cmd/compile: support structural typing in unified IR

This CL updates unified IR to look at the structural type of a
composite literal type, rather than merely the underlying type, to
determine if it's a structure. This fixes a number of currently
failing regress test cases.

Updates #50833.

Change-Id: I11c040c77ec86c23e8ffefcf1ce1aed548687dc5
Reviewed-on: https://go-review.googlesource.com/c/go/+/381074
Run-TryBot: Matthew Dempsky <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Trust: Matthew Dempsky <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>

Author: mdempsky

src/cmd/compile/internal/noder/writer.go

@@ -1218,6 +1218,7 @@ func (w *writer) expr(expr syntax.Expr) {
 		}
 
 		obj := obj.(*types2.Var)
+		assert(!obj.IsField())
 		assert(targs.Len() == 0)
 
 		w.code(exprLocal)
@@ -1337,10 +1338,11 @@ func (w *writer) compLit(lit *syntax.CompositeLit) {
 	w.typ(tv.Type)
 
 	typ := tv.Type
+	// TODO(mdempsky): Use types2.StructuralType here too? See #50833.
 	if ptr, ok := typ.Underlying().(*types2.Pointer); ok {
 		typ = ptr.Elem()
 	}
-	str, isStruct := typ.Underlying().(*types2.Struct)
+	str, isStruct := types2.StructuralType(typ).(*types2.Struct)
 
 	w.len(len(lit.ElemList))
 	for i, elem := range lit.ElemList {

test/run.go

@@ -2167,7 +2167,6 @@ var unifiedFailures = setOf(
 
 	"fixedbugs/issue42284.go",  // prints "T(0) does not escape", but test expects "a.I(a.T(0)) does not escape"
 	"fixedbugs/issue7921.go",   // prints "… escapes to heap", but test expects "string(…) escapes to heap"
-	"typeparam/issue48538.go",  // assertion failure, interprets struct key as closure variable
 	"typeparam/issue47631.go",  // unified IR can handle local type declarations
 	"fixedbugs/issue42058a.go", // unified IR doesn't report channel element too large
 	"fixedbugs/issue42058b.go", // unified IR doesn't report channel element too large
@@ -2178,10 +2177,7 @@ var unifiedFailures = setOf(
 	"typeparam/typeswitch2.go", // duplicate case failure due to stenciling
 	"typeparam/typeswitch3.go", // duplicate case failure due to stenciling
 	"typeparam/typeswitch4.go", // duplicate case failure due to stenciling
-	"typeparam/issue50417b.go", // Need to handle field access on a type param
 	"typeparam/issue50552.go",  // gives missing method for instantiated type
-	"typeparam/absdiff2.go",    // wrong assertion about closure variables
-	"typeparam/absdiffimp2.go", // wrong assertion about closure variables
 )
 
 func setOf(keys ...string) map[string]bool {