go/types: don't drop type in n:1 var decl if one is given

In n:1 variable declarations (multiple lhs variables with single
multi-valued initialization expression) where also a variable
type is provided, make sure that that type is assigned to all
variables on the lhs before the init expression assignment is
checked. Otherwise, (some) variables are assumed to take the type
of the corresponding value of the multi-valued init expression.

Fixes #15755.

Change-Id: I969cb5a95c85e28dbb38abd7fa7df16ff5554c03
Reviewed-on: https://go-review.googlesource.com/23313Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/decl.go

@@ -141,6 +141,14 @@ func (check *Checker) varDecl(obj *Var, lhs []*Var, typ, init ast.Expr) {
 	// determine type, if any
 	if typ != nil {
 		obj.typ = check.typ(typ)
+		// We cannot spread the type to all lhs variables if there
+		// are more than one since that would mark them as checked
+		// (see Checker.objDecl) and the assignment of init exprs,
+		// if any, would not be checked.
+		//
+		// TODO(gri) If we have no init expr, we should distribute
+		// a given type otherwise we need to re-evalate the type
+		// expr for each lhs variable, leading to duplicate work.
 	}
 
 	// check initialization
@@ -173,6 +181,17 @@ func (check *Checker) varDecl(obj *Var, lhs []*Var, typ, init ast.Expr) {
 			panic("inconsistent lhs")
 		}
 	}
+
+	// We have multiple variables on the lhs and one init expr.
+	// Make sure all variables have been given the same type if
+	// one was specified, otherwise they assume the type of the
+	// init expression values (was issue #15755).
+	if typ != nil {
+		for _, lhs := range lhs {
+			lhs.typ = obj.typ
+		}
+	}
+
 	check.initVars(lhs, []ast.Expr{init}, token.NoPos)
 }
 

src/go/types/testdata/issues.src

@@ -170,3 +170,19 @@ func issue14229() {
 		_ = b % a
 	)
 }
+
+// Check that in a n:1 variable declaration with type and initialization
+// expression the type is distributed to all variables of the lhs before
+// the initialization expression assignment is checked.
+func issue15755() {
+	// from issue
+	var i interface{}
+	type b bool
+	var x, y b = i.(b)
+	_ = x == y
+
+	// related: we should see an error since the result of f1 is ([]int, int)
+	var u, v []int = f1 /* ERROR cannot use f1 */ ()
+	_ = u
+	_ = v
+}