From 35ec948de766e7e8854b95edda3a0bb4723a63ec Mon Sep 17 00:00:00 2001
From: Rob Findley <rfindley@google.com>
Date: Fri, 14 Apr 2023 14:25:53 -0400
Subject: [PATCH] go/types,types2: fix panic in reverse type inference when
 -lang<go1.18

Due to reverse type inference, we may not have an index expression when
type-checking a function instantiation. Fix a panic when the index expr
is nil.

Fixes #59639

Change-Id: Ib5de5e49cdb7b339653e4fb775bf5c5fdb3c6907
Reviewed-on: https://go-review.googlesource.com/c/go/+/484757
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
---
 src/cmd/compile/internal/types2/call.go             |  8 +++++++-
 src/go/types/call.go                                |  8 +++++++-
 src/internal/types/testdata/fixedbugs/issue59639.go | 11 +++++++++++
 3 files changed, 25 insertions(+), 2 deletions(-)
 create mode 100644 src/internal/types/testdata/fixedbugs/issue59639.go

diff --git a/src/cmd/compile/internal/types2/call.go b/src/cmd/compile/internal/types2/call.go
index 08c90b9f8f..bf561f2c87 100644
--- a/src/cmd/compile/internal/types2/call.go
+++ b/src/cmd/compile/internal/types2/call.go
@@ -24,7 +24,13 @@ func (check *Checker) funcInst(tsig *Signature, pos syntax.Pos, x *operand, inst
 	assert(tsig != nil || inst != nil)
 
 	if !check.allowVersion(check.pkg, pos, 1, 18) {
-		check.versionErrorf(inst.Pos(), "go1.18", "function instantiation")
+		var posn poser
+		if inst != nil {
+			posn = inst.Pos()
+		} else {
+			posn = pos
+		}
+		check.versionErrorf(posn, "go1.18", "function instantiation")
 	}
 
 	// targs and xlist are the type arguments and corresponding type expressions, or nil.
diff --git a/src/go/types/call.go b/src/go/types/call.go
index f220efb240..854ce7e406 100644
--- a/src/go/types/call.go
+++ b/src/go/types/call.go
@@ -26,7 +26,13 @@ func (check *Checker) funcInst(tsig *Signature, pos token.Pos, x *operand, ix *t
 	assert(tsig != nil || ix != nil)
 
 	if !check.allowVersion(check.pkg, pos, 1, 18) {
-		check.softErrorf(inNode(ix.Orig, ix.Lbrack), UnsupportedFeature, "function instantiation requires go1.18 or later")
+		var posn positioner
+		if ix != nil {
+			posn = inNode(ix.Orig, ix.Lbrack)
+		} else {
+			posn = atPos(pos)
+		}
+		check.softErrorf(posn, UnsupportedFeature, "function instantiation requires go1.18 or later")
 	}
 
 	// targs and xlist are the type arguments and corresponding type expressions, or nil.
diff --git a/src/internal/types/testdata/fixedbugs/issue59639.go b/src/internal/types/testdata/fixedbugs/issue59639.go
new file mode 100644
index 0000000000..c82d5b10fa
--- /dev/null
+++ b/src/internal/types/testdata/fixedbugs/issue59639.go
@@ -0,0 +1,11 @@
+// -reverseTypeInference -lang=go1.17
+
+// Copyright 2023 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
+
+func f[P /* ERROR "requires go1.18" */ interface{}](P) {}
+
+var v func(int) = f /* ERROR "requires go1.18" */
-- 
2.48.1