go/types, types2: re-enable panic if unification stack-overflows

With all the unification/type-inference fixes in place now, we
should not see stack overflows anymore. Re-enable the panic if
we do overflow, so we can address those issues should they arise.

Fixes #51377.

Change-Id: Ied64435ea5936811504cb30bda1126c7d85980f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/392755
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -39,9 +39,10 @@ const (
 	// due to implementation issues (e.g., see issues #48619, #48656).
 	unificationDepthLimit = 50
 
-	// Whether to panic when unificationDepthLimit is reached. Turn on when
-	// investigating infinite recursion.
-	panicAtUnificationDepthLimit = false
+	// Whether to panic when unificationDepthLimit is reached.
+	// If disabled, a recursion depth overflow results in a (quiet)
+	// unification failure.
+	panicAtUnificationDepthLimit = true
 
 	// If enableCoreTypeUnification is set, unification will consider
 	// the core types, if any, of non-local (unbound) type parameters.

src/go/types/unify.go

@@ -39,9 +39,10 @@ const (
 	// due to implementation issues (e.g., see issues #48619, #48656).
 	unificationDepthLimit = 50
 
-	// Whether to panic when unificationDepthLimit is reached. Turn on when
-	// investigating infinite recursion.
-	panicAtUnificationDepthLimit = false
+	// Whether to panic when unificationDepthLimit is reached.
+	// If disabled, a recursion depth overflow results in a (quiet)
+	// unification failure.
+	panicAtUnificationDepthLimit = true
 
 	// If enableCoreTypeUnification is set, unification will consider
 	// the core types, if any, of non-local (unbound) type parameters.