cmd/compile: fix method expressions with anonymous receivers

Method expressions with anonymous receiver types like "struct { T }.m"
require wrapper functions, which we weren't always creating. This in
turn resulted in linker errors.

This CL ensures that we generate wrapper functions for any anonymous
receiver types used in a method expression.

Fixes #22444.

Change-Id: Ia8ac27f238c2898965e57b82a91d959792d2ddd4
Reviewed-on: https://go-review.googlesource.com/105044
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -1474,6 +1474,7 @@ func itabsym(it *obj.LSym, offset int64) *obj.LSym {
 	return syms[methodnum]
 }
 
+// addsignat ensures that a runtime type descriptor is emitted for t.
 func addsignat(t *types.Type) {
 	signatset[t] = struct{}{}
 }

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

@@ -2378,6 +2378,16 @@ func looktypedot(n *Node, t *types.Type, dostrcmp int) bool {
 		return false
 	}
 
+	// The method expression T.m requires a wrapper when T is
+	// different from m's declared receiver type. We normally
+	// generate these wrappers while writing out runtime type
+	// descriptors, which is always done for types declared at
+	// package scope. However, we need to make sure to generate
+	// wrappers for anonymous receiver types too.
+	if mt.Sym == nil {
+		addsignat(t)
+	}
+
 	n.Sym = methodSym(t, n.Sym)
 	n.Xoffset = f2.Offset
 	n.Type = f2.Type

test/method7.go

@@ -45,10 +45,9 @@ func main() {
 	interface{ m1(string) }.m1(x, "d")
 	want += " m1(d)"
 
-	// cannot link the call below - see #22444
+	g := struct{ T }.m2
-	// g := struct{ T }.m2
+	g(struct{ T }{})
-	// g(struct{T}{})
+	want += " m2()"
-	// want += " m2()"
 
 	if got != want {
 		panic("got" + got + ", want" + want)