
+   26 |         &v
+      |         ^
+   27 |     }
+   28 | }
+vlib/v/parser/tests/prefix_first.vv:13:6: error: `if` expression requires an expression as the last statement of every branch
+   11 |
+   12 |     // later this should compile correctly
+   13 |     _ = if true {
+      |         ~~~~~~~
+   14 |         v = 1
+   15 |         -1
+vlib/v/parser/tests/prefix_first.vv:24:6: error: last statement in the `or {}` block should be an expression of type `&int` or exit parent scope
+   22 |     // later this should compile correctly
+   23 |     v := 3
+   24 |     _ = opt() or {
+      |         ~~~~~
+   25 |         _ = 1
+   26 |         &v

vlib/v/parser/tests/prefix_first.vv

@@ -0,0 +1,28 @@
+// a prefix op can be parsed as an infix op if there's an expression on the line before
+// https://github.com/vlang/v/pull/6491
+fn test_prefix() {
+	mut v := 1
+	mut p := &v
+	// OK, special workaround
+	unsafe {
+		v = 1
+		*p = 2
+	}
+
+	// later this should compile correctly
+	_ = if true {
+		v = 1
+		-1
+	} else {1}
+}
+
+fn opt() ?&int {return none}
+
+fn test_prefix_or() {
+	// later this should compile correctly
+	v := 3
+	_ = opt() or {
+		_ = 1
+		&v
+	}
+}

vlib/v/token/token.v

@@ -409,6 +409,10 @@ pub fn (k Kind) is_start_of_type() bool {
 	return k in [.name, .lpar, .amp, .lsbr, .question]
 }
 
+pub fn (kind Kind) is_prefix() bool {
+	return kind in [.minus, .amp, .mul, .not, .bit_not]
+}
+
 pub fn (kind Kind) is_infix() bool {
 	return kind in [.plus, .minus, .mod, .mul, .div, .eq, .ne, .gt, .lt, .key_in,
 	//