go/types, types2: ensure that signature type bounds are interfaces

Do this by running verification for instantiated signatures
later, after the delayed type parameter set-up had a chance
to wrap type bounds in implicit interfaces where needed.

Fixes #50450

Change-Id: If3ff7dc0be6af14af854830bfddb81112ac575cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/375737
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/call.go

@@ -74,17 +74,21 @@ func (check *Checker) instantiateSignature(pos syntax.Pos, typ *Signature, targs
 
 	inst := check.instance(pos, typ, targs, check.bestContext(nil)).(*Signature)
 	assert(len(xlist) <= len(targs))
-	tparams := typ.TypeParams().list()
-	if i, err := check.verify(pos, tparams, targs); err != nil {
-		// best position for error reporting
-		pos := pos
-		if i < len(xlist) {
-			pos = syntax.StartPos(xlist[i])
+
+	// verify instantiation lazily (was issue #50450)
+	check.later(func() {
+		tparams := typ.TypeParams().list()
+		if i, err := check.verify(pos, tparams, targs); err != nil {
+			// best position for error reporting
+			pos := pos
+			if i < len(xlist) {
+				pos = syntax.StartPos(xlist[i])
+			}
+			check.softErrorf(pos, "%s", err)
+		} else {
+			check.mono.recordInstance(check.pkg, pos, tparams, targs, xlist)
 		}
-		check.softErrorf(pos, "%s", err)
-	} else {
-		check.mono.recordInstance(check.pkg, pos, tparams, targs, xlist)
-	}
+	})
 
 	return inst
 }

src/cmd/compile/internal/types2/testdata/fixedbugs/issue50450.go2

@@ -0,0 +1,11 @@
+// Copyright 2022 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 p
+
+type S struct{}
+
+func f[P S]() {}
+
+var _ = f[S]

src/go/types/call.go

@@ -75,17 +75,21 @@ func (check *Checker) instantiateSignature(pos token.Pos, typ *Signature, targs
 
 	inst := check.instance(pos, typ, targs, check.bestContext(nil)).(*Signature)
 	assert(len(xlist) <= len(targs))
-	tparams := typ.TypeParams().list()
-	if i, err := check.verify(pos, tparams, targs); err != nil {
-		// best position for error reporting
-		pos := pos
-		if i < len(xlist) {
-			pos = xlist[i].Pos()
+
+	// verify instantiation lazily (was issue #50450)
+	check.later(func() {
+		tparams := typ.TypeParams().list()
+		if i, err := check.verify(pos, tparams, targs); err != nil {
+			// best position for error reporting
+			pos := pos
+			if i < len(xlist) {
+				pos = xlist[i].Pos()
+			}
+			check.softErrorf(atPos(pos), _InvalidTypeArg, "%s", err)
+		} else {
+			check.mono.recordInstance(check.pkg, pos, tparams, targs, xlist)
 		}
-		check.softErrorf(atPos(pos), _InvalidTypeArg, "%s", err)
-	} else {
-		check.mono.recordInstance(check.pkg, pos, tparams, targs, xlist)
-	}
+	})
 
 	return inst
 }

src/go/types/testdata/fixedbugs/issue50450.go2

@@ -0,0 +1,11 @@
+// Copyright 2022 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 p
+
+type S struct{}
+
+func f[P S]() {}
+
+var _ = f[S]