cmd/compile: use "method T.m already declared" for method redeclaration errors

Compromise between old compiler error "T.m redeclared in this block"
(where the "in this block" is not particularly helpful) and the old
type-checker error "method m already declared for type T ...".
In the case where we have position information for the original
declaration, the error message is "method T.m already declared at
<position>". The new message is both shorter and more precise.

For #55326.

Change-Id: Id4a7f326fe631b11db9e8030eccb417c72d6c7db
Reviewed-on: https://go-review.googlesource.com/c/go/+/435016
Run-TryBot: Robert Griesemer <[email protected]>
Auto-Submit: Robert Griesemer <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: griesemer

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

@@ -675,15 +675,11 @@ func (check *Checker) collectMethods(obj *TypeName) {
 		// to it must be unique."
 		assert(m.name != "_")
 		if alt := mset.insert(m); alt != nil {
-			var err error_
-			err.code = _DuplicateMethod
-			if check.conf.CompilerErrorMessages {
-				err.errorf(m.pos, "%s.%s redeclared in this block", obj.Name(), m.name)
+			if alt.Pos().IsKnown() {
+				check.errorf(m.pos, _DuplicateMethod, "method %s.%s already declared at %s", obj.Name(), m.name, alt.Pos())
 			} else {
-				err.errorf(m.pos, "method %s already declared for %s", m.name, obj)
+				check.errorf(m.pos, _DuplicateMethod, "method %s.%s already declared", obj.Name(), m.name)
 			}
-			err.recordAltDecl(alt)
-			check.report(&err)
 			continue
 		}
 

src/go/types/decl.go

@@ -749,8 +749,11 @@ func (check *Checker) collectMethods(obj *TypeName) {
 		// to it must be unique."
 		assert(m.name != "_")
 		if alt := mset.insert(m); alt != nil {
-			check.errorf(m, _DuplicateMethod, "method %s already declared for %s", m.name, obj)
-			check.reportAltDecl(alt)
+			if alt.Pos().IsValid() {
+				check.errorf(m, _DuplicateMethod, "method %s.%s already declared at %s", obj.Name(), m.name, alt.Pos())
+			} else {
+				check.errorf(m, _DuplicateMethod, "method %s.%s already declared", obj.Name(), m.name)
+			}
 			continue
 		}
 

test/alias2.go

@@ -36,9 +36,9 @@ type (
 
 // Methods can be declared on the original named type and the alias.
 func (T0) m1()  {} // GCCGO_ERROR "previous"
-func (*T0) m1() {} // ERROR "method redeclared: T0\.m1|T0\.m1 redeclared in this block|redefinition of .m1."
-func (A0) m1()  {} // ERROR "T0\.m1 redeclared in this block|redefinition of .m1."
-func (A0) m1()  {} // ERROR "T0\.m1 redeclared in this block|redefinition of .m1."
+func (*T0) m1() {} // ERROR "method redeclared: T0\.m1|T0\.m1 already declared|redefinition of .m1."
+func (A0) m1()  {} // ERROR "T0\.m1 already declared|redefinition of .m1."
+func (A0) m1()  {} // ERROR "T0\.m1 already declared|redefinition of .m1."
 func (A0) m2()  {}
 
 // Type aliases and the original type name can be used interchangeably.

test/fixedbugs/bug350.go

@@ -9,7 +9,7 @@ package main
 type T int
 
 func (T) m() {} // GCCGO_ERROR "previous"
-func (T) m() {} // ERROR "T[.]m redeclared|redefinition"
+func (T) m() {} // ERROR "T\.m already declared|redefinition"
 
 func (*T) p() {} // GCCGO_ERROR "previous"
-func (*T) p() {} // ERROR "[(][*]T[)][.]p redeclared|redefinition|redeclared"
+func (*T) p() {} // ERROR "T\.p already declared|redefinition"

test/fixedbugs/issue18655.go

@@ -11,12 +11,12 @@ type A = T
 type B = T
 
 func (T) m() {}
-func (T) m() {} // ERROR "redeclared|redefinition"
-func (A) m() {} // ERROR "redeclared|redefinition"
-func (A) m() {} // ERROR "redeclared|redefinition"
-func (B) m() {} // ERROR "redeclared|redefinition"
-func (B) m() {} // ERROR "redeclared|redefinition"
+func (T) m() {} // ERROR "already declared|redefinition"
+func (A) m() {} // ERROR "already declared|redefinition"
+func (A) m() {} // ERROR "already declared|redefinition"
+func (B) m() {} // ERROR "already declared|redefinition"
+func (B) m() {} // ERROR "already declared|redefinition"
 
-func (*T) m() {} // ERROR "redeclared|redefinition"
-func (*A) m() {} // ERROR "redeclared|redefinition"
-func (*B) m() {} // ERROR "redeclared|redefinition"
+func (*T) m() {} // ERROR "already declared|redefinition"
+func (*A) m() {} // ERROR "already declared|redefinition"
+func (*B) m() {} // ERROR "already declared|redefinition"

test/method1.go

@@ -12,10 +12,10 @@ package main
 type T struct{}
 
 func (t *T) M(int, string)  // GCCGO_ERROR "previous"
-func (t *T) M(int, float64) {} // ERROR "redeclared|redefinition"
+func (t *T) M(int, float64) {} // ERROR "already declared|redefinition"
 
 func (t T) H()  // GCCGO_ERROR "previous"
-func (t *T) H() {} // ERROR "redeclared|redefinition"
+func (t *T) H() {} // ERROR "already declared|redefinition"
 
 func f(int, string)  // GCCGO_ERROR "previous"
 func f(int, float64) {} // ERROR "redeclared|redefinition"