[dev.typeparams] cmd/compile/internal/syntax: not all index expressions can be instantiated types

An index expression followed by an opening "{" may indicate
a composite literal but only if the index expression can be
a type. Exclude cases where the index expression cannot be
a type (e.g. s[0], a[i+j], etc.).

This leads to a better error message in code that is erroneous.

Fixes #46558.

Change-Id: Ida9291ca30683c211812dfb95abe4969f44c474f
Reviewed-on: https://go-review.googlesource.com/c/go/+/325009
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/syntax/parser.go

@@ -1100,7 +1100,7 @@ loop:
 					complit_ok = true
 				}
 			case *IndexExpr:
-				if p.xnest >= 0 {
+				if p.xnest >= 0 && !isValue(t) {
 					// x is possibly a composite literal type
 					complit_ok = true
 				}
@@ -1127,6 +1127,21 @@ loop:
 	return x
 }
 
+// isValue reports whether x syntactically must be a value (and not a type) expression.
+func isValue(x Expr) bool {
+	switch x := x.(type) {
+	case *BasicLit, *CompositeLit, *FuncLit, *SliceExpr, *AssertExpr, *TypeSwitchGuard, *CallExpr:
+		return true
+	case *Operation:
+		return x.Op != Mul || x.Y != nil // *T may be a type
+	case *ParenExpr:
+		return isValue(x.X)
+	case *IndexExpr:
+		return isValue(x.X) || isValue(x.Index)
+	}
+	return false
+}
+
 // Element = Expression | LiteralValue .
 func (p *parser) bare_complitexpr() Expr {
 	if trace {

src/cmd/compile/internal/syntax/testdata/issue46558.src

@@ -0,0 +1,14 @@
+// Copyright 2021 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(s string) {
+	switch s[0] {
+	case 'a':
+		case s[2] { // ERROR unexpected {
+		case 'b':
+		}
+	}
+} // ERROR non-declaration statement