cmd/compile: do not emit code for discardable blank fields

Fixes #38690

Change-Id: I3544daf617fddc0f89636265c113001178d16b0c
Reviewed-on: https://go-review.googlesource.com/c/go/+/230121
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -528,6 +528,9 @@ func fixedlit(ctxt initContext, kind initKind, n *Node, var_ *Node, init *Nodes)
 
 	for _, r := range n.List.Slice() {
 		a, value := splitnode(r)
+		if a == nblank && candiscard(value) {
+			continue
+		}
 
 		switch value.Op {
 		case OSLICELIT:

test/fixedbugs/issue38690.go

@@ -0,0 +1,65 @@
+// compile
+
+// Copyright 2020 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.
+
+// Make sure that literal value can be passed to struct
+// blank field of array/struct type, see issue #38690.
+
+package main
+
+type A1 = [0]int
+type A2 = [1]int
+
+type S1 struct{}
+
+type S2 struct {
+	x int
+}
+
+type S3 = struct{}
+
+type S4 = struct{ x int }
+
+type S struct {
+	x int
+	_ [0]int
+	_ [1]int
+	_ A1
+	_ A2
+	_ S1
+	_ S2
+	_ S3
+	_ S4
+	_ [1]S4
+}
+
+var s = S{1, [0]int{}, [1]int{1}, A1{}, A2{1}, S1{}, S2{1}, S3{}, S4{1}, [1]S4{}}
+
+func main() {
+	f1()
+	mustPanic(f2)
+	mustPanic(f3)
+}
+
+func f1() {
+	_ = S{1, [0]int{}, [1]int{1}, A1{}, A2{1}, S1{}, S2{1}, S3{}, S4{1}, [1]S4{}}
+}
+
+func f2() {
+	_ = S{1, [0]int{}, [1]int{1}, A1{}, A2{1}, S1{}, S2{1}, S3{}, func() S4 { panic("") }(), [1]S4{}}
+}
+
+func f3() {
+	_ = S{1, [0]int{}, [1]int{1}, A1{}, A2{1}, S1{}, S2{1}, S3{}, S4{1}, func() [1]S4 { panic("") }()}
+}
+
+func mustPanic(f func()) {
+	defer func() {
+		if recover() == nil {
+			panic("expected panic, got nil")
+		}
+	}()
+	f()
+}