go/types: don't accept incorrect shift expression arguments

Under certain circumstances involving shifts, go/types didn't verify
that untyped constant values were representable by the relevant type,
leading to the acceptance of incorrect programs (see the issue).

Fixing this code exposed another problem with int-to-string conversions
which suddenly failed because now the type-checker complained that a
(constant) integer argument wasn't representable as a string. Fixed that
as well.

Added many additional tests covering the various scenarious.

Found two cmd/compile bugs in the process (#21979, #21981) and filed
a go/types TODO (#21982).

Fixes #21727.

Change-Id: If443ee0230979cd7d45d2fc669e623648caa70da
Reviewed-on: https://go-review.googlesource.com/65370
Reviewed-by: Alan Donovan <adonovan@google.com>

1 files changed, 14 insertions, 4 deletions

diff --git a/src/go/types/api_test.go b/src/go/types/api_test.go
index ab08a2669d..4f54f684b8 100644
--- a/src/go/types/api_test.go
+++ b/src/go/types/api_test.go
@@ -87,6 +87,10 @@ func TestValuesInfo(t *testing.T) {
 		{`package c5a; var _ = string("foo")`, `"foo"`, `string`, `"foo"`},
 		{`package c5b; var _ = string("foo")`, `string("foo")`, `string`, `"foo"`},
 		{`package c5c; type T string; var _ = T("foo")`, `T("foo")`, `c5c.T`, `"foo"`},
+		{`package c5d; var _ = string(65)`, `65`, `untyped int`, `65`},
+		{`package c5e; var _ = string('A')`, `'A'`, `untyped rune`, `65`},
+		{`package c5f; type T string; var _ = T('A')`, `'A'`, `untyped rune`, `65`},
+		{`package c5g; var s uint; var _ = string(1 << s)`, `1 << s`, `untyped int`, ``},
 
 		{`package d0; var _ = []byte("foo")`, `"foo"`, `string`, `"foo"`},
 		{`package d1; var _ = []byte(string("foo"))`, `"foo"`, `string`, `"foo"`},
@@ -122,7 +126,7 @@ func TestValuesInfo(t *testing.T) {
 		}
 		name := mustTypecheck(t, "ValuesInfo", test.src, &info)
 
-		// look for constant expression
+		// look for expression
 		var expr ast.Expr
 		for e := range info.Types {
 			if ExprString(e) == test.expr {
@@ -142,9 +146,15 @@ func TestValuesInfo(t *testing.T) {
 			continue
 		}
 
-		// check that value is correct
-		if got := tv.Value.ExactString(); got != test.val {
-			t.Errorf("package %s: got value %s; want %s", name, got, test.val)
+		// if we have a constant, check that value is correct
+		if tv.Value != nil {
+			if got := tv.Value.ExactString(); got != test.val {
+				t.Errorf("package %s: got value %s; want %s", name, got, test.val)
+			}
+		} else {
+			if test.val != "" {
+				t.Errorf("package %s: no constant found; want %s", name, test.val)
+			}
 		}
 	}
 }