c++: Avoid strict_aliasing_warning on dependent types or expressions [PR94951]

The following testcase gets a bogus warning during build_base_path,
when cp_build_indirect_ref* calls strict_aliasing_warning with a dependent
expression.  IMHO calling get_alias_set etc. on dependent types feels wrong
to me, we should just defer the warnings in those cases until instantiation
and only handle the cases where neither type nor expr are dependent.

2020-05-06  Jakub Jelinek  <jakub@redhat.com>

	PR c++/94951
	* typeck.c (cp_strict_aliasing_warning): New function.
	(cp_build_indirect_ref_1, build_reinterpret_cast_1): Use
	it instead of strict_aliasing_warning.

	* g++.dg/warn/Wstrict-aliasing-bogus-tmpl.C: New test.

gcc/cp/ChangeLog

 2020-05-06  Jakub Jelinek  <jakub@redhat.com>
 
+	PR c++/94951
+	* typeck.c (cp_strict_aliasing_warning): New function.
+	(cp_build_indirect_ref_1, build_reinterpret_cast_1): Use
+	it instead of strict_aliasing_warning.
+
+2020-05-06  Jakub Jelinek  <jakub@redhat.com>
+
 	PR c++/94907
 	* method.c (defaulted_late_check): Don't call synthesize_method
 	on constexpr sfk_comparison if it has been called on it already.

gcc/cp/typeck.c

@@ -3318,6 +3318,22 @@ build_x_indirect_ref (location_t loc, tree expr, ref_operator errorstring,
     return rval;
 }
 
+/* Like c-family strict_aliasing_warning, but don't warn for dependent
+   types or expressions.  */
+
+static bool
+cp_strict_aliasing_warning (location_t loc, tree type, tree expr)
+{
+  if (processing_template_decl)
+    {
+      tree e = expr;
+      STRIP_NOPS (e);
+      if (dependent_type_p (type) || type_dependent_expression_p (e))
+	return false;
+    }
+  return strict_aliasing_warning (loc, type, expr);
+}
+
 /* The implementation of the above, and of indirection implied by other
    constructs.  If DO_FOLD is true, fold away INDIRECT_REF of ADDR_EXPR.  */
 
@@ -3360,10 +3376,10 @@ cp_build_indirect_ref_1 (location_t loc, tree ptr, ref_operator errorstring,
 	  /* If a warning is issued, mark it to avoid duplicates from
 	     the backend.  This only needs to be done at
 	     warn_strict_aliasing > 2.  */
-	  if (warn_strict_aliasing > 2)
-	    if (strict_aliasing_warning (EXPR_LOCATION (ptr),
-					 type, TREE_OPERAND (ptr, 0)))
-	      TREE_NO_WARNING (ptr) = 1;
+	  if (warn_strict_aliasing > 2
+	      && cp_strict_aliasing_warning (EXPR_LOCATION (ptr),
+					     type, TREE_OPERAND (ptr, 0)))
+	    TREE_NO_WARNING (ptr) = 1;
 	}
 
       if (VOID_TYPE_P (t))
@@ -7777,7 +7793,7 @@ build_reinterpret_cast_1 (location_t loc, tree type, tree expr,
       expr = cp_build_addr_expr (expr, complain);
 
       if (warn_strict_aliasing > 2)
-	strict_aliasing_warning (EXPR_LOCATION (expr), type, expr);
+	cp_strict_aliasing_warning (EXPR_LOCATION (expr), type, expr);
 
       if (expr != error_mark_node)
 	expr = build_reinterpret_cast_1
@@ -7891,7 +7907,7 @@ build_reinterpret_cast_1 (location_t loc, tree type, tree expr,
 
       if (warn_strict_aliasing <= 2)
 	/* strict_aliasing_warning STRIP_NOPs its expr.  */
-	strict_aliasing_warning (EXPR_LOCATION (expr), type, expr);
+	cp_strict_aliasing_warning (EXPR_LOCATION (expr), type, expr);
 
       return build_nop_reinterpret (type, expr);
     }

gcc/testsuite/ChangeLog

 2020-05-06  Jakub Jelinek  <jakub@redhat.com>
 
+	PR c++/94951
+	* g++.dg/warn/Wstrict-aliasing-bogus-tmpl.C: New test.
+
+2020-05-06  Jakub Jelinek  <jakub@redhat.com>
+
 	PR c++/94907
 	* g++.dg/cpp2a/spaceship-synth8.C: New test.
 

gcc/testsuite/g++.dg/warn/Wstrict-aliasing-bogus-tmpl.C

+// PR c++/94951
+// { dg-do compile }
+// { dg-options "-O2 -Wall" }
+
+struct A { int a; };
+template <int N>
+struct B : public A
+{
+  static B<N> foo () { B<N> t; t.a = 4; return t; }	// { dg-bogus "dereferencing type-punned pointer will break strict-aliasing rules" }
+};
+
+B<0> b = B<0>::foo ();