Fix display of empty string constant (#3207).

Changes

@@ -22,6 +22,7 @@ Verilator 4.215 devel
 * Fix array method names with parenthesis (#3181) (#3183). [Teng Huang]
 * Fix split_var assign merging (#3177) (#3179). [Yutetsu TAKATSUKASA]
 * Fix nested generate if genblk naming (#3189). [yanx21]
+* Fix display of empty string constant (#3207). [Julie Schwartz]
 
 
 Verilator 4.214 2021-10-17

src/V3Number.h

@@ -233,6 +233,7 @@ public:
     V3Number(String, AstNode* nodep, const string& value) {
         init(nodep, 0);
         setString(value);
+        m_fromString = true;
     }
     class Null {};
     V3Number(Null, AstNode* nodep) {

src/verilog.y

@@ -5006,7 +5006,15 @@ str<strp>:			// yaSTRING but with \{escapes} need decoded
 
 strAsInt<nodep>:
 		yaSTRING
-			{ $$ = new AstConst{$<fl>1, AstConst::VerilogStringLiteral(), GRAMMARP->deQuote($<fl>1, *$1)}; }
+			{ if ($1->empty()) {
+			      // else "" is not representable as number as is width 0
+                              // TODO all strings should be represented this way
+			      // until V3Width converts as/if needed to a numerical constant
+			      $$ = new AstConst{$<fl>1, AstConst::String{}, GRAMMARP->deQuote($<fl>1, *$1)};
+			  } else {
+			      $$ = new AstConst{$<fl>1, AstConst::VerilogStringLiteral(), GRAMMARP->deQuote($<fl>1, *$1)};
+			  }
+			}
 	;
 
 strAsIntIgnore<nodep>:		// strAsInt, but never matches for when expr shouldn't parse strings

test_regress/t/t_display.out

@@ -56,6 +56,7 @@ hello, from a concatenated string.
 hello, from a concatenated format string [0].
 extra argument:                    0
                    0 : pre argument after
+empty: ><
 [0] Embedded tab '	' and <#013> return
 [0] Embedded
 multiline

test_regress/t/t_display.v

@@ -149,6 +149,7 @@ module t;
       $write("hel", "lo, fr", "om a concatenated format string [%0t].\n", $time);
       $display("extra argument: ", $time);
       $display($time,, ": pre argument",, "after");
+      $display("empty: >%s<", "");
       $write("[%0t] Embedded tab '\t' and \r return\n", $time);
       $display("[%0t] Embedded\
 multiline", $time);