go/types: check for non-negative index in tparamIndex

There are code paths (particularly error formatting or tracing) that
call tparamIndex before the type parameter is bound. We cannot rely on
the index being non-negative.

Change-Id: Ibad91c691b4f319b0c7b465a750b679a8a7af6a1
Reviewed-on: https://go-review.googlesource.com/c/go/+/364715
Trust: Robert Findley <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: findleyr

src/cmd/compile/internal/types2/unify.go

@@ -9,7 +9,6 @@ package types2
 import (
 	"bytes"
 	"fmt"
-	"internal/buildcfg"
 )
 
 // The unifier maintains two separate sets of type parameters x and y
@@ -162,13 +161,13 @@ func (d *tparamsList) index(typ Type) int {
 // If tpar is a type parameter in list, tparamIndex returns the type parameter index.
 // Otherwise, the result is < 0. tpar must not be nil.
 func tparamIndex(list []*TypeParam, tpar *TypeParam) int {
-	// Temporary work-around for getting around a crash
-	// with unified build.
-	// TODO(gri) investigate and implement proper fix
-	if buildcfg.Experiment.Unified && tpar.index < 0 {
-		return -1
-	}
-	if i := tpar.index; i < len(list) && list[i] == tpar {
+	// Once a type parameter is bound its index is >= 0. However, there are some
+	// code paths (namely tracing and type hashing) by which it is possible to
+	// arrive here with a type parameter that has not been bound, hence the check
+	// for 0 <= i below.
+	// TODO(rfindley): investigate a better approach for guarding against using
+	// unbound type parameters.
+	if i := tpar.index; 0 <= i && i < len(list) && list[i] == tpar {
 		return i
 	}
 	return -1

src/go/types/unify.go

@@ -159,9 +159,15 @@ func (d *tparamsList) index(typ Type) int {
 }
 
 // If tpar is a type parameter in list, tparamIndex returns the type parameter index.
-// Otherwise, the result is < 0. tpar must not be nil.
+// Otherwise, the result is < 0. tpar must not be nil.j
 func tparamIndex(list []*TypeParam, tpar *TypeParam) int {
-	if i := tpar.index; i < len(list) && list[i] == tpar {
+	// Once a type parameter is bound its index is >= 0. However, there are some
+	// code paths (namely tracing and type hashing) by which it is possible to
+	// arrive here with a type parameter that has not been bound, hence the check
+	// for 0 <= i below.
+	// TODO(rfindley): investigate a better approach for guarding against using
+	// unbound type parameters.
+	if i := tpar.index; 0 <= i && i < len(list) && list[i] == tpar {
 		return i
 	}
 	return -1