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>

src/cmd/compile/internal/noder/stmt.go

@@ -46,6 +46,10 @@ func (g *irgen) stmt(stmt syntax.Stmt) ir.Node {
 		n.SetTypecheck(1)
 		return n
 	case *syntax.DeclStmt:
+		if _, ok := stmt.DeclList[0].(*syntax.TypeDecl); ok && g.topFuncIsGeneric {
+			// TODO: remove this restriction. See issue 47631.
+			base.ErrorfAt(g.pos(stmt), "type declarations inside generic functions are not currently supported")
+		}
 		n := ir.NewBlockStmt(g.pos(stmt), nil)
 		g.decls(&n.List, stmt.DeclList)
 		return n

test/run.go

@@ -2186,6 +2186,7 @@ var unifiedFailures = setOf(
 	"fixedbugs/issue42284.go", // prints "T(0) does not escape", but test expects "a.I(a.T(0)) does not escape"
 	"fixedbugs/issue7921.go",  // prints "… escapes to heap", but test expects "string(…) escapes to heap"
 	"typeparam/issue48538.go", // assertion failure, interprets struct key as closure variable
+	"typeparam/issue47631.go", // unified IR can handle local type declarations
 )
 
 func setOf(keys ...string) map[string]bool {

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
 

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
+}

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) {