go/types: cleanup panic calls

This is a port of CL 339969 to go/types. It differs slightly in
errors.go, due to the differing API.

Change-Id: Ie2bf84ebf312ea3872ee6706615dfc6169a32405
Reviewed-on: https://go-review.googlesource.com/c/go/+/342431
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/decl.go

@@ -339,7 +339,7 @@ func (check *Checker) validType(typ Type, path []Object) typeInfo {
 			// cycle detected
 			for i, tn := range path {
 				if t.obj.pkg != check.pkg {
-					panic("internal error: type cycle via package-external type")
+					panic("type cycle via package-external type")
 				}
 				if tn == t.obj {
 					check.cycleError(path[i:])
@@ -347,7 +347,7 @@ func (check *Checker) validType(typ Type, path []Object) typeInfo {
 					return t.info
 				}
 			}
-			panic("internal error: cycle start not found")
+			panic("cycle start not found")
 		}
 		return t.info
 	}

src/go/types/errors.go

@@ -68,7 +68,7 @@ func (check *Checker) sprintf(format string, args ...interface{}) string {
 		case nil:
 			arg = "<nil>"
 		case operand:
-			panic("internal error: should always pass *operand")
+			panic("got operand instead of *operand")
 		case *operand:
 			arg = operandString(a, check.qualifier)
 		case token.Pos:
@@ -236,7 +236,7 @@ func (s atPos) Pos() token.Pos {
 func spanOf(at positioner) posSpan {
 	switch x := at.(type) {
 	case nil:
-		panic("internal error: nil")
+		panic("nil positioner")
 	case posSpan:
 		return x
 	case ast.Node:

src/go/types/lookup.go

@@ -320,7 +320,7 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 				return m, f
 			}
 			if ftyp.TParams().Len() > 0 {
-				panic("internal error: method with type parameters")
+				panic("method with type parameters")
 			}
 
 			// If the methods have type parameters we don't care whether they
@@ -372,7 +372,7 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 			return m, f
 		}
 		if ftyp.TParams().Len() > 0 {
-			panic("internal error: method with type parameters")
+			panic("method with type parameters")
 		}
 
 		// If V is a (instantiated) generic type, its methods are still

src/go/types/named.go

@@ -33,7 +33,7 @@ type Named struct {
 // The underlying type must not be a *Named.
 func NewNamed(obj *TypeName, underlying Type, methods []*Func) *Named {
 	if _, ok := underlying.(*Named); ok {
-		panic("types.NewNamed: underlying type must not be *Named")
+		panic("underlying type must not be *Named")
 	}
 	return (*Checker)(nil).newNamed(obj, nil, underlying, nil, methods)
 }
@@ -100,7 +100,7 @@ func (check *Checker) newNamed(obj *TypeName, orig *Named, underlying Type, tpar
 		check.later(func() {
 			switch typ.under().(type) {
 			case *Named:
-				panic("internal error: unexpanded underlying type")
+				panic("unexpanded underlying type")
 			}
 			typ.check = nil
 		})
@@ -144,10 +144,10 @@ func (t *Named) Method(i int) *Func { return t.load().methods[i] }
 // SetUnderlying sets the underlying type and marks t as complete.
 func (t *Named) SetUnderlying(underlying Type) {
 	if underlying == nil {
-		panic("types.Named.SetUnderlying: underlying type must not be nil")
+		panic("underlying type must not be nil")
 	}
 	if _, ok := underlying.(*Named); ok {
-		panic("types.Named.SetUnderlying: underlying type must not be *Named")
+		panic("underlying type must not be *Named")
 	}
 	t.load().underlying = underlying
 }
@@ -195,7 +195,7 @@ func (n0 *Named) under() Type {
 	}
 
 	if n0.check == nil {
-		panic("internal error: Named.check == nil but type is incomplete")
+		panic("Named.check == nil but type is incomplete")
 	}
 
 	// Invariant: after this point n0 as well as any named types in its
@@ -246,7 +246,7 @@ func (n0 *Named) under() Type {
 		// Also, doing so would lead to a race condition (was issue #31749).
 		// Do this check always, not just in debug mode (it's cheap).
 		if n.obj.pkg != check.pkg {
-			panic("internal error: imported type with unresolved underlying type")
+			panic("imported type with unresolved underlying type")
 		}
 		n.underlying = u
 	}

src/go/types/signature.go

@@ -38,10 +38,10 @@ func NewSignature(recv *Var, params, results *Tuple, variadic bool) *Signature {
 	if variadic {
 		n := params.Len()
 		if n == 0 {
-			panic("types.NewSignature: variadic function must have at least one parameter")
+			panic("variadic function must have at least one parameter")
 		}
 		if _, ok := params.At(n - 1).typ.(*Slice); !ok {
-			panic("types.NewSignature: variadic parameter must be of unnamed slice type")
+			panic("variadic parameter must be of unnamed slice type")
 		}
 	}
 	return &Signature{recv: recv, params: params, results: results, variadic: variadic}

src/go/types/stmt.go

@@ -15,7 +15,7 @@ import (
 
 func (check *Checker) funcBody(decl *declInfo, name string, sig *Signature, body *ast.BlockStmt, iota constant.Value) {
 	if check.conf.IgnoreFuncBodies {
-		panic("internal error: function body not ignored")
+		panic("function body not ignored")
 	}
 
 	if trace {

src/go/types/typeparam.go

@@ -77,7 +77,7 @@ func (t *TypeParam) Constraint() Type {
 // SetConstraint sets the type constraint for t.
 func (t *TypeParam) SetConstraint(bound Type) {
 	if bound == nil {
-		panic("types2.TypeParam.SetConstraint: bound must not be nil")
+		panic("nil constraint")
 	}
 	t.bound = bound
 }
@@ -113,7 +113,7 @@ func bindTParams(list []*TypeName) *TypeParams {
 	for i, tp := range list {
 		typ := tp.Type().(*TypeParam)
 		if typ.index >= 0 {
-			panic("internal error: type parameter bound more than once")
+			panic("type parameter bound more than once")
 		}
 		typ.index = i
 	}

src/go/types/typeset.go

@@ -323,10 +323,10 @@ func sortMethods(list []*Func) {
 
 func assertSortedMethods(list []*Func) {
 	if !debug {
-		panic("internal error: assertSortedMethods called outside debug mode")
+		panic("assertSortedMethods called outside debug mode")
 	}
 	if !sort.IsSorted(byUniqueMethodName(list)) {
-		panic("internal error: methods not sorted")
+		panic("methods not sorted")
 	}
 }
 

src/go/types/typestring.go

@@ -132,7 +132,7 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 		// Unions only appear as (syntactic) embedded elements
 		// in interfaces and syntactically cannot be empty.
 		if t.NumTerms() == 0 {
-			panic("internal error: empty union")
+			panic("empty union")
 		}
 		for i, t := range t.terms {
 			if i > 0 {
@@ -185,7 +185,7 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 		case RecvOnly:
 			s = "<-chan "
 		default:
-			panic("unreachable")
+			unreachable()
 		}
 		buf.WriteString(s)
 		if parens {
@@ -332,7 +332,7 @@ func writeTuple(buf *bytes.Buffer, tup *Tuple, variadic bool, qf Qualifier, visi
 					// special case:
 					// append(s, "foo"...) leads to signature func([]byte, string...)
 					if t := asBasic(typ); t == nil || t.kind != String {
-						panic("internal error: string type expected")
+						panic("expected string type")
 					}
 					writeType(buf, typ, qf, visited)
 					buf.WriteString("...")

src/go/types/union.go

@@ -133,7 +133,7 @@ func overlappingTerm(terms []*term, y *term) int {
 		// disjoint requires non-nil, non-top arguments
 		if debug {
 			if x == nil || x.typ == nil || y == nil || y.typ == nil {
-				panic("internal error: empty or top union term")
+				panic("empty or top union term")
 			}
 		}
 		if !x.disjoint(y) {

src/go/types/universe.go

@@ -259,6 +259,6 @@ func def(obj Object) {
 		}
 	}
 	if scope.Insert(obj) != nil {
-		panic("internal error: double declaration")
+		panic("double declaration of predeclared identifier")
 	}
 }