checker: disallow most statements in if/match expression branches (#6509)

vlib/v/ast/ast.v

@@ -34,6 +34,7 @@ pub struct Block {
 pub:
 	stmts     []Stmt
 	is_unsafe bool
+	pos       token.Position
 }
 
 // | IncDecStmt k
@@ -1097,6 +1098,19 @@ pub fn (expr Expr) is_expr() bool {
 	return true
 }
 
+// check if stmt can be an expression in C
+pub fn (stmt Stmt) check_c_expr()? {
+	match stmt {
+		AssignStmt {return}
+		ExprStmt {
+			if stmt.expr.is_expr() {return}
+			return error('unsupported statement (`${typeof(stmt.expr)}`)')
+		}
+		else {}
+	}
+	return error('unsupported statement (`${typeof(stmt)}`)')
+}
+
 pub fn (stmt Stmt) position() token.Position {
 	match stmt {
 		AssertStmt { return stmt.pos }
@@ -1104,8 +1118,9 @@ pub fn (stmt Stmt) position() token.Position {
 		/*
 		// Attr {
 		// }
-		// Block {
-		// }
+		*/
+		Block { return stmt.pos }
+		/*
 		// BranchStmt {
 		// }
 		*/

vlib/v/checker/checker.v

@@ -3002,6 +3002,13 @@ pub fn (mut c Checker) match_expr(mut node ast.MatchExpr) table.Type {
 	mut branch_without_return := false
 	for branch in node.branches {
 		c.stmts(branch.stmts)
+		if node.is_expr {
+			for st in branch.stmts {
+				st.check_c_expr() or {
+					c.error('`match` expression branch has $err', st.position())
+				}
+			}
+		}
 		// If the last statement is an expression, return its type
 		if branch.stmts.len > 0 {
 			match mut branch.stmts[branch.stmts.len - 1] as stmt {
@@ -3404,6 +3411,11 @@ pub fn (mut c Checker) if_expr(mut node ast.IfExpr) table.Type {
 				c.error('`$if_kind` expression requires an expression as the last statement of every branch',
 					branch.pos)
 			}
+			for st in branch.stmts {
+				st.check_c_expr() or {
+					c.error('`if` expression branch has $err', st.position())
+				}
+			}
 		}
 		// Also check for returns inside a comp.if's statements, even if its contents aren't parsed
 		if has_return := c.has_return(branch.stmts) {

vlib/v/checker/tests/if_match_expr.out

@@ -0,0 +1,28 @@
+vlib/v/checker/tests/if_match_expr.vv:8:6: error: `if` expression branch has unsupported statement (`v.ast.ForStmt`)
+    6 |     if true {1} else {-1} // OK
+    7 | } else {
+    8 |     for {break}
+      |         ^
+    9 |     {}
+   10 |     -1
+vlib/v/checker/tests/if_match_expr.vv:9:2: error: `if` expression branch has unsupported statement (`v.ast.Block`)
+    7 | } else {
+    8 |     for {break}
+    9 |     {}
+      |     ^
+   10 |     -1
+   11 | }
+vlib/v/checker/tests/if_match_expr.vv:15:10: error: `match` expression branch has unsupported statement (`v.ast.AssertStmt`)
+   13 | _ = match true {
+   14 |     true {
+   15 |         assert true
+      |                ~~~~
+   16 |         1
+   17 |     }
+vlib/v/checker/tests/if_match_expr.vv:19:3: error: `match` expression branch has unsupported statement (`v.ast.IfExpr`)
+   17 |     }
+   18 |     else {
+   19 |         if true {} // statement not expression
+      |         ~~
+   20 |         (-1) // parens needed ATM due to bug
+   21 |     }

vlib/v/checker/tests/if_match_expr.vv

@@ -0,0 +1,22 @@
+// only C expressions are allowed for each statement
+
+_ = if true {
+	if true {} // FIXME should error, if statement
+	_ = if true {1} else {-1} // OK
+	if true {1} else {-1} // OK
+} else {
+	for {break}
+	{}
+	-1
+}
+
+_ = match true {
+	true {
+		assert true
+		1
+	}
+	else {
+		if true {} // statement not expression
+		(-1) // parens needed ATM due to bug
+	}
+}

vlib/v/parser/parser.v

@@ -557,9 +557,11 @@ pub fn (mut p Parser) stmt(is_top_level bool) ast.Stmt {
 	p.is_stmt_ident = p.tok.kind == .name
 	match p.tok.kind {
 		.lcbr {
+			pos := p.tok.position()
 			stmts := p.parse_block()
 			return ast.Block{
 				stmts: stmts
+				pos: pos
 			}
 		}
 		.key_assert {
@@ -1989,6 +1991,7 @@ fn (mut p Parser) unsafe_stmt() ast.Stmt {
 		p.next()
 		return ast.Block{
 			is_unsafe: true
+			pos: pos
 		}
 	}
 	p.inside_unsafe = true
@@ -2025,5 +2028,6 @@ fn (mut p Parser) unsafe_stmt() ast.Stmt {
 	return ast.Block{
 		stmts: stmts
 		is_unsafe: true
+		pos: pos
 	}
 }