go/types, types2: remove remaining references to coreType in literals.go

For now, use commonUnder (formerly called sharedUnder) and update
error messages and comments. We can provide better error messages
in individual cases eventually.

For #70128.

Change-Id: I906ba9a0c768f6499c1683dc9be3ad27da8007a9
Reviewed-on: https://go-review.googlesource.com/c/go/+/653156
Reviewed-by: Robert Griesemer <[email protected]>
Auto-Submit: Robert Griesemer <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: griesemer

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

@@ -129,7 +129,7 @@ func (check *Checker) compositeLit(x *operand, e *syntax.CompositeLit, hint Type
 		typ = hint
 		base = typ
 		// *T implies &T{}
-		if b, ok := deref(coreType(base)); ok {
+		if b, ok := deref(commonUnder(check, base, nil)); ok {
 			base = b
 		}
 		isElem = true
@@ -142,7 +142,7 @@ func (check *Checker) compositeLit(x *operand, e *syntax.CompositeLit, hint Type
 		base = typ
 	}
 
-	switch utyp := coreType(base).(type) {
+	switch utyp := commonUnder(check, base, nil).(type) {
 	case *Struct:
 		// Prevent crash if the struct referred to is not yet set up.
 		// See analogous comment for *Array.
@@ -330,7 +330,7 @@ func (check *Checker) compositeLit(x *operand, e *syntax.CompositeLit, hint Type
 			}
 			var cause string
 			if utyp == nil {
-				cause = " (no core type)"
+				cause = " (no common underlying type)"
 			}
 			check.errorf(e, InvalidLit, "invalid composite literal%s type %s%s", qualifier, typ, cause)
 			x.mode = invalid

src/go/types/literals.go

@@ -8,9 +8,9 @@ type S1 struct{}
 type S2 struct{}
 
 func _[P *S1|*S2]() {
-	_= []P{{ /* ERROR "invalid composite literal element type P (no core type)" */ }}
+	_= []P{{ /* ERROR "invalid composite literal element type P (no common underlying type)" */ }}
 }
 
 func _[P *S1|S1]() {
-	_= []P{{ /* ERROR "invalid composite literal element type P (no core type)" */ }}
+	_= []P{{ /* ERROR "invalid composite literal element type P (no common underlying type)" */ }}
 }