cmd/compile: don't report redundant error for invalid integer literals

Fixes #30722.

Change-Id: Ia4c6e37282edc44788cd8af3f6cfa10895a19e4f
Reviewed-on: https://go-review.googlesource.com/c/go/+/166519Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/mpint.go

@@ -283,15 +283,11 @@ func (a *Mpint) SetInt64(c int64) {
 func (a *Mpint) SetString(as string) {
 	_, ok := a.Val.SetString(as, 0)
 	if !ok {
-		// required syntax is [+-][0[x]]d*
-		// At the moment we lose precise error cause;
-		// the old code distinguished between:
-		// - malformed hex constant
-		// - malformed octal constant
-		// - malformed decimal constant
-		// TODO(gri) use different conversion function
-		yyerror("malformed integer constant: %s", as)
-		a.Val.SetUint64(0)
+		// The lexer checks for correct syntax of the literal
+		// and reports detailed errors. Thus SetString should
+		// never fail (in theory it might run out of memory,
+		// but that wouldn't be reported as an error here).
+		Fatalf("malformed integer constant: %s", as)
 		return
 	}
 	if a.checkOverflow(0) {

test/fixedbugs/issue30722.go

+// errorcheck
+
+// Copyright 2019 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.
+
+// Verify that we only get one error per invalid integer literal.
+
+package p
+
+const (
+	_ = 1_       // ERROR "'_' must separate successive digits"
+	_ = 0b       // ERROR "binary literal has no digits"
+	_ = 0o       // ERROR "octal literal has no digits"
+	_ = 0x       // ERROR "hexadecimal literal has no digits"
+	_ = 0xde__ad // ERROR "'_' must separate successive digits"
+)