checker: unreachable code warning

vlib/v/ast/ast.v

@@ -852,3 +852,79 @@ fn (expr Expr) position() token.Position {
 		}
 	}
 }
+
+fn (stmt Stmt) position() token.Position {
+	match mut stmt {
+		AssertStmt {
+			return it.pos
+		}
+		AssignStmt {
+			return it.pos
+		}
+		// Attr {
+		// }
+		// Block {
+		// }
+		// BranchStmt {
+		// }
+		Comment {
+			return it.pos
+		}
+		CompIf {
+			return it.pos
+		}
+		ConstDecl {
+			return it.pos
+		}
+		// DeferStmt {
+		// }
+		EnumDecl {
+			return it.pos
+		}
+		ExprStmt {
+			return it.pos
+		}
+		FnDecl {
+			return it.pos
+		}
+		ForCStmt {
+			return it.pos
+		}
+		ForInStmt {
+			return it.pos
+		}
+		ForStmt {
+			return it.pos
+		}
+		// GlobalDecl {
+		// }
+		// GoStmt {
+		// }
+		// GotoLabel {
+		// }
+		// GotoStmt {
+		// }
+		// HashStmt {
+		// }
+		Import {
+			return it.pos
+		}
+		// InterfaceDecl {
+		// }
+		// Module {
+		// }
+		Return {
+			return it.pos
+		}
+		StructDecl {
+			return it.pos
+		}
+		// TypeDecl {
+		// }
+		// UnsafeStmt {
+		// }
+		else {
+			return token.Position{}
+		}
+	}
+}

vlib/v/checker/checker.v

@@ -34,6 +34,7 @@ mut:
 	// checked_ident  string // to avoid infinit checker loops
 	var_decl_name  string
 	returns        bool
+	scope_returns  bool
 	mod            string // current module name
 	is_builtin_mod bool // are we in `builtin`?
 }
@@ -1398,6 +1399,7 @@ fn (mut c Checker) stmt(node ast.Stmt) {
 		ast.Return {
 			c.returns = true
 			c.return_stmt(mut it)
+			c.scope_returns = true
 		}
 		ast.StructDecl {
 			c.struct_decl(it)
@@ -1416,10 +1418,20 @@ fn (mut c Checker) stmt(node ast.Stmt) {
 }
 
 fn (mut c Checker) stmts(stmts []ast.Stmt) {
+	mut unreachable := token.Position{line_nr: -1}
 	c.expected_type = table.void_type
 	for stmt in stmts {
+		if c.scope_returns {
+			if unreachable.line_nr == -1 {
+				unreachable = stmt.position()
+			}
+		}
 		c.stmt(stmt)
 	}
+	if unreachable.line_nr >= 0 {
+		c.warn('unreachable code', unreachable)
+	}
+	c.scope_returns = false
 	c.expected_type = table.void_type
 }
 

vlib/v/checker/tests/unreachable_code.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/unreachable_code.v:3:7: error: unreachable code 
+    1| fn foo() int {
+    2|     return if 1 == 1 { 1 } else { 2 }
+    3|     a := 1
+                ^
+    4|     println(a)
+    5| }

vlib/v/checker/tests/unreachable_code.vv

@@ -0,0 +1,8 @@
+fn foo() int {
+	return if 1 == 1 { 1 } else { 2 }
+	a := 1
+	println(a)
+}
+fn main() {
+	foo()
+}