cmd/compile: don't implicitly dereference pointer to arrays for real/imag

Fixes #20602.

Change-Id: Iac1589484dec626c018314e0cea7efce091fd87d
Reviewed-on: https://go-review.googlesource.com/45075Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/typecheck.go

@@ -1349,7 +1349,9 @@ OpSwitch:
 		}
 		n.Left = typecheck(n.Left, Erv)
 		n.Left = defaultlit(n.Left, nil)
-		n.Left = implicitstar(n.Left)
+		if n.Op == OCAP || n.Op == OLEN {
+			n.Left = implicitstar(n.Left)
+		}
 		l := n.Left
 		t := l.Type
 		if t == nil {

test/fixedbugs/issue20602.go

+// errorcheck
+
+// Copyright 2017 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.
+
+// Verify that the correct (not implicitly dereferenced)
+// type is reported in the error message.
+
+package p
+
+var p = &[1]complex128{0}
+var _ = real(p)  // ERROR "type \*\[1\]complex128"
+var _ = imag(p)	 // ERROR "type \*\[1\]complex128"