go/types: fix incorrect string(int) conversion (regression)

The bug was introduced by https://golang.org/cl/220844.

Fixes #42790.

Change-Id: I44d619a1a4d3f2aee1c5575d5cfddcc4ba10895f
Reviewed-on: https://go-review.googlesource.com/c/go/+/272666
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/go/types/conversions.go

@@ -6,7 +6,10 @@
 
 package types
 
-import "go/constant"
+import (
+	"go/constant"
+	"unicode"
+)
 
 // Conversion type-checks the conversion T(x).
 // The result is in x.
@@ -21,14 +24,11 @@ func (check *Checker) conversion(x *operand, T Type) {
 		case representableConst(x.val, check, t, &x.val):
 			ok = true
 		case isInteger(x.typ) && isString(t):
-			codepoint := int64(-1)
-			if i, ok := constant.Int64Val(x.val); ok {
-				codepoint = i
+			codepoint := unicode.ReplacementChar
+			if i, ok := constant.Uint64Val(x.val); ok && i <= unicode.MaxRune {
+				codepoint = rune(i)
 			}
-			// If codepoint < 0 the absolute value is too large (or unknown) for
-			// conversion. This is the same as converting any other out-of-range
-			// value - let string(codepoint) do the work.
-			x.val = constant.MakeString(string(rune(codepoint)))
+			x.val = constant.MakeString(string(codepoint))
 			ok = true
 		}
 	case x.convertibleTo(check, T):

test/fixedbugs/issue42790.go

@@ -0,0 +1,9 @@
+// compile
+
+// Copyright 2020 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
+
+const _ = -uint(len(string(1<<32)) - len("\uFFFD"))