cmd/compile/internal/syntax: fix printing of ambiguous constraint literals

Without this change, the type parameter list "[P T | T]" is printed
as "[P T | T,]" in an attempt to avoid an ambiguity. But the type
parameter P cannot syntactically combine with the constraint T | T
and make a new valid expression.

This change introduces a specific combinesWithName predicate that
reports whether a constraint expression can combine with a type
parameter name to form a new valid (value) expression.

Use combinesWithName to accurately determine when a comma is needed.

For #49482.

Change-Id: Id1d17a18f0c9af04495da7b0453e83798f32b04a
Reviewed-on: https://go-review.googlesource.com/c/go/+/404397
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -919,16 +919,38 @@ func (p *printer) printParameterList(list []*Field, tok token) {
 		}
 		p.printNode(unparen(f.Type)) // no need for (extra) parentheses around parameter types
 	}
-	// A type parameter list [P *T] where T is not a type element requires a comma as in [P *T,]
-	// so that it's not parsed as [P*T].
-	if tok == _Type && len(list) == 1 {
-		if t, _ := list[0].Type.(*Operation); t != nil && !isTypeElem(t) {
-			p.print(_Comma)
-		}
+	// A type parameter list [P T] where the name P and the type expression T syntactically
+	// combine to another valid (value) expression requires a trailing comma, as in [P *T,]
+	// (or an enclosing interface as in [P interface(*T)]), so that the type parameter list
+	// is not parsed as an array length [P*T].
+	if tok == _Type && len(list) == 1 && combinesWithName(list[0].Type) {
+		p.print(_Comma)
 	}
 	p.print(close)
 }
 
+// combinesWithName reports whether a name followed by the expression x
+// syntactically combines to another valid (value) expression. For instance
+// using *T for x, "name *T" syntactically appears as the expression x*T.
+// On the other hand, using  P|Q or *P|~Q for x, "name P|Q" or name *P|~Q"
+// cannot be combined into a valid (value) expression.
+func combinesWithName(x Expr) bool {
+	switch x := x.(type) {
+	case *Operation:
+		if x.Y == nil {
+			// name *x.X combines to name*x.X if x.X is not a type element
+			return x.Op == Mul && !isTypeElem(x.X)
+		}
+		// binary expressions
+		return combinesWithName(x.X) && !isTypeElem(x.Y)
+	case *ParenExpr:
+		// name(x) combines but we are making sure at
+		// the call site that x is never parenthesized.
+		panic("unexpected parenthesized expression")
+	}
+	return false
+}
+
 func (p *printer) printStmtList(list []Stmt, braces bool) {
 	for i, x := range list {
 		p.print(x, _Semi)

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

@@ -76,6 +76,8 @@ var stringTests = [][2]string{
 
 	// a type literal in an |-expression indicates a type parameter list (blank after type parameter list and type)
 	dup("package p; type _[P *[]int] struct{}"),
+	dup("package p; type _[P T | T] struct{}"),
+	dup("package p; type _[P T | T | T | T] struct{}"),
 	dup("package p; type _[P *T | T, Q T] struct{}"),
 	dup("package p; type _[P *[]T | T] struct{}"),
 	dup("package p; type _[P *T | T | T | T | ~T] struct{}"),
@@ -84,7 +86,7 @@ var stringTests = [][2]string{
 	dup("package p; type _[P <-chan int] struct{}"),
 	dup("package p; type _[P *T | struct{} | T] struct{}"),
 
-	// a trailing comma always indicates a type parameter list (blank after type parameter list and type)
+	// a trailing comma always indicates a (possibly invalid) type parameter list (blank after type parameter list and type)
 	dup("package p; type _[P *T,] struct{}"),
 	dup("package p; type _[P *T | T,] struct{}"),
 	dup("package p; type _[P *T | <-T | T,] struct{}"),