cmd/compile: error when using internal type declarations in generic functions

We hope to support this feature one day, but it doesn't work currently.
Issue a nice error message instead of having the compiler crash.

Update #47631

Change-Id: I0359411410acbaf9a5b9dbb988cd933de1bb8438
Reviewed-on: https://go-review.googlesource.com/c/go/+/364054
Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>

3 files changed, 43 insertions, 7 deletions

diff --git a/test/typeparam/builtins.go b/test/typeparam/builtins.go
index 844cdae8ab..73dda77e0e 100644
--- a/test/typeparam/builtins.go
+++ b/test/typeparam/builtins.go
@@ -69,24 +69,25 @@ func m1[
 	C1 interface{ chan int },
 	C2 interface{ chan int | chan string },
 ]() {
-	type S0 []int
 	_ = make([]int, 10)
-	_ = make(S0, 10)
+	_ = make(m1S0, 10)
 	_ = make(S1, 10)
 	_ = make(S1, 10, 20)
 
-	type M0 map[string]int
 	_ = make(map[string]int)
-	_ = make(M0)
+	_ = make(m1M0)
 	_ = make(M1)
 	_ = make(M1, 10)
 
-	type C0 chan int
 	_ = make(chan int)
-	_ = make(C0)
+	_ = make(m1C0)
 	_ = make(C1)
 	_ = make(C1, 10)
 }
+// TODO: put these type declarations back inside m1 when issue 47631 is fixed.
+type m1S0 []int
+type m1M0 map[string]int
+type m1C0 chan int
 
 // len/cap
 
diff --git a/test/typeparam/issue47631.go b/test/typeparam/issue47631.go
new file mode 100644
index 0000000000..7f7cfa6abb
--- /dev/null
+++ b/test/typeparam/issue47631.go
@@ -0,0 +1,34 @@
+// errorcheck -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.
+
+// TODO: one day we will support internal type declarations, at which time this test will be removed.
+
+package p
+
+func g[T any]() {
+	type U []T // ERROR "type declarations inside generic functions are not currently supported"
+	type V []int // ERROR "type declarations inside generic functions are not currently supported"
+}
+
+type S[T any] struct {
+}
+
+func (s S[T]) m() {
+	type U []T // ERROR "type declarations inside generic functions are not currently supported"
+	type V []int // ERROR "type declarations inside generic functions are not currently supported"
+}
+
+
+func f() {
+	type U []int // ok
+}
+
+type X struct {
+}
+
+func (x X) m() {
+	type U []int // ok
+}
diff --git a/test/typeparam/typelist.go b/test/typeparam/typelist.go
index 8d6a228de5..34ea4b8aa9 100644
--- a/test/typeparam/typelist.go
+++ b/test/typeparam/typelist.go
@@ -26,11 +26,12 @@ func at[T interface{ ~[]E }, E any](x T, i int) E {
 // type is itself, its "operational type" is defined by the type list in
 // the tybe bound, if any.
 func _[T interface{ ~int }](x T) {
-	type myint int
 	var _ int = int(x)
 	var _ T = 42
 	var _ T = T(myint(42))
 }
+// TODO: put this type declaration back inside the above function when issue 47631 is fixed.
+type myint int
 
 // Indexing a generic type which has a structural contraints to be an array.
 func _[T interface{ ~[10]int }](x T) {