cmd/compile: fix transform.AssignOp to deal with tricky case

When going to dictionary formats derived from the function
instantiations, I had broken out noder.Assignop() to deal specially with
shape types, but didn't quite get the tricky case right. We still need
to allow conversion between shape types, but if the destination is an
interface, we need to use CONVIFACE rather than CONVNOP.

Fixes #48453.

Change-Id: I8c4b39c2e628172ac34f493f1dd682cbac1e55ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/350949
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

2 files changed, 33 insertions, 3 deletions

diff --git a/src/cmd/compile/internal/noder/transform.go b/src/cmd/compile/internal/noder/transform.go
index a117e310ea..3e5cfacb97 100644
--- a/src/cmd/compile/internal/noder/transform.go
+++ b/src/cmd/compile/internal/noder/transform.go
@@ -431,9 +431,18 @@ func Assignop(src, dst *types.Type) (ir.Op, string) {
 		return ir.OXXX, ""
 	}
 
-	// 1. src type is identical to dst.
-	if types.IdenticalStrict(src, dst) {
-		return ir.OCONVNOP, ""
+	// 1. src type is identical to dst (taking shapes into account)
+	if types.Identical(src, dst) {
+		// We already know from assignconvfn above that IdenticalStrict(src,
+		// dst) is false, so the types are not exactly the same and one of
+		// src or dst is a shape. If dst is an interface (which means src is
+		// an interface too), we need a real OCONVIFACE op; otherwise we need a
+		// OCONVNOP. See issue #48453.
+		if dst.IsInterface() {
+			return ir.OCONVIFACE, ""
+		} else {
+			return ir.OCONVNOP, ""
+		}
 	}
 	return typecheck.Assignop1(src, dst)
 }
diff --git a/test/typeparam/issue48453.go b/test/typeparam/issue48453.go
new file mode 100644
index 0000000000..0f751d38ed
--- /dev/null
+++ b/test/typeparam/issue48453.go
@@ -0,0 +1,21 @@
+// run -gcflags=-G=3
+
+// Copyright 2021 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.
+
+package main
+
+//go:noinline
+func CopyMap[M interface{ ~map[K]V }, K comparable, V any](m M) M {
+	out := make(M, len(m))
+	for k, v := range m {
+		out[k] = v
+	}
+	return out
+}
+
+func main() {
+	var m map[*string]int
+	CopyMap(m)
+}