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 <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Robert Findley <[email protected]>

Author: findleyr

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.

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.

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" */