go/types: typecheck loop body even if range clause is broken

Be more tolerant in the presence of incorrect range clauses
if for loops and type-check loop body with minimal assumptions
about iteration variables. (Before, in some cases we would simply
ignore the loop body in such cases).

Fixes #10148.

Change-Id: I0b66f81875348088c1a7fa04ccdcbfe768f2eb6c
Reviewed-on: https://go-review.googlesource.com/7525
Reviewed-by: Alan Donovan <adonovan@google.com>

go/types/stmt.go

@@ -611,20 +611,12 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 		defer check.closeScope()
 
 		// check expression to iterate over
-		decl := s.Tok == token.DEFINE
 		var x operand
 		check.expr(&x, s.X)
-		if x.mode == invalid {
-			// if we don't have a declaration, we can still check the loop's body
-			// (otherwise we can't because we are missing the declared variables)
-			if !decl {
-				check.stmt(inner, s.Body)
-			}
-			return
-		}
 
 		// determine key/value types
 		var key, val Type
+		if x.mode != invalid {
 			switch typ := x.typ.Underlying().(type) {
 			case *Basic:
 				if isString(typ) {
@@ -657,14 +649,11 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 					// ok to continue
 				}
 			}
+		}
 
 		if key == nil {
 			check.errorf(x.pos(), "cannot range over %s", &x)
-			// if we don't have a declaration, we can still check the loop's body
+			// ok to continue
-			if !decl {
-				check.stmt(inner, s.Body)
-			}
-			return
 		}
 
 		// check assignment to/declaration of iteration variables
@@ -672,9 +661,9 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 
 		// lhs expressions and initialization value (rhs) types
 		lhs := [2]ast.Expr{s.Key, s.Value}
-		rhs := [2]Type{key, val}
+		rhs := [2]Type{key, val} // key, val may be nil
 
-		if decl {
+		if s.Tok == token.DEFINE {
 			// short variable declaration; variable scope starts after the range clause
 			// (the for loop opens a new scope, so variables on the lhs never redeclare
 			// previously declared variables)
@@ -701,10 +690,15 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 				}
 
 				// initialize lhs variable
+				if typ := rhs[i]; typ != nil {
 					x.mode = value
 					x.expr = lhs // we don't have a better rhs expression to use here
-				x.typ = rhs[i]
+					x.typ = typ
 					check.initVar(obj, &x, false)
+				} else {
+					obj.typ = Typ[Invalid]
+					obj.used = true // don't complain about unused variable
+				}
 			}
 
 			// declare variables
@@ -721,12 +715,14 @@ func (check *Checker) stmt(ctxt stmtContext, s ast.Stmt) {
 				if lhs == nil {
 					continue
 				}
+				if typ := rhs[i]; typ != nil {
 					x.mode = value
 					x.expr = lhs // we don't have a better rhs expression to use here
-				x.typ = rhs[i]
+					x.typ = typ
 					check.assignVar(lhs, &x)
 				}
 			}
+		}
 
 		check.stmt(inner, s.Body)
 

go/types/testdata/stmt0.src

@@ -785,6 +785,21 @@ func issue6766b() {
 	for a, a /* ERROR redeclared */ := range []int{1, 2, 3} { _ = a }
 }
 
+// Test that despite errors in the range clause,
+// the loop body is still type-checked (and thus
+// errors reported).
+func issue10148() {
+	for y /* ERROR declared but not used */ := range "" {
+		_ = "" /* ERROR cannot convert */ + 1
+	}
+	for range 1 /* ERROR cannot range over 1 */ {
+		_ = "" /* ERROR cannot convert */ + 1
+	}
+	for y := range 1 /* ERROR cannot range over 1 */ {
+		_ = "" /* ERROR cannot convert */ + 1
+	}
+}
+
 func labels0() {
 	goto L0
 	goto L1