cmd/compile/internal/syntax: more tolerant handling of missing function invocation in go/defer

Assume that an expression that is not a function call in a defer/go
statement is indeed a function that is just missing its invocation.
Report the error but continue with a sane syntax tree.

Fixes #23586.

Change-Id: Ib45ebac57c83b3e39ae4a1b137ffa291dec5b50d
Reviewed-on: https://go-review.googlesource.com/94156
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/syntax/parser.go

@@ -758,7 +758,7 @@ func (p *parser) callStmt() *CallStmt {
 		// already progressed, no need to advance
 		cx = new(CallExpr)
 		cx.pos = x.Pos()
-		cx.Fun = p.bad()
+		cx.Fun = x // assume common error of missing parentheses (function invocation)
 	}
 
 	s.Call = cx

test/fixedbugs/issue23586.go

@@ -0,0 +1,24 @@
+// errorcheck
+
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test that we type-check deferred/go functions even
+// if they are not called (a common error). Specifically,
+// we don't want to see errors such as import or variable
+// declared but not used.
+
+package p
+
+import (
+	"fmt"
+	"math"
+)
+
+func f() {
+	var i int
+	defer func() { fmt.Println() } // ERROR "must be function call"
+	go func() { _ = math.Sin(0) }  // ERROR "must be function call"
+	go func() { _ = i}             // ERROR "must be function call"
+}