go/types: record CallExpr result type even if argument is invalid

+ test

Fixes #15305

Change-Id: Ica657c00c92f0b19f0df7452cdbe5a95d23cc8a4
Reviewed-on: https://go-review.googlesource.com/22085Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/api_test.go

@@ -1059,3 +1059,28 @@ func TestIdentical_issue15173(t *testing.T) {
 		}
 	}
 }
+
+func TestIssue15305(t *testing.T) {
+	const src = "package p; func f() int16; var _ = f(undef)"
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "issue15305.go", src, 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+	conf := Config{
+		Error: func(err error) {}, // allow errors
+	}
+	info := &Info{
+		Types: make(map[ast.Expr]TypeAndValue),
+	}
+	conf.Check("p", fset, []*ast.File{f}, info) // ignore result
+	for e, tv := range info.Types {
+		if _, ok := e.(*ast.CallExpr); ok {
+			if tv.Type != Typ[Int16] {
+				t.Errorf("CallExpr has type %v, want int16", tv.Type)
+			}
+			return
+		}
+	}
+	t.Errorf("CallExpr has no type")
+}

src/go/types/call.go

@@ -62,14 +62,12 @@ func (check *Checker) call(x *operand, e *ast.CallExpr) exprKind {
 		}
 
 		arg, n, _ := unpack(func(x *operand, i int) { check.multiExpr(x, e.Args[i]) }, len(e.Args), false)
-		if arg == nil {
+		if arg != nil {
+			check.arguments(x, e, sig, arg, n)
+		} else {
 			x.mode = invalid
-			x.expr = e
-			return statement
 		}
 
-		check.arguments(x, e, sig, arg, n)
-
 		// determine result
 		switch sig.results.Len() {
 		case 0:
@@ -81,6 +79,7 @@ func (check *Checker) call(x *operand, e *ast.CallExpr) exprKind {
 			x.mode = value
 			x.typ = sig.results
 		}
+
 		x.expr = e
 		check.hasCallOrRecv = true