[dev.typeparams] all: merge master (46fd547) into dev.typeparams

Conflicts:

- src/go/types/check_test.go

  CL 324730 on dev.typeparams changed the directory paths in TestCheck,
  TestExamples, and TestFixedbugs and renamed checkFiles to testFiles;
  whereas CL 337529 on master added a new test case just above them and
  that used checkFiles.

Merge List:

+ 2021-08-12 46fd547d89 internal/goversion: update Version to 1.18
+ 2021-08-12 5805efc78e doc/go1.17: remove draft notice
+ 2021-08-12 39634e7dae CONTRIBUTORS: update for the Go 1.17 release
+ 2021-08-12 095bb790e1 os/exec: re-enable LookPathTest/16
+ 2021-08-11 dea23e9ca8 src/make.*: make --no-clean flag a no-op that prints a warning
+ 2021-08-11 d4c0ed26ac doc/go1.17: linker passes -I to extld as -Wl,--dynamic-linker
+ 2021-08-10 1f9c9d8530 doc: use "high address/low address" instead of "top/bottom"
+ 2021-08-09 f1dce319ff cmd/go: with -mod=vendor, don't panic if there are duplicate requirements
+ 2021-08-09 7aeaad5c86 runtime/cgo: when using msan explicitly unpoison cgoCallers
+ 2021-08-08 507cc341ec doc: add example for conversion from slice expressions to array ptr
+ 2021-08-07 891547e2d4 doc/go1.17: fix a typo introduced in CL 335135
+ 2021-08-06 8eaf4d16bc make.bash: do not overwrite GO_LDSO if already set
+ 2021-08-06 63b968f4f8 doc/go1.17: clarify Modules changes
+ 2021-08-06 70546f6404 runtime: allow arm64 SEH to be called if illegal instruction
+ 2021-08-05 fd45e267c2 runtime: warn that KeepAlive is not an unsafe.Pointer workaround
+ 2021-08-04 6e738868a7 net/http: speed up and deflake TestCancelRequestWhenSharingConnection
+ 2021-08-02 8a7ee4c51e io/fs: don't use absolute path in DirEntry.Name doc
+ 2021-07-31 b8ca6e59ed all: gofmt
+ 2021-07-30 b7a85e0003 net/http/httputil: close incoming ReverseProxy request body
+ 2021-07-29 70fd4e47d7 runtime: avoid possible preemption when returning from Go to C
+ 2021-07-28 9eee0ed439 cmd/go: fix go.mod file name printed in error messages for replacements
+ 2021-07-28 b39e0f461c runtime: don't crash on nil pointers in checkptrAlignment
+ 2021-07-27 7cd10c1149 cmd/go: use .mod instead of .zip to determine if version has go.mod file
+ 2021-07-27 c8cf0f74e4 cmd/go: add missing flag in UsageLine
+ 2021-07-27 7ba8e796c9 testing: clarify T.Name returns a distinct name of the running test
+ 2021-07-27 33ff155970 go/types: preserve untyped constants on the RHS of a shift expression
+ 2021-07-26 840e583ff3 runtime: correct variable name in comment
+ 2021-07-26 bfbb288574 runtime: remove adjustTimers counter
+ 2021-07-26 9c81fd53b3 cmd/vet: add missing copyright header

Change-Id: Ia80604d24c6f4205265683024e3100769cf32065

1 files changed, 25 insertions, 9 deletions

diff --git a/src/go/types/expr.go b/src/go/types/expr.go
index b55f51185f..c9a55aa871 100644
--- a/src/go/types/expr.go
+++ b/src/go/types/expr.go
@@ -795,32 +795,48 @@ func (check *Checker) shift(x, y *operand, e ast.Expr, op token.Token) {
 	// spec: "The right operand in a shift expression must have integer type
 	// or be an untyped constant representable by a value of type uint."
 
-	// Provide a good error message for negative shift counts.
+	// Check that constants are representable by uint, but do not convert them
+	// (see also issue #47243).
 	if y.mode == constant_ {
+		// Provide a good error message for negative shift counts.
 		yval := constant.ToInt(y.val) // consider -1, 1.0, but not -1.1
 		if yval.Kind() == constant.Int && constant.Sign(yval) < 0 {
 			check.invalidOp(y, _InvalidShiftCount, "negative shift count %s", y)
 			x.mode = invalid
 			return
 		}
+
+		if isUntyped(y.typ) {
+			// Caution: Check for representability here, rather than in the switch
+			// below, because isInteger includes untyped integers (was bug #43697).
+			check.representable(y, Typ[Uint])
+			if y.mode == invalid {
+				x.mode = invalid
+				return
+			}
+		}
 	}
 
-	// Caution: Check for isUntyped first because isInteger includes untyped
-	//          integers (was bug #43697).
-	if isUntyped(y.typ) {
+	// Check that RHS is otherwise at least of integer type.
+	switch {
+	case isInteger(y.typ):
+		if !isUnsigned(y.typ) && !check.allowVersion(check.pkg, 1, 13) {
+			check.invalidOp(y, _InvalidShiftCount, "signed shift count %s requires go1.13 or later", y)
+			x.mode = invalid
+			return
+		}
+	case isUntyped(y.typ):
+		// This is incorrect, but preserves pre-existing behavior.
+		// See also bug #47410.
 		check.convertUntyped(y, Typ[Uint])
 		if y.mode == invalid {
 			x.mode = invalid
 			return
 		}
-	} else if !isInteger(y.typ) {
+	default:
 		check.invalidOp(y, _InvalidShiftCount, "shift count %s must be integer", y)
 		x.mode = invalid
 		return
-	} else if !isUnsigned(y.typ) && !check.allowVersion(check.pkg, 1, 13) {
-		check.invalidOp(y, _InvalidShiftCount, "signed shift count %s requires go1.13 or later", y)
-		x.mode = invalid
-		return
 	}
 
 	if x.mode == constant_ {