go/types, types2: report type name in comp. literal error, if possible

When reporting an error for the element type of a struct literal, use
the element type's type name rather than it's underlying/core type.

Also, combine error reporting for invalid composite literal types in
one place, at the end.

Fixes golang#68184.

Change-Id: I1f407d5403777948da9a0eca95aacc1389f4bd44
Reviewed-on: https://go-review.googlesource.com/c/go/+/595075
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Robert Griesemer <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
Reviewed-by: Tim King <[email protected]>

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

@@ -1151,6 +1151,7 @@ func (check *Checker) exprInternal(T *target, x *operand, e syntax.Expr, hint Ty
 
 	case *syntax.CompositeLit:
 		var typ, base Type
+		var isElem bool // true if composite literal is an element of an enclosing composite literal
 
 		switch {
 		case e.Type != nil:
@@ -1171,11 +1172,12 @@ func (check *Checker) exprInternal(T *target, x *operand, e syntax.Expr, hint Ty
 		case hint != nil:
 			// no composite literal type present - use hint (element type of enclosing type)
 			typ = hint
-			base, _ = deref(coreType(typ)) // *T implies &T{}
-			if base == nil {
-				check.errorf(e, InvalidLit, "invalid composite literal element type %s (no core type)", typ)
-				goto Error
+			base = typ
+			// *T implies &T{}
+			if b, ok := deref(coreType(base)); ok {
+				base = b
 			}
+			isElem = true
 
 		default:
 			// TODO(gri) provide better error messages depending on context
@@ -1361,7 +1363,15 @@ func (check *Checker) exprInternal(T *target, x *operand, e syntax.Expr, hint Ty
 			}
 			// if utyp is invalid, an error was reported before
 			if isValid(utyp) {
-				check.errorf(e, InvalidLit, "invalid composite literal type %s", typ)
+				var qualifier string
+				if isElem {
+					qualifier = " element"
+				}
+				var cause string
+				if utyp == nil {
+					cause = " (no core type)"
+				}
+				check.errorf(e, InvalidLit, "invalid composite literal%s type %s%s", qualifier, typ, cause)
 				goto Error
 			}
 		}

src/go/types/expr.go

@@ -1129,6 +1129,7 @@ func (check *Checker) exprInternal(T *target, x *operand, e ast.Expr, hint Type)
 
 	case *ast.CompositeLit:
 		var typ, base Type
+		var isElem bool // true if composite literal is an element of an enclosing composite literal
 
 		switch {
 		case e.Type != nil:
@@ -1151,11 +1152,12 @@ func (check *Checker) exprInternal(T *target, x *operand, e ast.Expr, hint Type)
 		case hint != nil:
 			// no composite literal type present - use hint (element type of enclosing type)
 			typ = hint
-			base, _ = deref(coreType(typ)) // *T implies &T{}
-			if base == nil {
-				check.errorf(e, InvalidLit, "invalid composite literal element type %s (no core type)", typ)
-				goto Error
+			base = typ
+			// *T implies &T{}
+			if b, ok := deref(coreType(base)); ok {
+				base = b
 			}
+			isElem = true
 
 		default:
 			// TODO(gri) provide better error messages depending on context
@@ -1343,7 +1345,15 @@ func (check *Checker) exprInternal(T *target, x *operand, e ast.Expr, hint Type)
 			}
 			// if utyp is invalid, an error was reported before
 			if isValid(utyp) {
-				check.errorf(e, InvalidLit, "invalid composite literal type %s", typ)
+				var qualifier string
+				if isElem {
+					qualifier = " element"
+				}
+				var cause string
+				if utyp == nil {
+					cause = " (no core type)"
+				}
+				check.errorf(e, InvalidLit, "invalid composite literal%s type %s%s", qualifier, typ, cause)
 				goto Error
 			}
 		}

src/internal/types/testdata/fixedbugs/issue68184.go

@@ -0,0 +1,38 @@
+// Copyright 2024 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 p
+
+type VeryLongStruct struct {
+	A1  int
+	A2  int
+	A3  int
+	A4  int
+	A5  int
+	A6  int
+	A7  int
+	A8  int
+	A9  int
+	A10 int
+	A11 int
+	A12 int
+	A13 int
+	A14 int
+	A15 int
+	A16 int
+	A17 int
+	A18 int
+	A19 int
+	A20 int
+}
+
+func _() {
+	// The error messages in both these cases should print the
+	// struct name rather than the struct's underlying type.
+
+	var x VeryLongStruct
+	x.B2 /* ERROR "x.B2 undefined (type VeryLongStruct has no field or method B2)" */ = false
+
+	_ = []VeryLongStruct{{B2 /* ERROR "unknown field B2 in struct literal of type VeryLongStruct" */ : false}}
+}