go/types: fix regression with short variable declarations

The variables on the lhs of a short variable declaration are
only in scope after the variable declaration. Specifically,
function literals on the rhs of a short variable declaration
must not see newly declared variables on the lhs.

This used to work and this bug was likely introduced with
https://go-review.googlesource.com/c/go/+/83397 for go1.11.
Luckily this is just an oversight and the fix is trivial:
Simply use the mechanism for delayed type-checkin of function
literals introduced in the before-mentioned change here as well.

Fixes #24026.

Change-Id: I74ce3a0d05c5a2a42ce4b27601645964f906e82d
Reviewed-on: https://go-review.googlesource.com/96177Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/assignments.go

@@ -279,6 +279,7 @@ func (check *Checker) assignVars(lhs, rhs []ast.Expr) {
 }
 
 func (check *Checker) shortVarDecl(pos token.Pos, lhs, rhs []ast.Expr) {
+	top := len(check.delayed)
 	scope := check.scope
 
 	// collect lhs variables
@@ -319,6 +320,9 @@ func (check *Checker) shortVarDecl(pos token.Pos, lhs, rhs []ast.Expr) {
 
 	check.initVars(lhsVars, rhs, token.NoPos)
 
+	// process function literals in rhs expressions before scope changes
+	check.processDelayed(top)
+
 	// declare new variables
 	if len(newVars) > 0 {
 		// spec: "The scope of a constant or variable identifier declared inside

src/go/types/testdata/issues.src

@@ -207,3 +207,25 @@ func issue20358() {
 	_ = T{t}
 	_ = P{f: p}
 }
+
+// Test that we don't declare lhs variables in short variable
+// declarations before we type-check function literals on the
+// rhs.
+func issue24026() {
+	f := func() int { f(0) /* must refer to outer f */; return 0 }
+	_ = f
+
+	_ = func() {
+		f := func() { _ = f() /* must refer to outer f */ }
+		_ = f
+	}
+
+	// b and c must not be visible inside function literal
+	a := 0
+	a, b, c := func() (int, int, int) {
+		return a, b /* ERROR undeclared */ , c /* ERROR undeclared */
+	}()
+	_, _ = b, c
+}
+
+func f(int) {} // for issue24026