diff --git a/src/go/types/expr.go b/src/go/types/expr.go
index 11f94112846..b026e99ce28 100644
--- a/src/go/types/expr.go
+++ b/src/go/types/expr.go
@@ -1073,7 +1073,11 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 	case *ast.BasicLit:
 		x.setConst(e.Kind, e.Value)
 		if x.mode == invalid {
-			check.invalidAST(e, "invalid literal %v", e.Value)
+			// The parser already establishes syntactic correctness.
+			// If we reach here it's because of number under-/overflow.
+			// TODO(gri) setConst (and in turn the go/constant package)
+			// should return an error describing the issue.
+			check.errorf(e, _InvalidConstVal, "malformed constant: %s", e.Value)
 			goto Error
 		}
 
diff --git a/src/go/types/fixedbugs/issue42695.src b/src/go/types/fixedbugs/issue42695.src
new file mode 100644
index 00000000000..d0d62009696
--- /dev/null
+++ b/src/go/types/fixedbugs/issue42695.src
@@ -0,0 +1,17 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package issue42695
+
+const _ = 6e5518446744 // ERROR malformed constant
+const _ uint8 = 6e5518446744 // ERROR malformed constant
+
+var _ = 6e5518446744 // ERROR malformed constant
+var _ uint8 = 6e5518446744 // ERROR malformed constant
+
+func f(x int) int {
+        return x + 6e5518446744 // ERROR malformed constant
+}
+
+var _ = f(6e5518446744 /* ERROR malformed constant */ )
diff --git a/src/go/types/operand.go b/src/go/types/operand.go
index 2d30dbd024c..3e1ac312d9a 100644
--- a/src/go/types/operand.go
+++ b/src/go/types/operand.go
@@ -195,9 +195,15 @@ func (x *operand) setConst(tok token.Token, lit string) {
 		unreachable()
 	}
 
+	val := constant.MakeFromLiteral(lit, tok, 0)
+	if val.Kind() == constant.Unknown {
+		x.mode = invalid
+		x.typ = Typ[Invalid]
+		return
+	}
 	x.mode = constant_
 	x.typ = Typ[kind]
-	x.val = constant.MakeFromLiteral(lit, tok, 0)
+	x.val = val
 }
 
 // isNil reports whether x is the nil value.