[dev.typeparams] cmd/compile/internal/types2: cleanup panic calls

End-users are not expected to deal with the details of panics,
so providing extra information such as an "internal error" prefix
or the name of the function invoking the panic are not helpful.

Remove unnecessary panic verbiage if it is readily available from
a stack trace (such as the function where it happens, and the fact
that is is an "internal error").

Change-Id: I5f86bae6d2cca7c04ce692d17257da7ddee206d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/339969
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

11 files changed, 24 insertions, 24 deletions

diff --git a/src/cmd/compile/internal/types2/decl.go b/src/cmd/compile/internal/types2/decl.go
index cf4d4c95a7..bb33c287f3 100644
--- a/src/cmd/compile/internal/types2/decl.go
+++ b/src/cmd/compile/internal/types2/decl.go
@@ -340,7 +340,7 @@ 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:])
@@ -348,7 +348,7 @@ 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
 	}
diff --git a/src/cmd/compile/internal/types2/errors.go b/src/cmd/compile/internal/types2/errors.go
index 8c5e185f6c..a68273271b 100644
--- a/src/cmd/compile/internal/types2/errors.go
+++ b/src/cmd/compile/internal/types2/errors.go
@@ -88,7 +88,7 @@ func sprintf(qf Qualifier, 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, qf)
 		case syntax.Pos:
@@ -148,7 +148,7 @@ func (check *Checker) sprintf(format string, args ...interface{}) string {
 
 func (check *Checker) report(err *error_) {
 	if err.empty() {
-		panic("internal error: reporting no error")
+		panic("no error to report")
 	}
 	check.err(err.pos(), err.msg(check.qualifier), err.soft)
 }
diff --git a/src/cmd/compile/internal/types2/lookup.go b/src/cmd/compile/internal/types2/lookup.go
index f62c3771d2..0363008ad9 100644
--- a/src/cmd/compile/internal/types2/lookup.go
+++ b/src/cmd/compile/internal/types2/lookup.go
@@ -322,7 +322,7 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 				return m, f
 			}
 			if !acceptMethodTypeParams && 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
@@ -374,7 +374,7 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 			return m, f
 		}
 		if !acceptMethodTypeParams && 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
diff --git a/src/cmd/compile/internal/types2/named.go b/src/cmd/compile/internal/types2/named.go
index 8ded197df5..14e073bfae 100644
--- a/src/cmd/compile/internal/types2/named.go
+++ b/src/cmd/compile/internal/types2/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("types2.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
 		})
@@ -140,10 +140,10 @@ 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("types2.Named.SetUnderlying: underlying type must not be nil")
+		panic("underlying type must not be nil")
 	}
 	if _, ok := underlying.(*Named); ok {
-		panic("types2.Named.SetUnderlying: underlying type must not be *Named")
+		panic("underlying type must not be *Named")
 	}
 	t.load().underlying = underlying
 }
@@ -191,7 +191,7 @@ 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
@@ -242,7 +242,7 @@ 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
 	}
diff --git a/src/cmd/compile/internal/types2/signature.go b/src/cmd/compile/internal/types2/signature.go
index 832f37a6af..14112462e1 100644
--- a/src/cmd/compile/internal/types2/signature.go
+++ b/src/cmd/compile/internal/types2/signature.go
@@ -36,10 +36,10 @@ func NewSignature(recv *Var, params, results *Tuple, variadic bool) *Signature {
 	if variadic {
 		n := params.Len()
 		if n == 0 {
-			panic("types2.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("types2.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}
diff --git a/src/cmd/compile/internal/types2/stmt.go b/src/cmd/compile/internal/types2/stmt.go
index 1efce511f1..ad8efa91f8 100644
--- a/src/cmd/compile/internal/types2/stmt.go
+++ b/src/cmd/compile/internal/types2/stmt.go
@@ -14,7 +14,7 @@ import (
 
 func (check *Checker) funcBody(decl *declInfo, name string, sig *Signature, body *syntax.BlockStmt, iota constant.Value) {
 	if check.conf.IgnoreFuncBodies {
-		panic("internal error: function body not ignored")
+		panic("function body not ignored")
 	}
 
 	if check.conf.Trace {
diff --git a/src/cmd/compile/internal/types2/typeparam.go b/src/cmd/compile/internal/types2/typeparam.go
index 27e6e35588..12513ed6dd 100644
--- a/src/cmd/compile/internal/types2/typeparam.go
+++ b/src/cmd/compile/internal/types2/typeparam.go
@@ -75,7 +75,7 @@ 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
 }
@@ -118,7 +118,7 @@ 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
 	}
diff --git a/src/cmd/compile/internal/types2/typeset.go b/src/cmd/compile/internal/types2/typeset.go
index 6e19115ff5..c5fcb97ff9 100644
--- a/src/cmd/compile/internal/types2/typeset.go
+++ b/src/cmd/compile/internal/types2/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")
 	}
 }
 
diff --git a/src/cmd/compile/internal/types2/typestring.go b/src/cmd/compile/internal/types2/typestring.go
index 628eeaf3dd..b3675424a5 100644
--- a/src/cmd/compile/internal/types2/typestring.go
+++ b/src/cmd/compile/internal/types2/typestring.go
@@ -130,7 +130,7 @@ 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 {
@@ -183,7 +183,7 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 		case RecvOnly:
 			s = "<-chan "
 		default:
-			panic("unreachable")
+			unreachable()
 		}
 		buf.WriteString(s)
 		if parens {
@@ -329,7 +329,7 @@ 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("...")
diff --git a/src/cmd/compile/internal/types2/union.go b/src/cmd/compile/internal/types2/union.go
index fcd83ce688..0325c72dbb 100644
--- a/src/cmd/compile/internal/types2/union.go
+++ b/src/cmd/compile/internal/types2/union.go
@@ -129,7 +129,7 @@ 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) {
diff --git a/src/cmd/compile/internal/types2/universe.go b/src/cmd/compile/internal/types2/universe.go
index 7b6c297d05..55bf0982b3 100644
--- a/src/cmd/compile/internal/types2/universe.go
+++ b/src/cmd/compile/internal/types2/universe.go
@@ -258,6 +258,6 @@ func def(obj Object) {
 		}
 	}
 	if scope.Insert(obj) != nil {
-		panic("internal error: double declaration")
+		panic("double declaration of predeclared identifier")
 	}
 }